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Highly prevalent MDR, frequently carrying virulence genes and antimicrobial resistance genes in Salmonella enterica serovar 4,[5],12:i:- isolates from Guizhou Province, China

  • Li Long ,

    Contributed equally to this work with: Li Long, Lv You

    Roles Data curation, Formal analysis, Validation, Writing – original draft

    Affiliation Laboratory of Bacterial Disease, Experimental Center, Guizhou Provincial Center for Disease Control and Prevention, Guiyang, People’s Republic of China

  • Lv You ,

    Contributed equally to this work with: Li Long, Lv You

    Roles Data curation, Formal analysis, Investigation, Validation

    Affiliation Laboratory of Bacterial Disease, Experimental Center, Guizhou Provincial Center for Disease Control and Prevention, Guiyang, People’s Republic of China

  • Dan Wang,

    Roles Data curation, Software

    Affiliation Institute of Communicable Disease Control and Prevention, Guizhou Provincial Center for Disease Control and Prevention, Guiyang, People’s Republic of China

  • Ming Wang,

    Roles Software

    Affiliation Laboratory of Bacterial Disease, Experimental Center, Guizhou Provincial Center for Disease Control and Prevention, Guiyang, People’s Republic of China

  • Junhua Wang,

    Roles Software, Visualization

    Affiliation School of Public Health, the Key Laboratory of Environmental Pollution Monitoring and Disease Control, Ministry of Education, Guizhou Medical University, Guiyang, China

  • Guihuan Bai,

    Roles Validation

    Affiliation School of Public Health, the Key Laboratory of Environmental Pollution Monitoring and Disease Control, Ministry of Education, Guizhou Medical University, Guiyang, China

  • Jianhua Li,

    Roles Software, Visualization

    Affiliation Tongren City Center for Disease Control and Prevention, Tongren, People’s Republic of China

  • Xiaoyu Wei ,

    Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Writing – original draft, Writing – review & editing

    weixyuse@foxmail.com (XW); zjumedjun@163.com (SL)

    Affiliation Laboratory of Bacterial Disease, Experimental Center, Guizhou Provincial Center for Disease Control and Prevention, Guiyang, People’s Republic of China

  • Shijun Li

    Roles Methodology, Supervision, Visualization

    weixyuse@foxmail.com (XW); zjumedjun@163.com (SL)

    Affiliation Laboratory of Bacterial Disease, Experimental Center, Guizhou Provincial Center for Disease Control and Prevention, Guiyang, People’s Republic of China

Abstract

Salmonella enterica serovar 4,[5],12:i:-, a monophasic variant of Salmonella Typhimurium lacking the phase 2 flagellin, is one of the common serotypes causing Salmonellosis worldwide. However, information on Salmonella serovar 4,[5],12:i:- from Guizhou Province has lacked so far. This study aimed to investigate the antimicrobial resistance, the presence of antimicrobial resistance genes and virulence genes, and characterize the MLST genotypes of Salmonella serovar 4,[5],12:i:- isolates from Guizhou province, China. We collected 363 non-typhoid Salmonella (NTS) isolates of Guizhou from 2013 to 2018. Biochemical identification, serogroups testing, and specific multiplex polymerase chain reaction (mPCR) assay were conducted to identify Salmonella 4,[5],12:i:- isolates. Isolates were determined the antimicrobial resistance by the micro broth dilution method, detected the presence of antimicrobial resistance genes and virulence genes by PCR, and examined the molecular genotyping by Multilocus sequence typing (MLST). Eighty-seven Salmonella 4,[5],12:i:- isolates were detected, accounting for 23.9% (87/363) of the total NTS isolates. All Salmonella 4,[5],12:i:- isolates showed highly resistant to sulfaoxazole (93.1%), streptomycin (90.8%), ampicillin (88.5%), tetracycline (86.2%) and doxycycline (86.2%). A high proportion (94.2%) of multi-drug resistance (MDR) isolates were found. Most (83.9%) Salmonella 4,[5],12:i:- isolates carried four antimicrobial resistance genes, especially blaTEM-1, strA-strB, sul2, and tetB genes. Salmonella 4,[5],12:i:- isolates showed a high rate of invA, sseL, mgtC, siiE, sopB, gipA, gtgB, sspH1, and sspH2 (72.4%~98.9%). On the contrary, none of the isolates were detected the spvC and pefA genes. MLST analysis revealed three sequence types (STs), and ST34 (97.7%) was the dominant sequence type. This study is the first report of Salmonella 4,[5],12:i:- in humans from Guizhou province, China. The data might be useful for rational antimicrobial usage against Salmonella 4,[5],12:i:- infections, risk management, and public health strategies in Guizhou.

Introduction

Non-typhoid Salmonella (NTS) is one of the most common causes of human infectious diarrheal diseases and causes a severe disease burden [1]. In the late 1980s, a Salmonella serovar 4,[5],12:i:-, closely linked to the antigenic structure and genetic characterization of Salmonella Typhimurium, was first identified from poultry in Portugal [2]. Since then, this serotype has been increasingly spread around the world and has caused more significant outbreaks in Luxembourg and Italy [3, 4].

Multi-drug resistance (MDR) has been increased all worldwide that is considered a public health threat. Several recent investigations reported the emergence of multidrug-resistant bacterial pathogens from different origins including humans, birds, fish, and cattles that increase the need for routine application of the antimicrobial susceptibility testing to detect the antibiotic of choice as well as the screening of the emerging MDR strains [58]. The rapid dissemination of Salmonella 4,[5],12:i:- is related to the increasing MDR, which is a significant problem of public health, and may represent the advantage of its pathogenesis [9, 10]. Significantly, the horizontal transfer of resistance genes mediated by mobile genetic elements such as transposons and plasmids may enhance the survival adaptability of this serotype [11].

Salmonella infects the host by first attaching to the host tissue and then invading the host cells through virulence factors, mainly including Salmonella pathogenicity island (SPI), plasmid virulence, prophage virulence and cell swelling toxins [12]. SpI-1 is necessary to invade of host non-phagocytes, and plays an essential role in salmonella invasion of macrophages and intestinal epithelial cells [13]. SpI-2 encodes a type III secretory system associated with systemic infection. It allows Salmonella to survive inside the macrophage and it facilitates spreading through the host body [13]. Salmonella plasmid virulence genes enhance the ability to spread and proliferate in the host, and most of them are associated with extra-intestinal infection in humans and animals [12, 13]. In addition, previous investigations indicated that Salmonella 4,[5],12:i:- serotype was more virulent than other serotypes [14]. The presence of multiple virulence determinants, along with the formation of biofilm, enables Salmonella 4,[5],12:i:- to infect humans and results in disease or death [14, 15].

Salmonella 4,[5],12:i:- is a monophasic serotype due to its lack of phase 2 flagellar antigen expression [10]. Several distinct molecular subtyping methods, including phage typing, multilocus sequence typing (MLST), pulsed-field gel electrophoresis (PFGE), and whole genome sequencing (WGS) were used to identify Salmonella 4,[5],12:i:–isolates [1618]. Studies indicated that Salmonella 4,[5],12:i:- isolates belong to multiple clonal lines, which evolved from different Salmonella Typhimurium clonal ancestors through different genetic events [19, 20].

In Guizhou, the Southwest of China, NTS was one of the primary pathogens causing infectious diarrhea, and the distribution of serotypes was diverse [21, 22]. However, information on Salmonella serovar 4,[5],12:i:- from Guizhou is lacking. To provide a better understanding of the characterization Salmonella 4,[5],12:i:-, we characterized the antimicrobial resistance, the presence of antimicrobial resistance genes and virulence genes, and the genetic characterization of Salmonella 4,[5],12:i:- isolates in Guizhou from 2013 to 2018.

Materials and methods

Ethics statement

This study was reviewed and approved by the Ethics Review Committee of Guizhou Provincial Center for Disease Control and Prevention. All data were analyzed anonymously.

Bacterial isolates and identification

A total of 363 Salmonella isolates were collected from nine different cities between 2013 and 2018 in Guizhou, including Anshun (n = 54), Bijie (n = 10), Guiyang (n = 76), Liupanshui (n = 10), Qiandongnan (n = 22), Qiannan (n = 19), Qianxinan (n = 12), Tongren (n = 97), and Zunyi (n = 63), and they were all obtained from clinical patients. Most isolates were available from stool samples of outpatients, and the source of a few isolates was unknown. The obtained samples were inoculated into selenite brilliant green sulfa enrichment (SBG), cultured at 36°C ± 1°C for 6–8 h (Youkang Biological, China). The enriched bacteria solution was inoculated on the Salmonella chromogenic medium, incubated at 36°Cfor 18–24 h (CHROMagar, France). Pale purple or purple colonies were selected and inoculated onto krebs disaccharide iron medium (KIA) and motility indol urea iron medium (MIU) at 36°C for 18–24 h (Cyclokay Biological, China). The isolates were systematically identified by API20E identification kits (Biomerieux, France). According to the White- Kaufmann- Le Minor Scheme [23], the confirmed Salmonella isolates were serotyped by slide agglutination test for O and H antigens (SSI, Denmark). Flagellar induction testing was performed on the isolates with antigenic formula (4,[5],12:i:-). If induction testing showed negative for H2 phase, the flagellar antigen gene (fljB) and IS200 fragment (fljB-fljA) of H2 phase were further detected by a multiple PCR as described by Tennant et al [24]. Primers used to amplify fljB and fljB-fljA genes were listed in S1 Table. Bacterial DNA was extracted by the boiled lysis method. The supernatant was taken as DNA template and stored at -80°C for use. The multiple PCR reaction conditions were as follows: initial denaturation at 95°C for 2 min, 30 cycles of denaturation at 95°C for 30 s, annealing at 58°C for 30 s, extension at 72°C for 90 s, and a final delay at 72°C for 10 min.

Antimicrobial susceptibility test

Antimicrobial susceptibility was evaluated by micro broth dilution method with ten classes 16 antimicrobials (Xingbai Biological, China), including: Penicillin (Ampicillin), Phenicols (Chloramphenicol), Aminoglycosides (Streptomycin, Gentamicin), Carbapenems (Imipenem), β-lactamase inhibitor (Amoxicillin/clavulanic acid), Cephems (Cefoxitin, Ceftriaxone, Cefepime), Sulfonamides (Sulfamethoxazole, Trimethoprim /sulfamethoxazole), Tetracyclines (Tetracycline, Doxycycline), Quinolones and Fluoroquinolones (Nalidixic acid, Ciprofloxacine), Macrolides (Azithromycin). Escherichia coli ATCC 25922 was used as a control strain. The breakpoints for antimicrobials followed interpretive standards provided by Clinical Laboratory Standards Institute guidelines [25]. The phenotypic resistance profiles were classified into MDR, XDR, and PDR as described by Magiorakos et al [26].

Detection of antimicrobial resistance genes

Genes coding for resistance to β-lactamase (blaTEM, blaOXA-1, blaCTX-M), phenicols (floR, cmlA1), aminoglycosides (aac (3)-IV, strA-strB, aadA2), sulfonamides (sul2), and tetracyclines (tetB) were evaluated by PCR using primers and conditions as previously described [2729]. The reaction volume was 20 μl. Primers used to amplify the antimicrobial resistance genes and PCR reaction conditions in this study were listed in S2 Table. The agar gel electrophoresis was carried out to separate the obtained PCR products using 1.0% agarose, followed by photographing the gel. All positive PCR products of blaTEM and blaCTX-M genes were sequenced and aligned with the National Centre for Biotechnology Information (NCBI) database sequences using the BLAST program to identify resistance gene subtypes. The correlation between phenotypic and genotypic was performed.

Detection of virulence genes

Fourteen virulence genes were detected by PCR. These virulence genes are related to the presence of Salmonella pathogenicity island (invA, sseL, mgtC, siiE, sopB), prophages (gipA, gtgB, sopE, sspH1, sspH2), and plasmids (spvB, spvC, spvR, pefA). The primer sequences as previously described [13, 30, 31] were listed in S3 Table. The PCR reaction conditions were as follows: initial denaturation at 94°C for 5 min, 28 cycles of denaturation at 94°C for 30 s, annealing at 58°C for 30 s, extension at 72°C for 1min, and a final delay at 72°C for 5 min. The PCR products were analyzed by electrophoresis and visualized under ultraviolet light.

Multilocus sequence typing (MLST)

MLST typing was executed for all Salmonella 4,[5],12:i:- isolates based on seven housekeeping genes, including thrA, purE, sucA, hisD, hemD, aroC and dnaN. Primers used to amplify the seven housekeeping genes in this study were listed in S4 Table. The amplification conditions were as follows: initial denaturation at 94°C for 5min, 30 cycles of denaturation at 94°C for 30 S, annealing at 56°C for 1 min, extension at 72°C for 1 min, and a final delay at 72°C for 10 min. Alleles and ST types of isolates were obtained from the Salmonella database on the PubMLST website. The phylogenetic tree was constructed using BioNumerics 8.0 software (Applied-Maths, Belgium).

Statistical analysis

We used Cohen’s kappa coefficient to assess the correlation between phenotypic and genotypic resistance. The agreement, as expressed by the kappa coefficient, was interpreted as follows: values ≤ 0 as indicating no agreement and 0.01–0.20 as none to slight, 0.21–0.40 as fair, 0.41–0.60 as moderate, 0.61–0.80 as substantial, and 0.81–1.00 as almost perfect agreement [32]. All P values were two-tailed, and the level of statistical significance was specified as 0.05. Statistical analyses were performed using version 26.0 SPSS statistical software.

Results

Phenotypic characteristics of the recovered Samonella isolates

As described in the methods section, samples were enriched using SBG. A single colony purple or pale purple colony was observed on the Salmonella chromogenic medium. Biochemical results for Salmonella on KIA and MIU were slant (K), butt (A), H2S (+), gas (+), dynamic (+), indole (-), urea (-). After the multiple PCR detection, all Salmonella Typhimurium isolates produced two amplification bands (1 000 bp and 1 389 bp), specific to Salmonella Typhimurium. However, the Salmonella 4,[5],12:i:- isolates produced only one 1 000 bp amplification band (S1 Fig). Among the 363 NTS isolates, 23.9% (87/363) were confirmed as Salmonella 4,[5],12:i:-. They were distributed in seven of the nine cities in Guizhou, which were Anshun (n = 10), Guiyang (n = 14), Liupanshui (n = 2), Qiandongnan (n = 3), Qiannan (n = 2), Tongren (n = 37), and Zunyi (n = 20), respectively. The prevalence of Salmonella 4,[5],12:i:- ranged between 18.1% to 29.4% in 2013–2018 with the highest prevalence in 2015 (Fig 1).

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Fig 1. Detection of Salmonella 4,[5],12:i:- isolates from Guizhou, 2013–2018.

https://doi.org/10.1371/journal.pone.0266443.g001

Antimicrobial resistance

Antimicrobial resistance testing showed that Salmonella 4,[5],12:i:- isolates were shown to be the most resistant to sulfaoxazole (93.1%), followed by streptomycin (90.8%), ampicillin (88.5%), tetracycline (86.2%), and doxycycline (86.2%). Furthermore, Salmonella 4,[5],12:i:- isolates showed resistance to chloramphenicol (42.5%), trimethoprim /sulfamethoxazole (35.6%), nalidixic acid (34.5%), and amoxicillin/clavulanic acid (31.0%), respectively (Table 1). The resistance to ciprofloxacin was 13.8%. However, 35.6% of isolates showed decreased sensitivity to ciprofloxacin (MIC≥0.12 μg/mL). Notably, Salmonella 4,[5],12:i:- isolates showed resistance to imipenem and azithromycin in this study. More importantly, three isolates were co-resistant to ciprofloxacin, the third and fourth-generation cephalosporins, and azithromycin.

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Table 1. Antimicrobial resistance of 87 Salmonella 4,[5],12:i:- isolates.

https://doi.org/10.1371/journal.pone.0266443.t001

A percentage of 98.9% (86/87) isolates were resistant to at least one antimicrobial agent. Most (62.0%) isolates were resistant to 5–7 among the 16 antimicrobial agents tested. Notably, 12 isolates (13.8%) were resistant to ten or more antimicrobial agents, among which two isolates were resistant to 13 antimicrobial agents in 2016–2017. A total of 89.6% (78/87) of the isolates were MDR. Four isolates (4.5%) showed XDR (Table 2). PDR isolates were not observed. Fourty-five antimicrobial resistance profiles were observed, of which AMP+STR+SOX+TCR+ DOX+AMC (16.1%,14/87) and AMP+STR+SOX+TCR+DOX (14.9%, 14/87) were the predominant antimicrobial resistance profiles.

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Table 2. Antimicrobial resistance profile of Salmonella 4,[5],12:i:- isolates.

https://doi.org/10.1371/journal.pone.0266443.t002

Antimicrobial resistance genes distribution

A great majority (83.9%) of Salmonella 4,[5],12:i:- isolates contained at least four antimicrobial resistance genes. As regards antimicrobial resistance genes, genes more often detected were tetB (94.2%), strA-strB (93.1%), sul2 (91.9%) and blaTEM-1 (74.7%). The existance of other resistance genes including blaOXA-1, blaCTX-M, aac (3)-IV, aadA2, cmlA1, and floR were 5.7%, 13.8%, 5.7%, 31.0%, 30.0%, and 20.2%, respectively. Among the positive isolates to β-lactamase resistance gene, nine isolates contained two β-lactamase genes (blaTEM-1/blaCTX-M, blaTEM-1/blaOXA-1), and one isolate contained blaTEM-1, blaCTX-M and blaOXA-1 genes. Sequencing of the blaCTX-M genes revealed presence of various types, including blaCTX-M-55 (58.3%, 7/12), blaCTX-M-65 (12.7%, 2/12), blaCTX-M-14 (8.3%, 1/12), blaCTX-M-15 (8.3%, 1/12), and blaCTX-M-27 (8.3%, 1/12) (S2 Fig).

The correlation between the phenotypic and genotypic MDR profiles

Our findings revealed that 12.6% (11/87) isolates were MDR to five antimicrobial classes (AMP, STR, SOX, TCR, DOX, AMC) and harbored blaTEM-1, strA-strB, sul2, tetB. Nine (10.3%) MDR isolates to four antimicrobial classes (AMP, STR, SOX, TCR, DOX) and harbored blaTEM-1, strA-strB, sul2, tetB. Besides, two (2.3%) XDR isolates to eight antimicrobial classes (AMP, CHL, STR, GEN, SOX, TCR, DOX, NAL, CIP, AMC, AZM, CRO, FEP) and harbored blaTEM-1, blaCTX-M-55, aac(3) -IV, strA-strB, sul2, tetB (Fig 2). The kappa correlation between phenotypic and genotypic resistance was showed in Table 3. Cohen’s kappa was the highest for blaCTX-M vs CRO (Kappa = 0.794) and blaCTX-M vs FEP (Kappa = 0.673), followed by cmlA1 (Kappa = 0.509) vs CHL, strA-strB vs STR (Kappa = 0.418), and tetB vs TCY (Kappa = 0.388) (Table 3). In general, a certain correlation was seen between the antimicrobial phenotypes with genotypes.

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Fig 2. MLST clustering tree of the 87 Salmonella 4,[5],12:i:- isolates in Guizhou from 2013 to 2018 with the antimicrobial resistance profile, antimicrobial resistance genes, and virulence gene profile.

https://doi.org/10.1371/journal.pone.0266443.g002

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Table 3. The correlation between the phenotypic and genotypic of Salmonella 4,[5],12:i:- isolates.

https://doi.org/10.1371/journal.pone.0266443.t003

Virulence genes distribution

All Salmonella 4,[5],12:i:- isolates carried invA, siiE and sopB genes. The existence of other virulence genes including sseL, mgtC, gipA, gtgB, sspH1, and sspH2 were 72.4%, 98.9%, 97.7%, 95.4%, 79.3%, and 89.7%, respectively. SopE gene was present in 33 isolates with a detection rate of 37.9%. SpvB and spvR genes were detected in one isolate, while neither spvC nor pefA genes were present. All isolates harbored at least six virulence genes, and 62.1% (54/87) isolates were positive to nine or more virulence genes. A total of 20 different virulence gene profiles (VP1~VP20) among the 87 Salmonella 4,[5],12:i:- isolates were observed, and VP1 (invA-sseL-mgtC-siiE-sopB-gipA-gtgB-sspH1-sspH2) was the primary one, accounting for 33.3% (29/87), as shown in Table 4 and S3 Fig.

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Table 4. Virulence gene profiles of the Salmonella 4,[5],12:i:- isolates.

https://doi.org/10.1371/journal.pone.0266443.t004

MLST typing

All the 87 Salmonella 4,[5],12:i:- isolates were classified into three STs by MLST typing (Fig 2 and S4 Fig). ST34 was the dominant sequence type of Salmonella 4,[5],12:i:-, accounting for 97.7% (85/87). The other two STs were ST19 and ST1746, respectively. ST34 and ST1746 were a single-locus variant of ST19, with only one allele locus difference. The dnaN allele was distinct between ST34 and ST19 (dnaN19 replaced dnaN7). The purE allele was distinct between ST1746 and ST19 (purE6 replaced purE5).

Discussion

Salmonella 4,[5],12:i:- has increased significantly in human cases of Salmonellosis within the past two decades [10]. In Europe, Salmonella 4,[5],12:i:- was the third most common serotype of human Salmonellosis, accounting for 7.9% of foodborne disease outbreaks [33]. In recent years, this serotype has been increasing in China, and it has become one of the four most common serotypes causing human Salmonellosis [34]. Previous studies suggested that most Salmonella 4,[5],12:i:- isolates were from pigs and pork products, while other sources were thought to be rare [33, 35]. Here, the prevalence of Salmonella 1,4,[5],12:i:- clinical isolates in Guizhou over six years was higher compared with other regions and countries [16, 34].

Tests of susceptibility to 16 antimicrobial agents showed that isolates exhibited high resistance to sulfamethoxazole, streptomycin, ampicillin, tetracycline, and doxycycline (82.5%~92.0%), which was similar to other studies in China [36, 37], but much higher than that reported in Korea [16] and Japan [38]. Extended-spectrum cephalosporins and fluoroquinolones are the two most crucial antimicrobials for the treatment of invasive and severe infection of Salmonella, and the third/fourth-generation cephalosporins are mainly known as "Critically important antimicrobials" [39]. In the present study, we observed 19.5% and 14.9% isolates were resistant to ceftriaxone and cefepime, respectively. In addition, the sensitivity of ciprofloxacin decreased significantly, which may affect the clinical treatment effect. Azithromycin is a significant antibacterial drug with safety and excellent activity in treating Salmonellosis [40]. Notably, we found 3.4% Salmonella 4,[5],12:i:- isolates were co-resistance to azithromycin, ciprofloxacin, third /fourth-generation cephalosporins, and azithromycin. Carbapenems are atypical β-lactamase antimicrobial with the broadest antimicrobial spectrum, and it is still infrequent in the treatment of Salmonellosis [41]. Unfortunately, an imipenem-resistant Salmonella 4,[5],12:i:- isolate, collected from a 9-month-old boy in 2015 from Tongren, was found in this study. Our results indicated that the antimicrobial resistance phenomenon of Salmonella 4,[5],12:i:- in Guizhou was not optimistic and dynamic monitoring of antimicrobial resistance should be strengthened for this serotype.

Isolates showed a high-level MDR (89.6%) in our study, which was much higher than that of previous studies in China [34], Switzerland [42], and Denmark [35]. Over the last two decades, two primary MDR clones of Salmonella 4,[5],12:i:- were recognized as important for public health [43]. One clonal line (European clone) was characterized by chromosomally encoded resistance to ampicillin, streptomycin, sulfonamides, and tetracyclines (ASSuT), and another clonal line (Spanish clone) was characterized by plasmid-encoded resistance to ampicillin, chloramphenicol, sulfonamides, gentamicin, streptomycin, tetracycline, and trimethoprim (ACSuGSTTm) [10, 43]. In our study, 44.8% and 12.6% isolates displayed similar MDR profiles with European and Spanish clones, respectively. Moreover, isolates exhibited more comprehensive MDR profiles. In our study, isolates showed resistance to 13 of the 16 antimicrobial agents, further limiting the selection of antimicrobials for clinical treatment of Salmonella 4,[5],12:i:- infection.

Antimicrobial resistance gene detection showed that most isolates harbored blaTEM-1, strA-strB, sul2 and tetB genes regardless of origin. These genes are present in a chromosomal resistance island of Salmonella [10]. They are typically associated with the European clone [10, 15], which suggested that the resistant clone of Salmonella 4,[5],12:i:- isolates from Guizhou might be related to the European clone. There was a specific correlation between the antimicrobial phenotypes of β-lactamase, phenicols, aminoglycosides, sulfonamides, and tetracyclines with their resistance genotypes. Among the phenicols resistance genes, cmlAl and floR genes were found to have a moderate and fair correlation with phenotypic resistance of chloramphenicol, respectively. They can be located in large plasmids and transposons of Salmonella 4,[5],12:i:-, enabling the transfer of resistance genes between isolates [44]. Among the aminoglycosides resistance genes, the strA-strB gene has been found to have moderate correlation with phenotypic resistance of streptomycin. However, the streptomycin phenotype was mainly related to aph(3’)-Ia, aadA1, and aadA2 genes in Huang et al [45]. Among the sulfonamides resistance gene, sul2 gene was found to have fair correlation with phenotypic resistance of sulfaisoxazole. Sul2 gene was dominant in most sulfonamides resistance mechanisms of Salmonella 4,[5],12:i:- and always coexisted with sul1, sul3, and dfrA12 genes [44]. Among the tetracyclines resistance gene, tetB gene has been correlated with phenotypic resistance of tetracycline and doxycycline. This gene is the most common active efflux gene and ribosomal protective gene for resistance to tetracyclines in Salmonella 4,[5],12:i:-.

Significantly, extended-Spectrum β-lactamase (ESBLs) Salmonella is considered a severe global public health problem [10]. In this study, blaTEM-1 was the most frequent ESBLs gene consistent with Eastern China [37] and Thailand [46]. It is known that blaCTX-M is a plasmid-mediated ESBLs enzyme that preferentially hydrolyzes ceftriaxone or cefotaxime, becoming an effective mechanism of Salmonella resistance to broad-spectrum cephalosporin [47]. In our study, the blaCTX-M gene has been found to have a substantial correlation with the phenotypic resistance of ceftriaxone and cefepime. Five different blaCTX-M genes were identified, of which blaCTX-M-55 was the most prevalent in Guizhou. The detection rate of blaOXA-1 (5.7%) in this study was lower than that in Eastern China (18.4%) [37], but higher than that reported in Europe (0.21%) [48] and the United States (0.22%) [49]. It is worth noting that a proportion of isolates contained multiple ESBLs genes, which may enhance the adaptability of Salmonella 4,[5],12:i:- to cephalosporin drugs, thus affecting clinical treatment outcomes. Therefore, great attention should be paid to these isolates in future resistance monitoring. In our study, several isolates carried antimicrobial resistance genes without showing antimicrobial resistance phenotype. It might be because the drug resistance mechanism of Salmonella 4,[5],12:i:- was very complex and some resistance genes were not investigated in this study [50]. A more comprehensive drug resistance mechanism investigation of Salmonella 4,[5],12:i:- by whole genome sequencing needs to be performed in our future studies.

The pathogenicity of Salmonella involves different virulence genes, which contribute to the invasionand reproduction of Salmonella in a complex environment [13]. The invA, sseL, mgtC, siiE, and sopB genes were highly conserved and were genetic markers for the Salmonella pathogenicity island (SPI) in Salmonella [51]. The high prevalence of these virulence genes in our study also indicated widespread and highly conserved. The prophage virulence genes gipA, gtgB, sspH1, and sspH2 were highly prevalent, while the sopE gene was present in a few isolates. The sopE and gipA genes can be transferred by phages, then grow and survive in the Peyer’s patches, which will significantly increase the toxicity of Salmonella [9]. By contraries, we found that plasmid virulence genes had shallow detection in Salmonella 4,[5],12:i:- isolates. The plasmid virulence spvC and pefA genes were not detected in all Salmonella 4,[5],12:i:- isolates. Similar observations have been recorded in previous studies [34, 52]. The pefA gene product facilitates bacterial attachment to host epithelial cells [53]. In contrast, SpvC, a phosphothreonine lyase, is an effector protein involved in immune evasion in the early stages of infection and dissemination of the pathogen at the later stages [53]. These virulence genes were more prevalent in Salmonella Typhimurium and Salmonella Enteritidis isolates but were rare in Salmonella 4,[5],12:i:- [34, 54], which indicated that the presence of plasmid virulence in Salmonella might be related to specific serotypes. Generally, the plasmid virulence genes of Salmonella play a role in systemic infection, but not in the gastrointestinal form [55]. In our study, whether the absence of plasmid virulence genes in Salmonella 4,[5],12:i:- isolates were related to the origin of these isolates mainly from feces remains to be further confirmed.

In the last 20 years, the prevalent ST clones of Salmonella 4,[5],12:i:- have been changed. ST19 was the main ST clone in the US and Europe during 1991–2016, but the ST34 clone has become increasingly common since 2014 [10, 37]. In our study, all Salmonella 4,[5],12:i:- isolates were assigned to three STs and ST34 was the main clone, which was consistent with the European epidemic clone [11, 36]. MLST clustering tree showed that the genetic distance between ST34, ST1746 and ST19 was very close, with only one allele loci difference, indicating that Salmonella 4,[5],12:i:- ST19 was likely to be their clonal ancestors. Microevolution between different ST clones isolates remains determined by the Whole-genome sequencing technology and phylogenetic analysis.

Conclusions

In summary, we characterized the antimicrobial resistance, antimicrobial resistance gene, virulence profiles, and MLST of Salmonella 4,[5],12:i:- isolates from 2013 to 2018 in Guizhou, located in the southwest of China. Here, the prevalence of this serotype was at a high level. Isolates showed high rates of resistance to sulfamethoxazole, streptomycin, ampicillin, tetracycline, and doxycycline. A high burden of MDR was observed. Some isolates were co-resistant to ciprofloxacin, third and fourth-generation cephalosporins, and azithromycin had been found. Furthermore, the emergence of carbapenem-resistant and XDR isolates in Salmonella 4,[5],12:i:-. It is of great importance to strengthen the drug resistance monitoring of this serotype. Virulence genes and drug resistance genes were carried more frequently. The most common antimicrobial resistance genes were blaTEM-1, strA-strB, sul2 and tetB. A certain correlation between the antimicrobial phenotypes and genotypes was found. The examined Salmonella 4,[5],12:i:- isolates were mainly ST34. Our findings might be helpful to preliminary understand the characterization of this serotype in Guizhou. Further studies are needed to assess Salmonella 4,[5],12:i:- in more detail to better understand the antimicrobial resistance, pathogenicity, and genetic background.

Supporting information

S1 Fig. Identification of Salmonella Typhimurium and Salmonella 4,[5],12:i:- isolates by mPCR.

https://doi.org/10.1371/journal.pone.0266443.s001

(DOCX)

S2 Fig. The PCR figures of resistance genes tested in this study.

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S3 Fig. The PCR figures of virulence genes tested in this study.

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S4 Fig. The PCR figure of housekeeping genes tested in this study.

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S1 Table. The fljB and fljB-fljA genes primer and PCR cycling conditions information.

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S2 Table. Antimicrobial resistance genes primer and PCR cycling conditions information.

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S3 Table. Virulence genes primer and PCR cycling conditions information.

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S4 Table. Housekeeping genes primer and PCR cycling conditions information.

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Acknowledgments

We would like to thank nine Centers for Disease Control and Prevention in Guizhou, including Anshun, Bijie, Guiyang, Liupanshui, Qiandongnan, Qiannan, Qianxinan, Tongren and Zunyi, for providing Salmonella isolates. At the same time, we would also like to thank the suppliers for their consistent support.

References

  1. 1. Majowicz SE, Musto J, Scallan E, Angulo FJ, Kirk M, Brien SJ, et al. The global burden of nontyphoidal Salmonella gastroen teritis. Clin Infect Dis. 2010; 50(6): 882–89. Epub 2010/02/18. pmid:20158401.
  2. 2. JM F. Prevalence of Salmonella in chicken carcasses in Portugal. Journal of Applied Bacteriology. 1990; 69: 477–80. pmid:2292513
  3. 3. Mossong J, Marques P, Ragimbeau C, Huberty-Krau P, Losch S, Meyer G, et al. Outbreaks of monophasic Salmonella enterica serovar 4,[5],12:i:- in Luxembourg, 2006. Euro Surveill. 2007; 12(6): 11–12. Epub 2007/11/10. pmid:17991400.
  4. 4. Lettini AA, Saccardin C, Ramon E, Longo A, Cortini E, Dalla Pozza MC, et al. Characterization of an unusual Salmonella phage type DT7a and report of a foodborne outbreak of salmonellosis. Int J Food Microbiol. 2014; 189: 11–17. Epub 2014/08/12. pmid:25108760.
  5. 5. Hetta HF, Al-Kadmy IMS, Khazaal SS, Abbas S, Suhail A, El-Mokhtar MA, et al. Antibiofilm and antivirulence potential of silver nanoparticles against multidrug-resistant Acinetobacter baumannii. Sci Rep. 2021; 11(1): 10751. Epub 2021/05/26. pmid:34031472.
  6. 6. Algammal AM, Hashem HR, Al-Otaibi AS, Alfifi KJ, El-Dawody EM, Mahrous E, et al. Emerging MDR-Mycobacterium avium subsp. avium in house-reared domestic birds as the first report in Egypt. BMC Microbiol. 2021; 21(1): 237. Epub 2021/08/28. pmid:34445951.
  7. 7. Algammal AM, Mabrok M, Sivaramasamy E, Youssef FM, Atwa MH, El-Kholy AW, et al. Emerging MDR-Pseudomonas ae ruginosa in fish commonly harbor oprL and toxA virulence genes and blaTEM, blaCTX-M, and tetA antibiotic-resistance genes. Sci Rep. 2020; 10(1): 15961. Epub 2020/10/01. pmid:32994450.
  8. 8. Algammal AM, Hetta HF, Batiha GE, Hozzein WN, El Kazzaz WM, Hashem HR, et al. Virulence-determinants and antibiotic-resistance genes of MDR-E. coli isolated from secondary infections following FMD-outbreak in cattle. Sci Rep. 2020; 10(1): 19779. Epub 2020/11/15. pmid:33188216.
  9. 9. Nguyen Thi H, Pham TT, Turchi B, Fratini F, Ebani VV, Cerri D, et al. Characterization of Salmonella spp. Isolates from Swine: Virulence and Antimicrobial Resistance. Animals (Basel). 2020; 10(12). Epub 2020/12/23. pmid:33348681.
  10. 10. Sun H, Wan Y, Du P, Bai L. The Epidemiology of Monophasic Salmonella Typhimurium. Foodborne Pathog Dis. 2020; 17(2): 87–97. Epub 2019/09/19. pmid:31532231.
  11. 11. Antunes P, Mourao J, Pestana N, Peixe L. Leakage of emerging clinically relevant multidrug-resistant Salmonella clones from pig farms. J Antimicrob Chemother. 2011; 66(9): 2028–32. Epub 2011/06/24. pmid:21697179.
  12. 12. Wang Y, Chen X, Wang R, Chen J, Sun C, Cao H. Study on the serotype, drug sensitivity and virulence genes of clinical isolates of Salmonella in Zhejiang Province. Chin J Infect P is. 2020; 38(10): 646–50.
  13. 13. Cao C, Chen Q, Chen X, Zhong Y, Li M, Chen H. Virulence profile in 239 Salmonella isolates in Longyan, Fujian Province. Chinese Journal of Zoonoses 2019, 35, 720–5.
  14. 14. Hauser E, Tietze E, Helmuth R, Junker E, Blank K, Prager R, et al. Pork Contaminated with Salmonella enterica Serovar 4,[5],12:i:-, an Emerging Health Risk for Humans. Applied and Environmental Microbiology. 2010; 76(14): 4601–10. pmid:20472721
  15. 15. Seixas R, Santos TR, Machado J, Tavares L, Bernardo F, Semedo-Lemsaddek T, et al. Phenotypic and Molecular Characteriza tion of Salmonella 1,4,[5],12:i:- R-Type ASSuT Isolates from Humans, Animals, and Environment in Portugal, 2006–2011. Foodborne Pathog Dis. 2016; 13(11): 633–41. Epub 2016/10/22. pmid:27768382.
  16. 16. Kim A, Lim SK, Lee K, Jung SC, Cho YS, Yun SJ, et al. Characterization of Salmonella enterica Serovar 4,[5],12:i:- Isolates from Korean Food Animals. Foodborne Pathog Dis. 2015; 12(9): 766–9. Epub 2015/07/21. pmid:26192872.
  17. 17. Nobuo Arai TS, Tamamura Yukino, Tanaka Kiyoshi, Barco Lisa, Izumiya Hidemasa, Kusumoto Masahiro, et al. Phylogenetic Characterization of Salmonella enterica Serovar Typhimurium and Its Monophasic Variant Isolated from Food Animals in Japan Revealed Replacement of Major Epidemic Clones in the Last 4 Decades. J Clin Microbiol. 2018; 56(5): e01758–01717. pmid:29491013
  18. 18. Wasyl D, Hoszowski A. Occurrence and characterization of monophasic Salmonella enterica serovar typhimurium (1,4,[5],12:i:-) of non-human origin in Poland. Foodborne Pathog Dis. 2012; 9(11): 1037–43. Epub 2012/09/27. pmid:23009171.
  19. 19. Cuenca-Arias P, Montano LA, Villarreal JM, Wiesner M. Molecular and phenotypic characterization of Salmonella Typhi murium monophasic variant (1,4,[5],12:i:-) from Colombian clinical isolates. Biomedica. 2020; 40(4): 722–33. Epub 2020/12/05. pmid:33275350.
  20. 20. Mandilara G, Sideroglou T, Chrysostomou A, Rentifis I, Papadopoulos T, Polemis M, et al. The Rising Burden of Salmonellosis Caused by Monophasic Salmonella Typhimurium (1,4,[5],12:i:-) in Greece and New Food Vehicles. Antibiotics (Basel). 2021; 10(2). Epub 2021/03/07. pmid:33668483.
  21. 21. You L, He X, Wei X, Li S, Wang B, Huang H, et al. Surveillance and analysis on the pathogenic feature of Salmonella from infectious diarrheal patients in Guiyang,China,2013–2014. Chinese Journal of Zoonoses 2018, 34, 712–5.
  22. 22. Long L, You L, Wei X, Li S, Wang D, Huang J, et al. Antimicrobial resistance and molecular typing of non-typhoid Salmonella clinical isolates from Guizhou Province in 2017–2018. Chinese Journal of Zoonoses 2021, 37, 603–10.
  23. 23. Martine Guibourdenche PR, Mikoleit Matthew, Fields Patricia I, Bockemühl Jochen, Grimont Patrick A D, Weill François-Xavier. Supplement 2003–2007 (No. 47) to the White-Kauffmann-Le Minor scheme. Res Microbiol. 2010; 161(1): 26–29. Epub 2009 Oct 17. pmid:19840847
  24. 24. Tennant SM, Diallo S, Levy H, Livio S, Sow SO, Tapia M, et al. Identification by PCR of non-typhoidal Salmonella enterica serovars associated with invasive infections among febrile patients in Mali. PLoS Negl Trop Dis. 2010; 4(3): e621. Epub 2010/03/17. pmid:20231882.
  25. 25. Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing; 23th informa tional supplement. CLSI documents M100-S23. CLSI, 2017.
  26. 26. Magiorakos AP, Srinivasan A, Carey RB, Carmeli Y, Falagas ME, Giske CG, et al. Multidrug-resistant, extensively drug-re sistant and pandrug-resistant bacteria: an international expert proposal for interim standard definitions for acquired resistance. Clin Microbiol Infect. 2012; 18(3): 268–81. Epub 2011/07/29. pmid:21793988.
  27. 27. Hasman H, Mevius D, Veldman K, Olesen I, Aarestrup FM. β-Lactamases among extended-spectrum β-lactamase (ESBL)- resistant Salmonella from poultry, poultry products and human patients in The Netherlands. J Antimicrob Chemother. 2005; 56(1): 115–21. Epub 2005/06/09. pmid:15941775.
  28. 28. Abbas Doosti EM, Jami Mohammad-Saeid, Mokhtari-Farsani Abbas. Prevalence of aadA1, aadA2, aadB, strA and strB genes and their associations with multidrug resistance phenotype in Salmonella Typhimurium isolated from poultry carcasses. Thai J Vet Med. 2016; 46(4): 691–7.
  29. 29. Zhang Y, Luo W, Zhang H, Ren Y, Ge R, Yang J. Detection of antibiotic resistance and resistance genes of Salmonella in pigeons in some areas in Sichuan. Animal Husbandry & Veterinary Medicine 2019, 51, 52–58.
  30. 30. Liu S, Wu K, Zhou C, Liang J, Wei P. Foodborn Salmonella Isolates: Pathogenicity Evaluation and Detection of Virulence Genes. Food science 2018, 39, 182–6.
  31. 31. Capuano F, Mancusi A, Capparelli R, Esposito S, Proroga YT. Characterization of drug resistance and virulotypes of Salmo nella strains isolated from food and humans. Foodborne Pathog Dis. 2013; 10(11): 963–8. Epub 2013/10/10. pmid:24102078.
  32. 32. McHugh ML. Interrater reliability: the kappa statistic. Biochem Med. 2012; 22(3): 276–82. pmid:23092060
  33. 33. Marin C, Chinillac MC, Cerda-Cuellar M, Montoro-Dasi L, Sevilla-Navarro S, Ayats T, et al. Contamination of pig carcass with Salmonella enterica serovar Typhimurium monophasic variant 1,4[5],12:i:- originates mainly in live animals. Sci Total Environ. 2020; 703: 134609. Epub 2019/11/23. pmid:31753504.
  34. 34. Yang X, Wu Q, Zhang J, Huang J, Guo W, Cai S. Prevalence and Characterization of Monophasic Salmonella Serovar 1,4,[5],12:i:- of Food Origin in China. PLoS One. 2015; 10(9): e0137967. Epub 2015/09/12. pmid:26360603.
  35. 35. Arguello H, Sorensen G, Carvajal A, Baggesen DL, Rubio P, Pedersen K. Characterization of the emerging Salmonella 4,[5],12:i:- in Danish animal production. Foodborne Pathog Dis. 2014; 11(5): 366–72. Epub 2014/03/29. pmid:24673107.
  36. 36. Huang J, Ke B, Li B, He D, Liu Z, Li Z, et al. Molecular epidemiological characteristics and antibiotic resistance of multi- drug resistant Salmonella 1,4,[5]12:i:- in Guangdong. Disease Surveillance 2021, 36, 501–8.
  37. 37. Zheng D, Ma K, Du J, Zhou Y, Wu G, Qiao X, et al. Characterization of Human Origin Salmonella Serovar 1,4,[5],12:i:- in Eastern China, 2014 to 2018. Foodborne Pathog Dis. 2021; 18(11): 790–7. Epub 2021/07/22. pmid:34287022.
  38. 38. Kijima M, Shirakawa T, Uchiyama M, Kawanishi M, Ozawa M, Koike R. Trends in the serovar and antimicrobial resistance in clinical isolates of Salmonella enterica from cattle and pigs between 2002 and 2016 in Japan. J Appl Microbiol. 2019; 127(6): 1869–1875. Epub 2019/08/29. pmid:31461201.
  39. 39. Collignon P, Powers JH, Chiller TM, Aidara-Kane A, Aarestrup FM. World Health Organization ranking of antimicrobials according to their importance in human medicine: A critical step for developing risk management strategies for the use of antimicrobials in food production animals. Clin Infect Dis. 2009; 49(1): 132–141. Epub 2009/06/06. pmid:19489713.
  40. 40. Sjolund-Karlsson M, Joyce K, Blickenstaff K, Ball T, Haro J, Medalla FM, et al. Antimicrobial susceptibility to azithromycin among Salmonella enterica isolates from the United States. Antimicrob Agents Chemother. 2011; 55(9): 3985–9. Epub 2011/06/22. pmid:21690279.
  41. 41. Zhang J, Liu J, Xu F. Research Progress on Resistance of Salmonella Kentucky to Fluoroquinolones and Carbapenems. Chi neseAnimal Husbandry and Veterinarian 2020, 47, 3739–48
  42. 42. Gallati C, Stephan R, Hachler H, Malorny B, Schroeter A, Nuesch-Inderbinen M. Characterization of Salmonella enterica subsp. enterica serovar 4,[5],12:i:- clones isolated from human and other sources in Switzerland between 2007 and 2011. Foodborne Pathog Dis. 2013; 10(6): 549–54. Epub 2013/04/26. pmid:23614800.
  43. 43. He J, Sun F, Sun D, Wang Z, Jin S, Pan Z, et al. Multidrug resistance and prevalence of quinolone resistance genes of Salmonella enterica serotypes 4,[5],12:i:- in China. Int J Food Microbiol. 2020; 330: 108692. Epub 2020/06/11. pmid:32521291.
  44. 44. Garcia P, Guerra B, Bances M, Mendoza MC, Rodicio MR. IncA/C plasmids mediate antimicrobial resistance linked to viru lence genes in the Spanish clone of the emerging Salmonella enterica serotype 4,[5],12:i. J Antimicrob Chemother. 2011; 66(3): 543–9. Epub 2010/12/24. pmid:21177672.
  45. 45. JinMing H. Molecular evolution characteristics and antibiotic resistance of Salmonella 1,4,[5],12:- in Guangdong. Southern Medical University. 2021.
  46. 46. Win AT, Supa-Amornkul S, Orsi RH, Carey JH, Wolfgang WJ, Chaturongakul S. Sequence Analyses and Phenotypic Charac terization Revealed Multidrug Resistant Gene Insertions in the Genomic Region Encompassing Phase 2 Flagellin Encoding fljAB Genes in Monophasic Variant Salmonella enterica Serovar 4,5,12:i:- Isolates From Various Sources in Thailand. Front Microbiol. 2021; 12: 720604. Epub 2021/10/23. pmid:34675896.
  47. 47. He D, Chiou J, Zeng Z, Liu L, Chen X, Zeng L, et al. Residues Distal to the Active Site Contribute to Enhanced Catalytic Activity of Variant and Hybrid beta-Lactamases Derived from CTX-M-14 and CTX-M-15. Antimicrob Agents Chemother. 2015; 59(10): 5976–5983. Epub 2015/07/15. pmid:26169409.
  48. 48. Elnekave E, Hong S, Mather AE, Boxrud D, Taylor AJ, Lappi V, et al. Salmonella enterica Serotype 4,[5],12:i:- in Swine in the United States Midwest: An Emerging Multidrug-Resistant Clade. Clin Infect Dis. 2018; 66(6): 877–85. Epub 2017/10/27. pmid:29069323.
  49. 49. Elnekave E, Hong SL, Lim S, Boxrud D, Rovira A, Mather AE, et al. Transmission of Multidrug-Resistant Salmonella enterica Subspecies enterica 4,[5],12:i:- Sequence Type 34 between Europe and the United States. Emerg Infect Dis. 2020; 26(12): 3034–3038. Epub 2020/11/22. pmid:33219795.
  50. 50. Xu Z, Wang M, Zhou C, Gu G, Liang J, Hou X, et al. Prevalence and antimicrobial resistance of retail-meat-borne Salmonella in southern China during the years 2009–2016: The diversity of contamination and the resistance evolution of multidrug-resistant isolates. International Journal of Food Microbiology. 2020; 333. pmid:32693316
  51. 51. Xu L, Zhou X, Xu X, Matthews KR, Liu Y, Kuang D, et al. Antimicrobial resistance, virulence genes and molecular subtypes of S. Enteritidis isolated from children in Shanghai. J Infect Dev Ctries. 2018; 12(7): 573–80. Epub 2018/07/31. pmid:31954007.
  52. 52. Proroga YTR, Mancusi A, Peruzy MF, Carullo MR, Montone AMI, Fulgione A, et al. Characterization of Salmonella Typhi murium and its monophasic variant 1,4, [5],12:i:- isolated from different sources. Folia Microbiol (Praha). 2019; 64(6): 711–8. Epub 2019/02/06. pmid:30721446.
  53. 53. Fardsanei F, Soltan Dallal MM, Zahraei Salehi T, Douraghi M, Memariani M, Memariani H. Antimicrobial resistance patterns, virulence gene profiles, and genetic diversity of Salmonella enterica serotype Enteritidis isolated from patients with gastroenteritis in various Iranian cities. Iran J Basic Med Sci. 2021; 24(7): 914–921. Epub 2021/10/30. pmid:34712421.
  54. 54. Proroga YTR, Capuano F, Capparelli R, Bilei S, Bernardo M, Cocco MP, et al. Characterization of non-typhoidal Salmonella enterica strains of human origin in central and southern Italy. Ital J Food Saf. 2018; 7(1): 6888. Epub 2018/05/08. pmid:29732321.
  55. 55. Ido N, Lee K, Iwabuchi K, Izumiya H, Uchida I, Kusumoto M, et al. Characteristics of Salmonella enterica serovar 4,[5],12:i:- as a monophasic variant of serovar Typhimurium. PLoS One. 2014; 9(8): e104380. Epub 2014/08/06. pmid:25093666.