Advertisement
Browse Subject Areas
?

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Antimicrobial activity of organic acids against Campylobacter spp. and development of combinations—A synergistic effect?

  • Elisa Peh,

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

    Affiliation Institute for Food Quality and Food Safety, University of Veterinary Medicine Hannover, Foundation, Hannover, Germany

  • Sophie Kittler,

    Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Methodology, Project administration, Supervision, Writing – review & editing

    Affiliation Institute for Food Quality and Food Safety, University of Veterinary Medicine Hannover, Foundation, Hannover, Germany

  • Felix Reich,

    Roles Conceptualization, Formal analysis, Funding acquisition, Supervision, Writing – review & editing

    Affiliations Institute for Food Quality and Food Safety, University of Veterinary Medicine Hannover, Foundation, Hannover, Germany, German Federal Institute for Risk Assessment (BfR), Berlin, Germany

  • Corinna Kehrenberg

    Roles Conceptualization, Formal analysis, Funding acquisition, Methodology, Project administration, Resources, Supervision, Writing – review & editing

    Corinna.Kehrenberg@vetmed.uni-giessen.de

    Affiliation Institute for Veterinary Food Science, Justus-Liebig-University Giessen, Giessen, Germany

Antimicrobial activity of organic acids against Campylobacter spp. and development of combinations—A synergistic effect?

  • Elisa Peh, 
  • Sophie Kittler, 
  • Felix Reich, 
  • Corinna Kehrenberg
PLOS
x

Abstract

Contaminated poultry meat is considered to be the main source of human infection with Campylobacter spp., a pathogen that asymptomatically colonizes broiler chickens during fattening and contaminates carcasses during slaughter. To prevent or reduce the colonization of broiler flocks with Campylobacter spp., applying different organic acids, especially in combinations, via feed or drinking water seems to be a promising approach. However, only very few combinations of organic acids have been tested for their antibacterial efficacy against Campylobacter spp. Therefore, the in vitro susceptibility of 30 Campylobacter spp. isolates (20 C. jejuni and ten C. coli) to ten organic acids and ten combinations was determined. The testing of minimum inhibitory concentration (MIC) values was performed at pH 6.0 and 7.3 by using the broth microdilution method and included the following organic acids: Caprylic acid, sorbic acid, caproic acid, benzoic acid, ascorbic acid, propionic acid, acetic acid, formic acid, fumaric acid and tartaric acid and combinations thereof. The lowest MIC values were seen for caprylic acid (MIC range at pH 7.3: 0.5–2 mmol/L) and sorbic acid (MIC range at pH 7.3: 1–4 mmol/L). One to two dilution steps lower MIC values were determined at the lower pH value of 6.0. Furthermore, ten combinations consisting of three to five organic acids were developed. In addition to the tested antibacterial activity, other criteria were included such as approval as feed additives, reported synergistic effects and chemical properties. For nine of ten combinations, the MIC90 values of the organic acids decreased 1.25- to 241.5-fold compared to the MIC90 values for the individual substances. Furthermore, nine of ten combinations exhibited synergistic activities against two or more of the tested C. jejuni and C. coli isolates. A combination of caprylic acid, sorbic acid and caproic acid exhibited synergistic activities against the largest number of Campylobacter spp. isolates (six C. jejuni and four C. coli) with fractional inhibitory concentration (FIC) indices (∑FIC) ranging from 0.33 to 1.42. This study shows in vitro synergistic activities of different organic acids in combinations against the major Campylobacter species and could therefore be a promising basis for reducing Campylobacter spp. in vivo.

Introduction

Campylobacteriosis is one of the leading foodborne gastrointestinal diseases worldwide [1] and the most frequently reported zoonosis in the European Union with more than 246,000 reported cases in 2018 [2]. Campylobacter (C.) jejuni and C. coli are the species that most frequently cause gastrointestinal diseases [3]. Clinical manifestations of human infection with Campylobacter spp. include acute aqueous or bloody diarrhea, fever and occasionally severe sequelae such as the Guillain-Barré syndrome and Miller Fisher syndrome [1]. It is assumed that contaminated poultry meat is the major source for human infection with Campylobacter spp. [4]. Contamination of carcasses is mainly caused by faecal contamination during the slaughter process and currently a large number of slaughter batches are affected [5]. Comprehensive strategies for reducing microbial contamination of carcasses and controlling Campylobacter infections are urgently needed. Reducing Campylobacter spp. in primary production is considered to be most effective for minimizing human infections [4]. Different in vivo studies demonstrated the potential of organic acids to decrease the susceptibility for colonization or to reduce caecal concentrations of Campylobacter spp. when applicated via feed or water [69] However, as pointed out in the scientific opinion published by the European Food Safety Authority [4], results of previous in vivo experiments concerning the effectiveness of organic acids were inconsistent, indicating that further research using standardized methods is urgently needed.

A couple of in vitro studies have addressed the problem and investigated the antibacterial effect of organic acids against Campylobacter spp. However, it is difficult to compare the results because different methods and techniques were used to determine the susceptibility of the isolates. For example, Molatová et al. [10] performed susceptibility tests with only one C. jejuni strain using a SYBR Green-based real-time PCR, while Grilli et al. [11] determined MIC values of three C. jejuni isolates by using the broth macrodilution method. Two other studies performed susceptibility tests by using the broth microdilution method, but MIC values were determined either at pH 6 or 7.5 [12] or without any adjustment of the pH [13]. Furthermore, despite promising results of studies using combinations of organic acids in vivo [8, 14, 15], very few in vitro studies investigated the susceptibility status of Campylobacter spp. isolates to combinations of organic acids. Additionally, these studies did not always provide complete information on the composition and the selection of substances for the combinations.

Therefore, this study aimed to evaluate the antibacterial effect of a variety of organic acids in vitro. Minimum inhibitory concentration (MIC) values of ten organic acids were determined individually and in various combinations against current C. jejuni and C. coli field isolates. Subsequently, we investigated the interactions between the compounds based on the fractional inhibitory concentration (FIC) index.

Materials and methods

Bacterial strains

A total of 20 C. jejuni and ten C. coli isolates were included in this study. The strain collection included type strains C. jejuni DSM 4688 and C. coli DSM 4689 (German Collection of Microorganisms and Cell Cultures, Leibnitz-Institute, Braunschweig, Germany), whole genome-sequenced strain C. jejuni BfR-CA-14430, C. coli strain BfR-CA-09557 and C. jejuni strain ATCC 81–176 (American Type Culture Collection (ATCC), Manassas, VA, USA). Additionally, 17 C. jejuni and eight C. coli field isolates of avian origin, representing part of the strain collection of the Institute of Food Quality and Food Safety, University of Veterinary Medicine Hannover, Hannover, Germany and the Institute for Veterinary Food Science, Giessen, Germany were used for susceptibility testing. The field isolates were collected between July 2005 and January 2018 on the basis of epidemiological unrelatedness. Species confirmation of all isolates was carried out by MALDI-TOF mass spectrometry (Bruker Daltoniks GmbH, Bremen, Germany). All isolates were stored in cryotubes (Carl Roth GmbH + Co. KG, Karlsruhe, Germany) at -80 °C. Prior to use, isolates were plated out on Columbia agar supplemented with sheep blood (Oxoid Deutschland GmbH, Wesel, Germany) and incubated for 48 hours at 42 ± 1 °C under microaerobic conditions (10% CO2, 5% O2, and 85% N2).

Organic acids and test ranges

Ten organic acids were tested for their antibacterial effect: formic acid, propionic acid, ascorbic acid, tartaric acid, sorbic acid, benzoic acid, fumaric acid, caprylic acid, caproic acid (Carl Roth GmbH + Co. KG, Karlsruhe, Germany) and acetic acid (E. Merck KG, Darmstadt, Germany). Since it is considered that the antimicrobial efficacy of organic acids is related to the pH value [16], susceptibility tests were performed at two different pH values. This took into account that the dissociation state plays an important role for their effectiveness and that an application in poultry primary production, e.g. via drinking water, is expected to cause a pH shift with acidification of the water. For better comparability with results of previous studies [11, 13, 17], the concentrations of organic acids are given in mmol/L. The stock solutions of organic acids were prepared at double strength of the respective first dilution level in cation adjusted Mueller-Hinton broth (CAMH, Carl Roth GmbH + Co. KG, Karlsruhe, Germany), and adjusted to pH 6.0 or pH 7.3 using 2 mol/L and 8 mol/L sodium hydroxide. Subsequently, two-fold serial dilution series were prepared in CAMH broth previously adjusted to pH 6.0 or pH 7.3. The following final concentrations (which are specified after inoculation) and ranges were included: 0.5–512 mmol/L (formic acid, acetic acid, propionic acid, tartaric acid, fumaric acid), 0.031–32 mmol/L (ascorbic acid, caprylic acid), and 0.063–64 mmol/L (benzoic acid, sorbic acid, caproic acid). A volume of 50 μL of each dilution was then added to the wells of a microtiter plate (Sarstedt AG & Co. KG, Nümbrecht, Germany).

Determining the susceptibility of Campylobacter isolates towards organic acids

For determining minimal inhibitory concentration (MIC) values of organic acids, the broth microdilution method was used. Procedures regarding inoculum density, growth medium, incubation time and conditions were performed in accordance with the recommendations given in the Clinical and Laboratory Standards Institute (CLSI) document VET01-A4 [18]. The tests were performed in U-shaped bottom 96-well microtiter plates (Sarstedt AG & Co. KG, Nümbrecht, Germany). Colonies from overnight cultures were suspended in sodium chloride (0.85%) and adjusted to a turbidity in accordance with McFarland standard 0.5. The suspension was diluted 1:100 in CAMH broth and adjusted to pH 6.0 or pH 7.3, respectively. A volume of 50 μL of this suspension was added into the wells of the microtiter plate containing 50 μL of the double concentrated organic acid to achieve a final bacterial concentration of 5 x 105 CFU/mL. The microtiter plates were incubated for 48 h at 42 ± 1 °C under microaerobic conditions. C. jejuni strain DSM 4688 and C. coli strain DSM 4689 served as quality control strains and were included in every batch of MIC determinations. The MICs of the two quality control strains were determined in advance in three independent experiments by using the broth microdilution method as well as the broth macrodilution method, similar to a previous study on biocide testing (Rensch et al. 2013).

Development of combinations of organic acids

In accordance with the following criteria, ten combinations of organic acids termed CA to CJ were chosen, each consisting of three to five components. The respective combinations comprised the following organic acids: CA (caprylic acid, sorbic acid, caproic acid), CB (caprylic acid, sorbic acid, caproic acid, ascorbic acid), CC (caprylic acid, sorbic acid, caproic acid, benzoic acid), CD (caprylic acid, sorbic acid, caproic acid, ascorbic acid, benzoic acid), CE (caprylic acid, sorbic acid, caproic acid, benzoic acid, propionic acid), CF (sorbic acid, ascorbic acid, benzoic acid), CG (sorbic acid, ascorbic acid, benzoic acid, propionic acid), CH (sorbic acid, benzoic acid, propionic acid), CI (sorbic acid, benzoic acid, propionic acid, acetic acid), CJ (sorbic acid, benzoic acid, propionic acid, acetic acid, formic acid). The ratios of organic acids were 3:2:1 for three components, 4:3:2:1 for four components, and 5:4:3:2:1 for five components (Table 1).

thumbnail
Table 1. Composition of the ten tested blends of organic acids based on the dilution level 64 mmol/L.

https://doi.org/10.1371/journal.pone.0239312.t001

Since in previous studies, synergistic effects were reported for short chain fatty acids in combination with both phenolic compounds and medium chain fatty acids [17, 19], all combinations at least included either caprylic acid as a medium chain fatty acid or benzoic acid as a phenolic acid, or combinations thereof in order to use these reported synergistic effects. The mixtures of organic acids consisted either exclusively of organic acids listed as authorized feed additives in the European Union (combinations CF, CG, CH, CI, CJ) or contained caprylic acid and caproic acid as substances yet to be approved (combinations CA, CB, CC, CD, CE) [20]. The organic acids were selected on the basis of the results of the testing of individual substances (as they showed very low MIC values). The separate grouping of approved versus non-approved acids was done under the consideration of regulatory and practical aspects, since non-approved acids might be effective but not directly applicable. Combinations were prepared with respect to the chemical structure and octanol/water partition coefficient as a measure of hydrophobicity (Table 2) to evaluate possible synergistic effects between organic acids with different chemical properties. In that respect, ascorbic acid was included in this study as the only vinylogous carboxylic acid with the lowest octanol/water partition coefficient of all tested organic acids being included in four combinations (CB, CD, CF, CG).

The antibacterial effectiveness of the organic acids determined their selection and the proportion of organic acids in the mixtures (Table 1). The ratios of the combinations were 3:2:1 for three components, 4:3:2:1 for four components and 5:4:3:2:1 for five components. In all combinations, the organic acid with the lowest MIC90 value (lowest concentration of the organic acid at which 90% of the bacteria were inhibited or killed) constituted the largest proportion, followed by organic acids with the next lowest MIC90 values, respectively. Due to their low effectiveness based on their high MIC50 and MIC90 values compared to the other substances, the dicarboxylic acids tartaric acid and fumaric acid were not included in the mixtures.

For each combination of organic acids, stock solutions were prepared in CAMH broth with a total organic acid concentration of 64 mmol/L according to Table 1. The stock solutions were adjusted to pH 7.3 using 2 mol/L and 8 mol/L sodium hydroxide and 11 serial two-fold dilutions were prepared in CAMH broth.

Susceptibility testing of combinations of organic acids

To determine the minimum inhibitory concentrations of the combinations of organic acids, broth microdilution assays were performed as described above. MIC90 values of the combinations of organic acids were calculated.

Then, the fractional inhibitory concentration (FIC) index was calculated for each combination of organic acids to evaluate possible synergistic activities [21, 22]. A synergistic effect refers to a combination of two or more components that causes a greater effect than the sum of their individual effects [23].

First, MICs of the respective organic acids in each combination were transformed into fractional inhibitory concentrations (FICs) as follows [21]:

Second, the fractional inhibitory concentration (FIC) index (∑FIC) was calculated using the standard formula as described earlier [21, 22]:

Synergistic activities were defined as ∑FIC ≤ 0.5, indifference was defined as ∑FIC > 0.5 to < 2, and antagonism was defined as ∑FIC ≥ 2 [21].

Results

Antimicrobial susceptibility testing of individual organic acids

The distribution of MIC values determined in the testing of single organic acids is shown in Table 3A and 3B. The tests were performed using 20 C. jejuni and ten C. coli isolates; therefore, the results were presented separately by species. The MIC50 and MIC90 values were defined as the lowest concentration of organic acids at which 50% and 90% of the isolates were inhibited, respectively. The MIC50 and MIC90 values were 1- to 3-fold lower at pH 6.0 than at pH 7.3, except for fumaric acid with identical MIC50 and MIC90 values of 256 mmol/L determined for both pH conditions and both bacterial species. The overall distribution of the MIC values for the C. jejuni and C. coli isolates was quite similar (Table 3A and 3B). A comparison of the susceptibility of both bacterial species based on the MIC90 values showed a similar ranking of organic acids. The only difference was seen for benzoic acid and ascorbic acid. Regarding these substances, C. jejuni, showed lower MICs for benzoic acid (MIC90 of 8 mmol/L at pH 7.3) than for ascorbic acid (MIC90 of 16 mmol/L at pH 7.3). In contrast, the tested C. coli isolates revealed lower MIC90 values for ascorbic acid at pH 7.3 (8 mmol/L) than for benzoic acid (16 mmol/L).

thumbnail
Table 3. Distribution of minimum inhibitory concentrations (MIC) values of 20 C. jejuni and ten C. coli isolates for ten organic acids at pH 6.0 and 7.3 using the broth microdilution method.

https://doi.org/10.1371/journal.pone.0239312.t003

The lowest MIC values were detected for caprylic acid, followed by sorbic acid. For both organic acids, C. jejuni and C. coli isolates yielded MIC50 values of 0.5 mmol/L at pH 6.0, 2 mmol/l at pH 7.3 and MIC90 values of 1 mmol/L at pH 6.0. At pH 7.3, caprylic acid revealed MIC90 values (2 mmol/L) half of these of sorbic acid (4 mmol/L) against C. jejuni and C. coli isolates. The highest MIC values were observed for fumaric acid (MIC50 values: 128 mmol/L at pH 6.0, 256 mmol/L at pH 7.3; MIC90 values: 256 mmol/L at pH 6.0, 256 mmol/L at pH 7.3 for C. jejuni and C. coli isolates) and tartaric acid with MIC90 values of 128 mmol/L at pH 6.0 and 256 mmol/L at pH 7.3 for the two species.

Antimicrobial susceptibility testing of combinations of organic acids

The MIC90 values of the combined organic acids are presented in Table 4. According to Grilli et al. [11], the MIC90 values of the organic acids in combination were calculated using the MIC90 values of the combination as a whole and the respective proportions of the individual components. Combining organic acids decreased the MIC90 values of each organic acid except sorbic acid in combination with ascorbic acid or benzoic acid (CF combination) when testing C. coli, with MIC90 values of 4 mmol/L achieved both alone and in combination. However, looking at the results for C. jejuni isolates, the MIC90 value of sorbic acid in the CF combination was reduced by a factor of 2. Due to their low MIC90 values, either caprylic acid (used in CA–CE combinations) or sorbic acid (CF–CJ combinations) had the largest proportion in the mixtures and, thus, showed the lowest reductions in MIC90 values compared to single testing. The MIC90 values of caprylic acid (CA–CE combinations) decreased only 1.25- to 2.5-fold for both bacterial species. Similarly, the CG, CH, CI and CJ combinations resulted in 1.5- to 2.5-fold reductions in the MIC90 values of sorbic acid for C. jejuni and C. coli isolates. The highest reduction in an MIC90 value was observed for formic acid in the CJ combination, showing a 241.5-fold decrease in the MIC90 value for C. jejuni isolates.

thumbnail
Table 4. MIC90 values of eight organic acids tested alone and in combinations (CA-CJ) against 20 C. jejuni (A) and ten C. coli (B) isolates determined by the broth microdilution method at pH 7.3.

https://doi.org/10.1371/journal.pone.0239312.t004

Fractional inhibitory concentration index

The ∑FIC of the organic acids in combination calculated after testing of C. jejuni and C. coli isolates is presented in Table 5. The results indicated synergistic or indifferent interactions, while no antagonistic interactions were observed. The ∑FIC ranged from 0.28 to 2.75 for all combinations of organic acids and isolates. The highest number of synergistic effects against the tested isolates was observed for the combination CA consisting of caprylic acid, sorbic acid and caproic acid. Results showed synergistic activities against six C. jejuni and four C. coli isolates with ∑FIC ranging from 0.33 to 1.42 for C. jejuni and from 0.34 to 1.42 for C. coli. The combination CI consisting of sorbic acid, benzoic acid, propionic acid and acetic acid showed synergism against five C. jejuni and four C. coli isolates, presenting ∑FIC ranges of 0.46 to 1.81 for C. jejuni and 0.46 to 1.79 for C. coli. The CD combination consisting of caprylic acid, sorbic acid, caproic acid, ascorbic acid and benzoic acid exhibited exclusively indifferent interactions against all tested isolates (Table 5).

thumbnail
Table 5. Results of testing combined organic acids for synergistic activity using 20 C. jejuni and ten C. coli isolates.

https://doi.org/10.1371/journal.pone.0239312.t005

Discussion

In this study, the antibacterial activities of organic acids both alone and in different combinations were determined by using the broth microdilution method.

The MIC values of several organic acids differed in part strongly to those determined for two C. jejuni strains in a previous study [11]. Compared to the results of the present study, the authors found higher MIC values for caprylic acid (62.5 mmol/L), sorbic acid (500 mmol/L), acetic acid (>1000 mmol/L), formic acid (>1000 mmol/L), fumaric acid (>1000 mmol/L) and tartaric acid (>1000 mmol/L) [11]. However, MIC values of propionic acid (62.5 mml/L) and benzoic acid (31.25 mmol/L) differed only slightly from our results. Most likely, the partly varying results were due to differences regarding the evaluation of the MIC values. According to the CLSI standards for antimicrobial susceptibility tests for bacteria isolated from animals, the MIC value is defined as the lowest concentration that inhibits visible growth. Deviating from this, Grilli et al. [11] defined the MIC value as the lowest concentration effective in killing more than 99.9% of the initial inoculum as determined by a colony-count technique. As a consequence, the observed concentrations were most likely higher for organic acids that are rather bacteriostatic than bactericidal. In addition, different methods and techniques were used for susceptibility testing. Unlike our study, the MIC values were determined by the broth macrodilution method at pH 6.5 and by using Brain Heart Infusion broth as a test medium [11], which could lead to different results. Compared to our study, more similar results were obtained by two other studies that defined the MIC values in accordance with the CLSI standards and performed susceptibility tests using the broth microdilution method [12, 13]. Consistent with our results, Hermans et al. [12] observed MIC values for caprylic acid and caproic acid ranging between 2 and 4 mmol/L at pH 7.3. Beier et al. [13] reported MIC90 values similar to our results for propionic acid (13.82 mmol/L), formic acid (44.5 mmol/L) and acetic acid (34.1 mmol/L), although the pH values had not been previously adjusted. Thus, they performed susceptibility tests at widely varying pH values depending on the concentration and the pKa value of the respective organic acid.

As expected, the present study demonstrated that the pH value affects the antibacterial activity of organic acids, as all organic acids yielded lower MIC values at pH 6.0 compared to pH 7.3. The widely assumed reason for this is that organic acids are only able to cross the cell membrane in an undissociated form [24]. The proportion of those in an undissociated form depends on the pKa value in combination with the external pH value of the medium [11]. In the bacterial cell, the higher pH value leads to a dissociation of the organic acids into their anions and protons. Cytoplasmic acidification caused by protons lead to disruption of certain cell functions [16]. Additionally, accumulation of anions in the cytoplasm has been proposed to disrupt metabolic functions and to cause increased osmotic pressure and cell death [25]. However, the enhancing effect of acidification on the antibacterial activity of organic acids might be limited to drinking water and feed themselves, as the pH value was observed to increase in the intestines due to the buffering effect of the intestinal contents [26].

In the present study, synergistic activities were shown for the organic acids in all combinations except for the CD combination. For example, the CJ combination consisting of sorbic acid, benzoic acid, propionic acid, acetic acid and formic acid exhibited synergistic interactions against six C. jejuni and two C. coli isolates. For three of these organic acids, a previous study showed strong synergistic activities against Campylobacter spp. in vitro when added to a mixture of water and broiler feed [27]. At pH 4.5, combinations of formic acid, acetic acid and propionic acid resulted in higher reduction rates of Campylobacter spp. than the individual organic acids [27]. Furthermore, in an in vivo study conducted by Skånseng et al. [8], neither the addition of formic acid nor potassium sorbate to broiler feed did lead to reduced contamination levels, whereas the application of a combination of 2.0% formic acid and 0.1% potassium sorbate prevented C. jejuni colonization in chickens. Kim and Rhee [17] observed synergistic activities of three medium chain fatty acids (MCFAs) including caprylic acid and four different short chain fatty acids (SCFAs) against E. coli as indicated by higher rates of bacterial reduction compared to the individual treatment. In this study, four of five combinations consisting of caprylic acid as a MCFA and different SCFA (combinations CA, CB, CC, CE) exhibited synergistic interactions against two or more Campylobacter spp. isolates. The underlying mechanisms for the reported synergistic activities between different organic acids are still unclear. Synergism can occur when organic acids with mutually reinforcing modes of actions are used in combination. In fact, the bacterial cell membrane was found to be a target for antibacterial action of several MCFA [17, 28, 29] and phenolic acids [30]. In contrast, SCFA exhibited antibacterial activities without causing damage to the cell wall [17, 27]. Accordingly, Kim and Rhee [17] proposed that MCFA are likely to damage the bacterial cell membrane and thus may accelerate the influx of SCFA.

The combination of organic acids allowed a reduction in the concentrations of nearly all components required for effective antimicrobial activity as shown in the reduced MIC90 values. This finding offers an important advantage. Several authors observed reduced feed consumption when organic acids exceeding a certain concentration were added to feed or water [3133]. It was suggested that the strong taste of organic acids might decrease palatability thereof [34]. Thus, concentrations of single organic acids required for effective antibacterial activity might exceed the level of acceptance in broilers if used individually. For example, propionic acid was observed to decrease the feed intake and weight gain of broilers when added to drinking water at a concentration of 90 mmol/L [35] which is only slightly higher than the MIC90 value determined for propionic acid against C. coli isolates (64 mmol/L). Considering the dilution effect of intestinal contents, acid concentrations far higher than those of the MIC values would be required for effective antimicrobial activity. This would most probably lead to adverse effects on broiler performance due to reduced intake of water or feed. Such a problem could be overcome by treatment with optimized combinations that reach antimicrobial efficacy, while the concentration of the individual components is sufficiently low for sensory acceptance, especially if they have synergistic activities. However, it should be noted that the concentrations of organic acids in the entire gastrointestinal tract have decreased due to absorption and metabolism processes [36]. As suggested by various authors, this can be counteracted by the use of coated organic acids [11, 12]. Accordingly, it was observed that encapsulation of MCFAs increased the efficiency in reducing C. jejuni counts in faecal samples [37]. Thus, with regard to future in vivo studies with optimized combinations, it might therefore be worth to consider the administration of organic acids in microencapsulated form.

In conclusion, the results of the present study demonstrated the high potential of combining organic acids against Campylobacter spp. in vitro using a systematic stratified approach for selection. Synergistic activities were proven for nine of ten combinations of organic acids, while combining different organic acids at least decreased the MIC90 values of nearly all individual compounds. This study provides a database of effective combinations of organic acids against Campylobacter spp. evaluated in vitro by a highly standardized method. Further research using animal models is necessary to verify the antibacterial efficacy of the combined organic acids in vivo when applied via feed or drinking water.

Supporting information

S1 Table. The fractional inhibitory concentration indices (∑FIC) of ten combinations of organic acids tested against 20 C. jejuni and ten C. coli isolates.

https://doi.org/10.1371/journal.pone.0239312.s001

(DOCX)

Acknowledgments

We would like to thank all people who were involved in the experiments.

References

  1. 1. Kaakoush NO, Castaño-Rodríguez N, Mitchell HM, Man SM. Global epidemiology of Campylobacter infection. Clin Microbiol Rev. 2015;28(3):687–720. pmid:26062576
  2. 2. EFSA. The European Union one health 2018 zoonoses report. EFSA Journal. 2019;17(12).
  3. 3. Epps SV, Harvey RB, Hume ME, Phillips TD, Anderson RC, Nisbet DJ. Foodborne Campylobacter: infections, metabolism, pathogenesis and reservoirs. Int J Environ Res Public Health. 2013;10(12):6292–304. pmid:24287853
  4. 4. EFSA. Scientific opinion on Campylobacter in broiler meat production: control options and performance objectives and/or targets at different stages of the food chain. EFSA Journal. 2011;9(4):2105.
  5. 5. Hermans D, Pasmans F, Messens W, Martel A, Van Immerseel F, Rasschaert G, et al. Poultry as a host for the zoonotic pathogen Campylobacter jejuni. Vector Borne Zoonotic Dis. 2012;12(2):89–98. pmid:22133236
  6. 6. Solis de Los Santos F, Donoghue AM, Venkitanarayanan K, Dirain ML, Reyes-Herrera I, Blore PJ, et al. Caprylic acid supplemented in feed reduces enteric Campylobacter jejuni colonization in ten-day-old broiler chickens. Poult Sci. 2008;87(4):800–4. pmid:18340004
  7. 7. Byrd J, Hargis BM, Caldwell DJ, Bailey R, Herron KL, McReynolds JL, et al. Effect of lactic acid administration in the drinking water during preslaughter feed withdrawal on Salmonella and Campylobacter contamination of broilers. Poult Sci. 2001;80:278–83. pmid:11261556
  8. 8. Skånseng B, Kaldhusdal M, Moen B, Gjevre A-G, Johannessen GS, Sekelja M, et al. Prevention of intestinal Campylobacter jejuni colonization in broilers by combinations of in-feed organic acids. J Appl Microbiol. 2010;109(4):1265–73. pmid:20522149
  9. 9. van Gerwe T, Bouma A, Klinkenberg D, Wagenaar JA, Jacobs-Reitsma WF, Stegeman A. Medium chain fatty acid feed supplementation reduces the probability of Campylobacter jejuni colonization in broilers. Vet Microbiol. 2010;143(2):314–8. pmid:20022713
  10. 10. Molatová Z, Skřivanová E, Macias B, Mcewan NR, Březina P, Marounek M. Susceptibility of Campylobacter jejuni to organic acids and monoacylglycerols. Folia Microbiol (Praha). 2010;55(3):215–20. pmid:20526832
  11. 11. Grilli E, Vitari F, Domeneghini C, Palmonari A, Tosi G, Fantinati P, et al. Development of a feed additive to reduce caecal Campylobacter jejuni in broilers at slaughter age: from in vitro to in vivo, a proof of concept. J Appl Microbiol. 2013;114(2):308–17. pmid:23110383
  12. 12. Hermans D, Martel A, Van Deun K, Verlinden M, Van Immerseel F, Garmyn A, et al. Intestinal mucus protects Campylobacter jejuni in the ceca of colonized broiler chickens against the bactericidal effects of medium-chain fatty acids. Poult Sci. 2010;89(6):1144–55. pmid:20460660
  13. 13. Beier RC, Byrd JA, Caldwell D, Andrews K, Crippen TL, Anderson RC, et al. Inhibition and interactions of Campylobacter jejuni from broiler chicken houses with organic acids. Microorganisms. 2019;7(8). pmid:31366094
  14. 14. Hermans D, Martel A, Garmyn A, Verlinden M, Heyndrickx M, Gantois I, et al. Application of medium-chain fatty acids in drinking water increases Campylobacter jejuni colonization threshold in broiler chicks. Poult Sci. 2012;91:1733–8. pmid:22700521
  15. 15. Guyard-Nicodème M, Keita A, Quesne S, Amelot M, Poezevara T, Le Berre B, et al. Efficacy of feed additives against Campylobacter in live broilers during the entire rearing period. Poult Sci. 2016;95(2):298–305. pmid:26706356
  16. 16. Ricke SC. Perspectives on the use of organic acids and short chain fatty acids as antimicrobials. Poult Sci. 2003;82(4):632–9. pmid:12710485
  17. 17. Kim SA, Rhee MS. Marked synergistic bactericidal effects and mode of action of medium-chain fatty acids in combination with organic acids against Escherichia coli O157:H7. Appl Environ Microbiol. 2013;79(21):6552–60. pmid:23956396
  18. 18. CLSI. VET01-A4. Performance standards for antimicrobial disk and dilution susceptibility tests for bacteria isolated from animals; Approved standard- Fourth Edition; Clinical and Labaoratory Standards Institute: Wayne, PA, USA. 2013.
  19. 19. de Oliveira CEV, Stamford TLM, Neto NJG, de Souza EL. Inhibition of Staphylococcus aureus in broth and meat broth using synergies of phenolics and organic acids. Int J Food Microbiol. 2010;137(2):312–6. pmid:20004993
  20. 20. European Commission. Regulation (EC) No 1831/2003. European Union register of feed additives. Edition 7/2019 (273). Annex I—07.08.2019. 2019.
  21. 21. Orhan G, Bayram A, Zer Y, Balci I. Synergy tests by E test and checkerboard methods of antimicrobial combinations against Brucella melitensis. J Clin Microbiol. 2005;43(1):140–3. pmid:15634962
  22. 22. Berenbaum MC. A method for testing for synergy with any number of agents. The Journal of Infectious Diseases. 1978;137(2):122–30. pmid:627734
  23. 23. Foucquier J, Guedj M. Analysis of drug combinations: current methodological landscape. Pharmacology Research & Perspectives. 2015;3(3):e00149. pmid:26171228
  24. 24. Dibner JJ, Buttin P. Use of organic acids as a model to study the impact of gut microflora on nutrition and metabolism. Journal of Applied Poultry Research. 2002;11(4):453–63.
  25. 25. Russell JB. Another explanation for the toxicity of fermentation acids at low pH: anion accumulation versus uncoupling. J Appl Bacteriol. 1992;73(5):363–70.
  26. 26. Cengiz Ö, Koksal B, Tatli O, Sevim Ö, Avci H, Epikmen E, et al. Influence of dietary organic acid blend supplementation and interaction with delayed feed access after hatch on broiler growth performance and intestinal health. Vet Med (Praha). 2012;57:515–28.
  27. 27. Chaveerach P, Keuzenkamp DA, Urlings HA, Lipman LJ, van Knapen F. In vitro study on the effect of organic acids on Campylobacter jejuni/coli populations in mixtures of water and feed. Poult Sci. 2002;81(5):621–8. pmid:12033410
  28. 28. Desbois AP, Smith VJ. Antibacterial free fatty acids: activities, mechanisms of action and biotechnological potential. Appl Microbiol Biotechnol. 2010;85(6):1629–42. pmid:19956944
  29. 29. Bergsson G, Arnfinnsson J, Steingrimsson O, Thormar H. Killing of gram-positive cocci by fatty acids and monoglycerides. APMIS. 2001;109(10):670–8. pmid:11890570
  30. 30. Campos FM, Couto JA, Figueiredo AR, Tóth IV, Rangel AOSS, Hogg TA. Cell membrane damage induced by phenolic acids on wine lactic acid bacteria. Int J Food Microbiol. 2009;135(2):144–51. pmid:19733929
  31. 31. Metcalf JH, Donoghue AM, Venkitanarayanan K, Reyes-Herrera I, Aguiar VF, Blore PJ, et al. Water administration of the medium-chain fatty acid caprylic acid produced variable efficacy against enteric Campylobacter colonization in broilers. Poult Sci. 2011;90(2):494–7. pmid:21248350
  32. 32. Leeson S, Namkung H, Antongiovanni M, Lee EH. Effect of butyric acid on the performance and carcass yield of broiler chickens. Poult Sci. 2005;84(9):1418–22. pmid:16206563
  33. 33. Vale M, Menten J, Daróz de Morais SC, Brainer MM. Mixture of formic and propionic acid as additives in broiler feeds. Scientia Agricola—SCI AGRIC. 2004;61.
  34. 34. Adil S, Banday MT, Bhat GA, Qureshi S, Wani SA. Effect of supplemental organic acids on growth performance and gut microbial population of broiler chicken. Livestock Research for Rural Development. 2011;23. pmid:20613998
  35. 35. Cave NAG. Effect of dietary propionic and lactic acids on feed intake by chicks. Poult Sci. 1984;63(1):131–4. pmid:6701138
  36. 36. Thompson JL, Hinton M. Antibacterial activity of formic and propionic acids in the diet of hens on salmonellas in the crop. Br Poult Sci. 1997;38(1):59–65. pmid:9088614
  37. 37. Molatová Z, Skřivanová E, Baré J, Houf K, Bruggeman G, Marounek M. Effect of coated and non-coated fatty acid supplementation on broiler chickens experimentally infected with Campylobacter jejuni. J Anim Physiol Anim Nutr (Berl). 2011;95(6):701–6. pmid:21114690