Figures
Abstract
A nationwide laboratory-based surveillance study of invasive S. pyogenes infections was conducted in Germany. Invasive isolates (n = 719) were obtained between 2009 and 2014. Most isolates were obtained from blood (92.1%). The proportions of isolates from cerebrospinal fluid, pleural fluid, synovial fluid and peritoneal fluid were 3.9%, 1.8%, 1.7% and 0.6%, respectively. The most common emm types were emm 1 (31.8%), emm 28 (15.4%) and emm 89 (14.5%). The most common superantigen genes (speA, speC, speG, speH, speI, speJ, speK, speL, speM, ssa) identified from S. pyogenes were speG (92.1%), speJ (50.9%), and speC (42.0%). Significant associations of superantigen genes with underlying conditions or risks were observed in speG, speH, speJ, and speK. Significant associations between emm types or superantigen genes with clinical complications were observed in emm type 3 and in superantigen gene speA 1–3. Most frequent clinical manifestations included sepsis 59.4%, STSS 6.3%, meningitis 5.4%, and necrotizing fasciitis 5.0% (significantly associated with emm1).
Citation: Imöhl M, Fitzner C, Perniciaro S, van der Linden M (2017) Epidemiology and distribution of 10 superantigens among invasive Streptococcus pyogenes disease in Germany from 2009 to 2014. PLoS ONE 12(7): e0180757. https://doi.org/10.1371/journal.pone.0180757
Editor: Jose Melo-Cristino, Universidade de Lisboa Faculdade de Medicina, PORTUGAL
Received: August 16, 2016; Accepted: June 21, 2017; Published: July 18, 2017
Copyright: © 2017 Imöhl et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper. A supplementary table with all primary data is provided.
Funding: The author(s) received no specific funding for this work.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Streptococcus pyogenes (Lancefield group A streptococcus; GAS) is a major human pathogen and responsible for a wide range of both suppurative and non-suppurative diseases, e.g. pharyngitis, erysipelas, septicaemia, meningitis, pneumonia and the notably severe manifestations necrotising fasciitis (NF) and streptococcal toxic shock syndrome (STSS). Suppurative infections and post-infection sequelae, e.g. acute rheumatic fever, rheumatic heart disease and glomerulonephritis, result in substantial human morbidity [1]. Invasive infections caused by S. pyogenes (iGAS) have been reported increasingly since the mid- to late 1980s [2], and recent upsurges in iGAS infections were reported from England [3], Ireland [4, 5] and Sweden [6]. The global burden of invasive S. pyogenes disease is high, and there are estimated to be at least 663,000 new cases and 163,000 deaths worldwide each year. Beyond this, there are more than 111 million cases of S. pyogenes pyoderma and over 616 million cases of pharyngitis annually [7]. Among the many virulence factors produced by S. pyogenes, the M protein is considered to be of major importance. The M protein is a fimbrial protein located on the cell surface. The emm gene, which encodes the M protein, is used as the basis for typing S. pyogenes. Marked changes in the distribution of emm types circulating in Europe have been noticed over the last three decades [2]. Furthermore, there seem to be huge differences concerning the global distribution of emm types. A systematic review of the global distribution of GAS emm types found the epidemiology in Africa and the Pacific region to be different from that in other regions, particularly high-income countries. In Africa and the Pacific, there were no dominant emm types and a higher diversity of emm types, and many of the emm types common in other parts of the world were less common (including emm 1, 4, 6 and 12) [8].
In particular, emm 1, and, to a lesser extent, emm 3, are associated with outbreaks and fatal outcomes [2, 9–12]. A recent study analysing the epidemiological patterns of severe S. pyogenes disease in 11 European countries found an overall 7-day case fatality rate of 19%, ascending to 44% among patients who developed streptococcal toxic shock syndrome [13]. In comparison, these case fatality rates are lower than those reported in iGAS infections during a previous surveillance period in Germany (1996–2002: overall 40.6%, STSS 57.9%) [9]. Other important virulence factors include the streptococcal superantigens (SAgs). SAgs are bacterial toxins which bind to major histocompatibility complex class II and T-cell receptors, thereby stimulating large numbers of T cells and causing a massive release of cytokines into the bloodstream. Overproduction of these cytokines can lead to tissue damage, organ failure, and shock [14]. Currently, eleven different superantigens (speA, speC, speG, speH, speI, speJ, speK, speL, speM, ssa, smeZ) have been identified from S. pyogenes [14–18].
In this study, we analysed the superantigens speA, speC, speG, speH, speI, speJ, speK, speL, speM, and ssa for all isolates. The present investigation compares the emm types and the superantigen toxin genes of 719 invasive S. pyogenes strains collected in a nationwide voluntary laboratory-based surveillance in Germany during 2009 to 2014. Clinical manifestations, clinical complications, underlying conditions and risk factors are analysed.
Materials and methods
Study design
German microbiological laboratories were invited to send their isolates to the German National Reference Center for Streptococci (NRCS; Aachen, Germany). In total, 719 isolates were sent by 130 laboratories located all over Germany between 2009 and 2014. Isolates were included into the study when they met the criteria of an invasive infection according to the definition of the Working Group on Severe Streptococcal Infections 1993 [19], i.e. isolation from a normally sterile site (e.g., blood, cerebrospinal fluid, synovial fluid).
In order to collect the underlying data, a detailed questionnaire was filled out for each specimen sent by the participating centers. In the few cases without enclosed questionnaires, the completion of the data sheet was requested retrospectively. The data included gender and age of the patient, the diagnoses (including certain specified diagnoses like STSS, NF, septicaemia, pneumonia, cellulitis and puerperal sepsis), and information about the clinical course (including information about presence of shock, adult respiratory distress syndrome, presence of artificial ventilation, renal failure, soft-tissue necrosis, disseminated intravascular coagulation, liver abnormality and exanthema). Possible risk factors analysed included the presence of immunosuppression, concomitant surgery, diabetes mellitus, chronic skin lesions, hospital acquired infection and intravenous drug abuse.
Microbiological investigations
Isolates were identified by β-haemolysis on sheep blood agar, Lancefield antigen grouping using a commercially available agglutination technique (Slidex Streptokit, bioMérieux, Marcy-L’Etoile, France; Prolex Streptococcal Grouping Latex Kits, Pro-Lab Diagnostics, Richmond Hill, Canada), and the pyrrolidonyl-arylamidase (PYR) test. The detection of emm genes was determined by PCR using ‘all M primers’ as described previously [20]. PCR products were purified and sequenced using an automated ABI Prism 310 DNA sequencer (Applied Biosystems, Weiterstadt, Germany). The nucleotide sequences encoding the N-terminal hyper-variable portion of the M protein were compared to the emm database and emm types were assigned as described on the CDC’s website (http://www2a.cdc.gov/ncidod/biotech/strepblast.asp). The presence of the ten different superantigen genes (speA, speC, speG, speH, speI, speJ, speK, speL, speM, ssa) was determined by PCR as described previously [16]. Concerning speA, the primer spea1-4 detects the speA alleles 1, 2, 3, and 4, whereas the primer spea1-3+5 detects the speA alleles 1, 2, 3, and 5. If spea1-4 and spea1-3+5 both yield positive results, then the isolate contains speA allele 1, 2, or 3. If only spea1-4 gives positive results, then the isolate contains the allele speA 4; if only spea1-3+5 gives positive results, then the isolate contains the allele speA 5 [14].
Statistical analysis
Continuous variables were summarized by means and corresponding standard deviations. Categorical variables were summarized by absolute and relative frequencies. Univariate logistic regression models were used for variable selection, and a selection criterion of p < 0.05 was used for inclusion into multivariate logistic regression models. For each emm type and each superantigen, we investigated the possible influence on each outcome parameter (all diagnoses and all clinical complications). In contrast, the influence of all risk factors was investigated for all emm types and all superantigens (here the outcome parameter). Influence factors with a p value of p < 0.05 as well as age and sex were selected for the corresponding multivariate models. Univariate models were also constructed to examine possible relationships between emm type and superantigen genes. For sex, odds ratios >1 correspond to relationships which occur more commonly in males, while odds ratios <1 occur more commonly in females. For age, odds ratios >1 correspond to a relationship with increasing age, and odds ratios <1 correspond to decreasing age. Only outcome variables with 15 or more events are analysed in regression models [21]. Further possible estimation problems are described in the discussion section. All tests were two-sided and assessed at the 5% significance level. Because of the exploratory nature of the study we made no adjustment to the significance level of the several multivariate models. Statistical analyses were performed using R software, version 3.3.2. The complete data set used for the analyses is included as a supplementary table, S1 Table.
Ethical statement
An ethical approval or patients’ consent was not required since the study only includes microbiological samples sent to the German National Reference Center for Streptococci on an anonymized basis by the sending microbiological laboratories, and did not involve human subjects or material.
Results
A total of 719 iGAS samples were collected between January 1st 2009 and December 31st 2014. The numbers of included cases for each year were: 2009, 91; 2010, 112; 2011, 108; 2012, 122; 2013, 155 and 2014, 131. The isolates were obtained from blood (662), cerebrospinal fluid (28), pleural fluid (13), synovial fluid (12), and peritoneal fluid (4). A seasonal variation was noted, with most cases reported in winter and early spring (Fig 1).
The age-specific incidence of iGAS infections is shown in Fig 2. For the 719 patients, the mean age was 53.5 years, the median 59 years (range 0–97 years). A higher amount of cases per 100,000 inhabitants in the respective age groups was found in children up to 5 years and in adults ≥ 60 years. Among the latter, in our study the incidence was relatively constant among adults in the age groups from 30 to 59 years, but rose with every decade of age among those aged ≥ 60 years. For adults from 70–79 years, and especially those aged ≥ 80 years, the incidence even exceeded those among children from 0–5 years of age.
Most isolates (53.1%) were obtained from male patients, 46.7% from females and for 0.1% no information on gender was available. All patient data are listed in S1 Table.
Information on the existence of underlying conditions/risk factors was available for 95 (13.2%) of the 719 cases; 2.5% of these patients had two, and 0.3% had three risk factors. Most frequent risk factors were diabetes (43.2%), immunosuppression (29.5%) and chronic skin lesions/wounds (24.2%) (Table 1). The distribution of clinical manifestations and clinical complications of iGAS is shown in Table 1 as well. The most common clinical manifestations were sepsis (59.4%), followed by erysipelas (7.6%), pneumonia (7.0%), STSS (6.3%), and meningitis (5.4%). Next frequent manifestations were NF (5.0%), phlegmon (4.0%) and septic arthritis (2.1%). Four cases of puerperal sepsis (0.6%) were reported from 2009 to 2014. Among the clinical complications of iGAS infections, hypotensive shock (17.8%) was the most common condition, followed by renal insufficiency (15.7%), disseminated intravascular coagulopathy (DIC) (11.8%), liver abnormality (10.2%) soft tissue necrosis (8.6%), and respiratory distress (6.5%).
Among the 719 isolates, 46 different emm types were identified (Table 2). The five most common types, emm 1 (31.8%), emm 28 (15.4%), emm 89 (14.5%), emm 3 (7.9%), and emm 12 (6.4%), are responsible for 76.1% of iGAS disease. The frequency of emm type 28 isolates was fairly constant from 2009 to 2014, whereas the four other most prominent emm types (1, 89, 3, and 12) were more variable, though yearly variations in emm types did not show any significant patterns (see Table 3). 92.1% of samples were positive for speG, 50.9% for speJ, 42.0% for speC, 39.9% for speA 1–3, no samples were positive for speA 4, 0.7% for speA 5, 14.3% for ssa, 14.5% for speK, 9.7% for speH, 7.4% for speI, 5.7% for speM, and 4.7% for speL.
Bold print indicates a statistically-significant positive association from univariate logistic regression analysis (p≤0.05) between the listed emm type and superantigen gene(s).
The correlations between risks/underlying conditions, diagnoses and clinical complications and emm types or superantigens found in the statistical analysis are shown in Tables 4, 5 and 6. Among underlying conditions and risk factors, speH, speJ, and speK were significantly associated with chronic skin lesions, and speG was significantly associated with diabetes. Among clinical complications, emm 1 was non-significantly associated with hypotensive shock, DIC, renal insufficiency, liver abnormality, soft tissue necrosis, and exanthema. Hypotensive shock, DIC, and renal insufficiency were non-significantly associated with emm 28. Superantigen speC was non-significantly associated with DIC, renal insufficiency, and exanthema; speA 1–3 with hypotensive shock, DIC, renal insufficiency, liver abnormality (significantly), soft tissue necrosis, and exanthema. Superantigen speJ was non-significantly associated with hypotensive shock and exanthema; speM was non-significantly associated with soft tissue necrosis. In meningitis cases, emm types 1 and 89, as well as speA 1–3 were predictors. emm 1, speC, and speJ were all non-significantly associated with NF (emm1 reached statistical significance for NF) and sepsis. Septic arthritis was significantly associated with emm 28.
Bold values indicate a statistically significant association in multivariate analysis.
Bold values indicate a statistically significant association in multivariate analysis.
Bold values indicate a statistically significant association in multivariate analysis.
Additionally, we established some correlations between age, sex, and emm types and superantigen genes.Superantigen speH was associated with decreasing age. Among clinical complications, coagulopathy, respiratory distress, and exanthema were significantly associated with decreasing age, while renal insufficiency was significantly associated with increasing age. Decreasing age was associated with speH and speJ as predictors of chronic skin lesions. And in the studied diagnoses, meningitis was associated with decreasing age.
Discussion
In this paper we present the results of 6 years of surveillance of iGAS disease in Germany.
Reported iGAS cases in Germany are low in comparison with surveillance programs from other countries. This might, at least in part, be explained by the voluntary nature of the German surveillance system, resulting in a smaller number of cases being referred to the reference laboratory and a potential underreporting of invasive S. pyogenes infections. In comparison with previous German surveillance periods, the incidence per year is slightly, but not significantly, higher in the current study (0.15 cases/100,000 individuals) than in previous surveillance periods (1996–2002, 0.1 cases [9]; 2003–2007, 0.13 cases [22]). The seasonal occurrence of iGAS disease with most cases reported in winter and early spring is congruent with the patterns observed in other countries [2].
In the present study, emm 1 was the most prevalent emm type, which is consistent with results from the USA [23], Australia [24], Japan [25], and across Europe [2, 9, 11], followed in frequency by the emm types 28, 89, 3 and 12. These five emm types are responsible for over three-fourths of iGAS disease in the current German surveillance period and these emm types have been reported to be among the most prevalent in the United States [23], Denmark [26, 27], and other European countries [11] as well. Compared to the two previous surveillance periods in Germany (1996–2002 [9] and 2003–2007 [22]), overall there seems to be no statistically-significant pattern in the frequency of emm type 1 (1996–2002, 37.3%; 2003–2007, 30.5%, 2009–2014, 31.8%), nor in emm type 28, which increased from 9.1% in 1996–2002 to 18.3% in 2003–2007 and decreased to 15.4% in 2009–2014, which is similar to results reported among adults in France [10]. Nevertheless, in other studies a re-emergence [26] or an increase [28] of emm type 1 has been described. However, the most prominent trend in comparison to the two previous surveillance periods in Germany is the increase of emm type 89 from 3.4% in 1996–2002 to 7.0% in 2003–2007 and 14.5% in 2009–2014. Comparable results have been reported from other countries [26, 28].
While our models are exploratory in nature, some underlying conditions are nevertheless clear risk factors for iGAS disease. Diabetes is a risk factor for infection with strains harbouring speG. Chronic skin lesions are a risk factor for infection with strains harboring speH, speJ, and speK. Among the studied clinical complications, significant associations were found only with speA 1–3 (with liver abnormality), and emm 3, with respiratory distress. Among the studied diagnoses, significant associations were found with emm type 1 (with NF), and emm type 28 (with septic arthritis).
The relevance of erythrogenic toxin- and superantigen genes relating to invasive infections remains inconclusive, despite extensive literature on this topic [17], particularly since they are also common in non-invasive isolates. In our study, even the statistically significant results did not result in odds ratios far above or below one. Most emm types were characterized by the presence of one or two specific toxin gene profiles [29, 30]. Hypothetically, at least one toxin gene is required in order for severe GAS disease to manifest [27]. Indeed, in our study, only 10 of 719 cases (1.4%) did not have any of the superantigen genes we examined, of which two cases were from patients with a co-occurring serious illness (diabetes). There are no clear statistical relationships between diagnosis or clinical complications and the samples without any detected superantigens. Samples without any of the studied superantigens were from only five emm types, emm77 (n = 5), emm60 (n = 2), emm3 (n = 1), emm63 (n = 1), and emm165 (n = 1). Since we did not examine superantigen smeZ, we cannot rule out the possibility that these ten samples harbour this superantigen. Further research is necessary to elucidate the interrelation between superantigen gene combinations, emm types and disease pattern of iGAS infections.
Supporting information
S1 Table. Diagnoses, complications, underlying conditions, emm types, and superantigen genes in 719 cases of invasive Group A Streptococcus disease in Germany from 2009–2014.
https://doi.org/10.1371/journal.pone.0180757.s001
(XLSX)
Acknowledgments
We thank the microbiological laboratories in Germany for their cooperation and for providing the isolates.
References
- 1. Cunningham MW. Pathogenesis of group A streptococcal infections. Clin Microbiol Rev. 2000;13(3):470–511. Epub 2000/07/25. pmid:10885988.
- 2. Lamagni TL, Efstratiou A, Vuopio-Varkila J, Jasir A, Schalen C. The epidemiology of severe Streptococcus pyogenes associated disease in Europe. Euro Surveill. 2005;10(9):179–84. Epub 2005/11/11. pmid:16280610.
- 3. Zakikhany K, Degail MA, Lamagni T, Waight P, Guy R, Zhao H, et al. Increase in invasive Streptococcus pyogenes and Streptococcus pneumoniae infections in England, December 2010 to January 2011. Euro Surveill. 2011;16(5). pmid:21315057.
- 4. Martin J, Murchan S, O'Flanagan D, Fitzpatrick F. Invasive Group A streptococcal disease in Ireland, 2004 to 2010. Euro Surveill. 2011;16(41). pmid:22008200.
- 5. Meehan M, Murchan S, Bergin S, O'Flanagan D, Cunney R. Increased incidence of invasive group A streptococcal disease in Ireland, 2012 to 2013. Euro Surveill. 2013;18(33):20556. pmid:23968875.
- 6. Vikerfors A, Haggar A, Darenberg J, Low A, Melhus A, Hedlund J, et al. Severe group A streptococcal infections in Uppsala County, Sweden: clinical and molecular characterization of a case cluster from 2006 to 2007. Scand J Infect Dis. 2009;41(11–12):823–30. pmid:19922064.
- 7. Carapetis JR, Steer AC, Mulholland EK, Weber M. The global burden of group A streptococcal diseases. Lancet Infect Dis. 2005;5(11):685–94. Epub 2005/10/29. pmid:16253886.
- 8. Steer AC, Law I, Matatolu L, Beall BW, Carapetis JR. Global emm type distribution of group A streptococci: systematic review and implications for vaccine development. Lancet Infect Dis. 2009;9(10):611–6. pmid:19778763.
- 9. Wahl RU, Lütticken R, Stanzel S, van der Linden M, Reinert RR. Epidemiology of invasive Streptococcus pyogenes infections in Germany, 1996–2002: results from a voluntary laboratory surveillance system. Clin Microbiol Infect. 2007;13(12):1173–8. Epub 2007/09/14. pmid:17850344.
- 10. Plainvert C, Doloy A, Loubinoux J, Lepoutre A, Collobert G, Touak G, et al. Invasive group A streptococcal infections in adults, France (2006–2010). Clin Microbiol Infect. 2012;18(7):702–10. pmid:21883669.
- 11. Luca-Harari B, Darenberg J, Neal S, Siljander T, Strakova L, Tanna A, et al. Clinical and microbiological characteristics of severe Streptococcus pyogenes disease in Europe. J Clin Microbiol. 2009;47(4):1155–65. pmid:19158266; PubMed Central PMCID: PMC2668334.
- 12. Siljander T, Lyytikainen O, Vahakuopus S, Snellman M, Jalava J, Vuopio J. Epidemiology, outcome and emm types of invasive group A streptococcal infections in Finland. Eur J Clin Microbiol Infect Dis. 2010;29(10):1229–35. pmid:20563620.
- 13. Lamagni TL, Darenberg J, Luca-Harari B, Siljander T, Efstratiou A, Henriques-Normark B, et al. Epidemiology of severe Streptococcus pyogenes disease in Europe. J Clin Microbiol. 2008;46(7):2359–67. Epub 2008/05/09. pmid:18463210.
- 14. Lintges M, van der Linden M, Hilgers RD, Arlt S, Al-Lahham A, Reinert RR, et al. Superantigen genes are more important than the emm type for the invasiveness of group A Streptococcus infection. J Infect Dis. 2010;202(1):20–8. pmid:20497047.
- 15. Fraser JD, Proft T. The bacterial superantigen and superantigen-like proteins. Immunol Rev. 2008;225:226–43. Epub 2008/10/08. pmid:18837785.
- 16. Lintges M, Arlt S, Uciechowski P, Plumakers B, Reinert RR, Al-Lahham A, et al. A new closed-tube multiplex real-time PCR to detect eleven superantigens of Streptococcus pyogenes identifies a strain without superantigen activity. Int J Med Microbiol. 2007;297(6):471–8. pmid:17481952.
- 17. Commons RJ, Smeesters PR, Proft T, Fraser JD, Robins-Browne R, Curtis N. Streptococcal superantigens: categorization and clinical associations. Trends in molecular medicine. 2014;20(1):48–62. pmid:24210845.
- 18. Spaulding AR, Salgado-Pabon W, Kohler PL, Horswill AR, Leung DY, Schlievert PM. Staphylococcal and streptococcal superantigen exotoxins. Clin Microbiol Rev. 2013;26(3):422–47. pmid:23824366; PubMed Central PMCID: PMC3719495.
- 19. Defining the group A streptococcal toxic shock syndrome. Rationale and consensus definition. The Working Group on Severe Streptococcal Infections. JAMA. 1993;269(3):390–1. Epub 1993/01/20. pmid:8418347.
- 20. Podbielski A, Melzer B, Lütticken R. Application of the polymerase chain reaction to study the M protein(-like) gene family in beta-hemolytic streptococci. Medical Microbiology and Immunology. 1991;180(4):213–27. pmid:1784271
- 21. Vittinghoff E, McCulloch CE. Relaxing the rule of ten events per variable in logistic and Cox regression. Am J Epidemiol. 2007;165(6):710–8. pmid:17182981.
- 22. Imöhl M, Reinert RR, Ocklenburg C, van der Linden M. Epidemiology of invasive Streptococcus pyogenes disease in Germany during 2003–2007. FEMS Immunol Med Microbiol. 2010;58(3):389–96. pmid:20146737.
- 23. O'Loughlin RE, Roberson A, Cieslak PR, Lynfield R, Gershman K, Craig A, et al. The epidemiology of invasive group A streptococcal infection and potential vaccine implications: United States, 2000–2004. Clin Infect Dis. 2007;45(7):853–62. Epub 2007/09/07. pmid:17806049.
- 24. O'Grady KA, Kelpie L, Andrews RM, Curtis N, Nolan TM, Selvaraj G, et al. The epidemiology of invasive group A streptococcal disease in Victoria, Australia. Med J Aust. 2007;186(11):565–9. Epub 2007/06/06. pmid:17547544.
- 25. Ikebe T, Hirasawa K, Suzuki R, Ohya H, Isobe J, Tanaka D, et al. Distribution of emm genotypes among group A streptococcus isolates from patients with severe invasive streptococcal infections in Japan, 2001–2005. Epidemiol Infect. 2007;135(7):1227–9. Epub 2007/02/10. pmid:17288642.
- 26. Ekelund K, Skinhoj P, Madsen J, Konradsen HB. Reemergence of emm1 and a changed superantigen profile for group A streptococci causing invasive infections: results from a nationwide study. J Clin Microbiol. 2005;43(4):1789–96. Epub 2005/04/09. pmid:15815000.
- 27. Luca-Harari B, Ekelund K, van der Linden M, Staum-Kaltoft M, Hammerum AM, Jasir A. Clinical and Epidemiological Aspects of Invasive Streptococcus pyogenes Infections in Denmark during 2003 and 2004. J Clin Microbiol. 2008;46(1):79–86. Epub 2007/10/26. pmid:17959766.
- 28. Friaes A, Lopes JP, Melo-Cristino J, Ramirez M, Portuguese Group for the Study of Streptococcal I. Changes in Streptococcus pyogenes causing invasive disease in Portugal: evidence for superantigen gene loss and acquisition. Int J Med Microbiol. 2013;303(8):505–13. pmid:23932912.
- 29. Schmitz FJ, Beyer A, Charpentier E, Normark BH, Schade M, Fluit AC, et al. Toxin-gene profile heterogeneity among endemic invasive European group A streptococcal isolates. J Infect Dis. 2003;188(10):1578–86. Epub 2003/11/19. pmid:14624385.
- 30. Vlaminckx BJ, Mascini EM, Schellekens J, Schouls LM, Paauw A, Fluit AC, et al. Site-specific manifestations of invasive group a streptococcal disease: type distribution and corresponding patterns of virulence determinants. J Clin Microbiol. 2003;41(11):4941–9. Epub 2003/11/08. pmid:14605121.