Relationships between toxin gene content and genetic background in nasal carried isolates of Staphylococcus aureus from Asturias, Spain

Induction of Immunosuppressive CD8+CD25+FOXP3+ Regulatory T Cells by Suboptimal Stimulation with Staphylococcal Enterotoxin C1

Superantigens (SAgs) produced by Staphylococcus aureus at high concentrations induce proliferation of T cells bearing specific TCR Vβ sequences and massive cytokinemia that cause toxic shock syndrome. However, the biological relevance of SAgs produced at very low concentrations during asymptomatic colonization or chronic infections is not understood. In this study, we demonstrate that suboptimal stimulation of human PBMCs with a low concentration (1 ng/ml) of staphylococcal enterotoxin C1, at which half-maximal T cell proliferation was observed, induced CD8⁺CD25⁺ T cells expressing markers related to regulatory T cells (Tregs), such as IFN-γ, IL-10, TGF-β, FOXP3, CD28, CTLA4, TNFR2, CD45RO, and HLA-DR. Importantly, these CD8⁺CD25⁺ T cells suppressed responder cell proliferation mediated in contact-dependent and soluble factor-dependent manners, involving galectin-1 and granzymes, respectively. In contrast, optimal stimulation of human PBMCs with a high concentration (1 μg/ml) of staphylococcal enterotoxin C1, at which maximal T cell proliferation was observed, also induced similar expression of markers related to Tregs, including FOXP3 in CD8⁺CD25⁺ cells, but these T cells were not functionally immunosuppressive. We further demonstrated that SAg-induced TCR Vβ-restricted and MHC class II-restricted expansion of immunosuppressive CD8⁺CD25⁺ T cells is independent of CD4⁺ T cells. Our results suggest that the concentration of SAg strongly affects the functional characteristics of activated T cells, and low concentrations of SAg produced during asymptomatic colonization or chronic S. aureus infection induce immunosuppressive CD8⁺ Tregs, potentially promoting colonization, propagation, and invasion of S. aureus in the host.

Staphylococcus aureus toxins--their functions and genetics

The outcome of encounters between Staphylococcus (S.) aureus and its human host ranges from life-threatening infection through allergic reactions to symptom-free colonization. The pan-genome of this bacterial species encodes numerous toxins, known or strongly suspected to cause specific diseases or symptoms. Three toxin families are in the focus of this review, namely (i) pore-forming toxins, (ii) exfoliative toxins and (iii) superantigens. The majority of toxin-encoding genes are located on mobile genetic elements (MGEs), resulting in a pronounced heterogeneity in the endowment with toxin genes of individual S. aureus strains. Recent population genomic analysis have provided a framework for an improved understanding of the temporal and spatial scales of the motility of MGEs and their associated toxin genes. The distribution of toxin genes among clonal lineages within the species S. aureus is not random, and phylogenetic (sub-)lineages within clonal complexes feature characteristic toxin signatures. When studying pathogenesis, this lineage association, which is caused by the clonal nature of S. aureus makes it difficult to discriminate effects of specific toxins from contributions of the genetic background and/or other associated genetic factors.

Population structure and exotoxin gene content of methicillin-susceptible Staphylococcus aureus from Spanish healthy carriers

The population structure of 111 methicillin-susceptible Staphylococcus aureus (MSSA), recovered in Spain from healthy and risk-free carriers was investigated using pulsed-field gel electrophoresis (PFGE), spa (staphylococcal protein A) typing, multi locus sequence typing (MLST) and the accessory gene regulator (agr). Results from the different techniques were highly concordant, and revealed twelve clonal complexes (CCs): CC30 (27%), CC5 (18.9%), CC45 (16.2%), CC15 (11.7%), CC25 (8.1%), CC1, CC9 (3.6% each), CC59, CC97 and CC121 (2.7% each), CC72 (1.8%) and CC8 (0.9%). Isolates with genetic backgrounds of hospital-acquired MSSA were detected and, consistent with the ability of diverse MSSA to act as recipients of the SCCmec cassette, a MSSA isolate from a healthy carrier shared the ST, spa-type and agr-type of a MRSA clone recovered in a hospital of the same region. All except two fragments of the PGFE-profiles of these isolates were identical, and the differential fragment of the MRSA carried mecA. Analyses of the exotoxin gene content of the nasal isolates revealed an increase in the number of exotoxin genes over time. This, together with the detection of lukPV and the high frequency of tst, exfoliatin and enterotoxin genes, is worrisome and requires further surveillance.

Report of toxic shock syndrome toxin 1 (TSST-1) from Staphylococcus aureus isolated in food handlers and surfaces from foodservice establishments

A set of 53 Staphylococcus aureus isolates collected from food handlers and foodservice establishments in Spain was analyzed for toxic shock syndrome toxin (TSST-1) production. S. aureus strains were isolated from 908 samples collected from different surfaces such as dish towels, workers' hands, cutting boards, stainless steel tables and slicers, but they were not detected neither in clean plates nor in kitchen knives. Only one food worker hand has been reported to be contaminated by TSST-1 in a restaurant. Despite this, proper hygiene practices should be respected for the surfaces of contact with food, as well as for the hands of the manipulators This is the first article, in Spain, that reports the detection of TSST-1 in a restaurant worker hand.

Repeated intranasal instillation with staphylococcal enterotoxin B induces nasal allergic inflammation in guinea pigs

BACKGROUND

Staphylococcal enterotoxins (SEs) appear to play a role in the pathogenesis of allergic disease. However, no animal models have been reported to show nasal allergic inflammation by repeated inhalation with staphylococcal enterotoxin B (SEB) in the absence of adjuvant. This study was designed to determine whether intranasal instillation of guinea pigs with SEB results in nasal allergic inflammation.

METHODS

Guinea pigs were intranasally instilled with 40 μL of 4-μg SEB once every 4 days 11 times. For the control group, guinea pigs were prepared with saline instead of SEB. Sneezing and nasal scratching frequency were evaluated after each intranasal instillation. The production of antigen-specific antibodies including IgE, nasal eosinophilia, and cytokines in the nasal cavity lavage fluid (NCLF) were measured after the 11th intranasal immunization.

RESULTS

In the model group, symptoms of sneezing and nasal scratching were induced at the 8th to 11th challenges. Intranasal immunization with SEB elicited a local nasal mucosa inflammatory response characterized by apparent eosinophil infiltration. In the NCLF, the expression of IL-4 but not interferon-gamma was increased after challenges. The serum levels of total and SEB-specific IgE and IgG1 were higher in model groups in comparison with the control groups (p < 0.01).

CONCLUSION

These results indicate that repeated intranasal instillation with SEB leads to Th2 immune response, allergic nasal inflammation, and increased antigen-specific IgE production that are characteristic of allergic rhinitis (AR). The model in this study could be valuable in analyzing the pathogenesis of AR infected with Staphylococcus aureus.

Expression of staphylococcal superantigens during nasal colonization is not sufficient to induce a systemic neutralizing antibody response in humans

Staphylococcus aureus carriers have high-titer serum antibodies against non-enterotoxin gene cluster (egc) superantigens, whereas they lack anti-egc antibodies, suggesting different superantigen expression profiles in vivo. We measured the superantigen transcripts in S. aureus directly isolated from the nose of persistent carriers and correlated them with the superantigen-neutralizing antibody response. While neutralizing serum antibodies against the staphylococcal enterotoxins A and C (SEA and SEC) were found in carriers, antibodies against the egc-encoded staphylococcal enterotoxin-like toxin O (SElO) were rare. Surprisingly, the transcription of selo was comparable to sea and sec during nasal colonization. Thus, egc superantigens are transcribed during nasal colonization, but this is not sufficient to induce a serum antibody response.

Characterization of infecting strains and superantigen-neutralizing antibodies in Staphylococcus aureus bacteremia

Staphylococcus aureus superantigens (SAgs) are highly potent T cell mitogens. Antibodies against non-enterotoxin gene cluster (non-egc) SAgs are common in healthy adults, whereas neutralizing antibodies against egc SAgs are rare. We investigated the infecting S. aureus strains and the anti-SAg antibody response during S. aureus bacteremia (SAB). This prospective clinical study (www.clinicaltrials.gov, NCT00548002) included 43 injection drug users (IDUs) and 44 group-matched nonaddicts with SAB. spa genotypes and SAg gene patterns (multiplex PCR) of the S. aureus isolates were determined. The neutralizing capacities of sera obtained at the acute phase and the convalescent phase of SAB were tested against the SAg cocktail of the respective infecting strain and a panel of recombinant SAgs. The lineages CC59 and CC30 were more prevalent among bacteremia strains from IDUs than among strains from nonaddicts. SAg gene patterns in isolates from IDUs and nonaddicts were similar. At the acute phase of bacteremia, IDUs had more neutralizing antibodies against non-egc SAgs than did nonaddicts. Antibody titers frequently increased during infection. In contrast, there were no neutralizing antibodies against egc SAgs at disease onset and such antibodies were not induced by SAB. SAB triggers an antibody response only against non-egc SAgs. Preimmunization in IDU patients is probably due to previous exposure to the infecting strain.

Food poisoning and Staphylococcus aureus enterotoxins

Staphylococcus aureus produces a wide variety of toxins including staphylococcal enterotoxins (SEs; SEA to SEE, SEG to SEI, SER to SET) with demonstrated emetic activity, and staphylococcal-like (SEl) proteins, which are not emetic in a primate model (SElL and SElQ) or have yet to be tested (SElJ, SElK, SElM to SElP, SElU, SElU2 and SElV). SEs and SEls have been traditionally subdivided into classical (SEA to SEE) and new (SEG to SElU2) types. All possess superantigenic activity and are encoded by accessory genetic elements, including plasmids, prophages, pathogenicity islands, vSa genomic islands, or by genes located next to the staphylococcal cassette chromosome (SCC) implicated in methicillin resistance. SEs are a major cause of food poisoning, which typically occurs after ingestion of different foods, particularly processed meat and dairy products, contaminated with S. aureus by improper handling and subsequent storage at elevated temperatures. Symptoms are of rapid onset and include nausea and violent vomiting, with or without diarrhea. The illness is usually self-limiting and only occasionally it is severe enough to warrant hospitalization. SEA is the most common cause of staphylococcal food poisoning worldwide, but the involvement of other classical SEs has been also demonstrated. Of the new SE/SEls, only SEH have clearly been associated with food poisoning. However, genes encoding novel SEs as well as SEls with untested emetic activity are widely represented in S. aureus, and their role in pathogenesis may be underestimated.

Clonal complexes and diversity of exotoxin gene profiles in methicillin-resistant and methicillin-susceptible Staphylococcus aureus isolates from patients in a Spanish hospital

Molecular epidemiology studies have allowed the identification of the methicillin (meticillin)-resistant (MRSA) and methicillin-susceptible (MSSA) clonal complexes (CCs) and clones of Staphylococcus aureus circulating in a Spanish hospital recently. Of 81 isolates tested, 32.1% were MRSA. Most of them carried staphylococcal cassette chromosome mec (SCCmec) IVc (88.5%) and belonged to CC5 (88.5%; multilocus sequence typing types ST125 [mainly associated with spa type t067], ST5, and ST228). A higher diversity was found among MSSA isolates (67.9%). Eighty percent shared the genetic background of major MRSA lineages (CC5 [38.2%; ST125 and ST5], CC30 [25.5%; ST30], CC45 [14.5%; ST45 and ST47], and CC8 [1.8%; ST8]), but CC12, CC15, CC51, and CC59 were also detected. Many exotoxin genes were present in each of the 81 isolates, independent of whether they were involved in sepsis (11 to 22) or other types of infections (13 to 21), and they appeared in 73 combinations. The relevant data are that (i) all isolates were positive for hemolysin and leukotoxin genes (98.8% for lukED and 25.9% for lukPV); (ii) all contained an enterotoxin gene cluster (egc with or without seu), frequently with one or more genes encoding classical enterotoxins; (iii) about half were positive for tst and 95% were positive for exfoliatin-encoding genes (eta, etb, and/or etd); and (iv) the four agr groups were detected, with agrII (55.6%) and agrIII (23.5%) being the most frequent. Taken together, results of the present study suggest a frequent acquisition and/or loss of exotoxin genes, which may be mediated by efficient intralineage transfer of mobile genetic elements and exotoxin genes therein and by eventual breakage of interlineage barriers.

Distribution of superantigenic toxin genes in Staphylococcus aureus isolates from milk samples of bovine subclinical mastitis cases in two major diary production regions of China

To evaluate the distribution of most known staphylococcal superantigen (SAg) genes in Staphylococcus aureus isolated from bovine mastitis cases, a genetic analysis of 15 SAg genes and genotypes was performed in a total of 283 S. aureus isolates collected from milk samples of cows with subclinical mastitis in two major diary production regions of China. Almost 65% of the isolates possessed at least one toxin gene. The most frequently found genes were sea (36.0%) followed by sei (31.8%), seg (31.4%) and selm (26.9%). The genes see, selk, or selo were not found in any of the isolates tested. Overall, 28 SAg genotypes were observed, among which the genotypes sea-seg-sei-selm, seg-sei-selm-seln, and sea-sed-selj predominated at the rate of 8.8%, 7.4%, and 6.7%, respectively. Marked geographical variations were noticed in the distribution of individual SAg genes and genotypes among S. aureus isolates from the two different regions. The relationship between toxin genotypes and toxin genes encoding profiles of mobile genetic elements (MGEs) was analyzed, revealing that majority of SAg genes were present in certain MGEs, which were in accordance with current knowledge about MGEs carrying those genes. However, some gene combinations suggest the possibility of the existence of variants or new types of MGEs.

Distribution of enterotoxin genes among carriage- and infection-associated isolates of Staphylococcus aureus

AIMS

To compare the distribution of genes encoding classical and newly described enterotoxins among Staphylococcus aureus, associated with carriage and infection.

METHODS AND RESULTS

Forty-five nasal isolates from carriers and 42 clinical isolates were included. The genes sea to see and seg to sei as well as sem, sen, seo and seu were tested using multiplex and conventional PCR. The most frequently found toxin genes were egc-related genes, in particular the combination seg and sei (n = 55, 63.1%), followed by sen and seu (n = 54, 62.1%), sem (n = 51, 58.6%) and seo (n = 48, 55.2%). Significant differences were found for seg and sei combination (33 of the nasal vs 22 of the infection isolates, P = 0.048) as well as for the genes sem (P = 0.004), sen (P = 0.029) and seo (P = 0.032). Regarding the classical toxin genes no significant differences between the two groups of isolates were found.

CONCLUSIONS

Significant differences between infection and carriage strains were found only for the egc-related genes, which were more common in the nasal isolates.

SIGNIFICANCE AND IMPACT OF THE STUDY

The egc-related enterotoxin genes seem to be more prevalent in carriage- than in infection-associated S. aureus isolates. The possible contribution of egc-related genes in determining the potential for nasal carriage requires further investigation.

Superantigen types in Staphylococcus aureus isolated from patients with cystic fibrosis

The screening of 17 SAg genes of S. aureus isolated from the sputum of cystic fibrosis (CF) patients revealed that among 47 genetically different strains, 39 (83 %) carried SAg genes. Superantigens forming enterotoxin gene cluster were detected in 20 strains. The 2nd most common superantigen type was selk detected in 13 strains. In 9 strains, selk occurred together with the sea gene. Out of 74 strains recovered from nasal carriers, 56 (75 %) were found to carry SAg genes, 38 carried egc genes, while selk was detected in 5 strains. The predominant SAg types in both investigated S. aureus populations were egc and selk/sea, but selk gene frequency was significantly higher in the CF-derived strains.

Clonal distribution of superantigen genes in clinical Staphylococcus aureus isolates

Staphylococcus aureus is both a successful human commensal and a major pathogen. The elucidation of the molecular determinants of virulence, in particular assessment of the contributions of the genetic background versus those of mobile genetic elements (MGEs), has proved difficult in this variable species. To address this, we simultaneously determined the genetic backgrounds (spa typing) and the distributions of all 19 known superantigens and the exfoliative toxins A and D (multiplex PCR) as markers for MGEs. Methicillin- sensitive S. aureus strains from Pomerania, 107 nasal and 88 blood culture isolates, were investigated. All superantigen-encoding MGEs were linked more or less tightly to the genetic background. Thus, each S. aureus clonal complex was characterized by a typical repertoire of superantigen and exfoliative toxin genes. However, within each S. aureus clonal complex and even within the same spa type, virulence gene profiles varied remarkably. Therefore, virulence genes of nasal and blood culture isolates were separately compared in each clonal complex. The results indicated a role in infection for the MGE harboring the exfoliative toxin D gene. In contrast, there was no association of superantigen genes with bloodstream invasion. In summary, we show here that the simultaneous assessment of virulence gene profiles and the genetic background increases the discriminatory power of genetic investigations into the mechanisms of S. aureus pathogenesis.

The distribution of enterotoxin and enterotoxin-like genes in Staphylococcus aureus strains isolated from nasal carriers and food samples

Staphylococcus aureus strains were examined for the presence of 18 enterotoxin (se) and enterotoxin-like (sel) genes by PCR and four classic enterotoxins SEA to SED by reverse passive latex agglutination. We screened three groups of isolates: 53 recovered from food samples collected in years 2004-2005, 18--from food samples collected in the 1960s and 1970s and 30--from nasal carrier samples collected in the years 2000-2002. Eighty percent of all tested strains were se and sel positive, but the frequency of these genes was higher among nasal carrier strains (93%) than among food samples isolates (circa 76%). The enterotoxin genes cluster (egc) was the most prevalent among carrier strains (18/30-60%) and the least--among food strains isolated in the 1960s and 1970s (6/18 strains--33%). sea coding enterotoxin A, which was recognised as the major cause of staphylococcal food poisoning so far, was more often found among carrier strains than among the years 2004-2005 food strains (27% versus 11%), but it was the most frequent se/sel gene among food strains isolated in the 1960s and 1970s (10/18 strains). Moreover in our results certain se and sel genes coexisted, which was in accordance with current knowledge about movable genetic elements carrying those genes. The exception was for only one strain, which harboured the sole selr gene on a possibly new genetic element not yet described. As we found both types of egc, as well as seh (especially food samples strains) occurred alone in a majority of positive strains for each of those genes. The analysis of the results obtained by SET-RPLA method for the productivities of classical enterotoxins A-D and the results obtained by PCR for the presence of sea-sed genes revealed the correlation between each other. Only three of all sea-sed positive strains had silent genes--sed.

Staphylococcal enterotoxin-like toxins U2 and V, two new staphylococcal superantigens arising from recombination within the enterotoxin gene cluster

To test the hypothesis that the Staphylococcus aureus enterotoxin gene cluster (egc) can generate new enterotoxin genes by recombination, we analyzed the egc locus in a broad panel of 666 clinical isolates of S. aureus. egc was present in 63% of isolates, confirming its high prevalence. The archetypal organization of the egc locus, consisting of five enterotoxin genes plus two pseudogenes, was found in 409 of 421 egc-positive strains. The egc locus was incomplete in a few strains and occasionally harbored an insertion sequence and transposase genes. These strains may represent evolutionary intermediates of the egc locus. One strain with an atypical egc locus produced two new enterotoxins, designated SElV and SElU2, generated by (i) recombination between selm and sei, producing selv, and (ii) a limited deletion in the varphient1-varphient2 pseudogenes, producing selu2. Recombinant SElV and SElU2 had superantigen activity, as they specifically activated the T-cell families Vbeta 6, Vbeta 18, and Vbeta 21 (SElV) and Vbeta 13.2 and Vbeta 14 (SElU2). Immunoscope analysis showed a Gaussian CDR3 size distribution of T-cell receptor Vbeta chain junctional transcripts of expanded Vbeta subsets in toxin-stimulated cultures, reflecting a high level of polyclonality. These data show that egc is indeed capable of generating new superantigen genes through recombination.

The profiles of enterotoxin genes in Staphylococcus aureus from nasal carriers

AIMS

To evaluate the occurrence of enterotoxin genes in Staphylococcus aureus recovered from nasal carriers.

METHODS AND RESULTS

Eighty S. aureus strains were tested for the presence of 17 new enterotoxin genes using multiplex-PCR. Sixty-one isolates were found to carry enterotoxin genes. The majority of the enterotoxigenic isolates carried enterotoxin gene cluster (egc) genes, namely seg, sei, sem, sen and seo. The egc type containing the seu gene was found in 19 of the 47 isolates with egc-like genes. Interestingly, no seu-containing egc coexisted with sec and sel, as was the case for a considerable portion of the isolates carrying a seu-negative egc. The tst gene was detected in two isolates carrying sec and sel only and in eight isolates carrying seu, but not in the isolates containing the seu-negative egc type.

CONCLUSIONS

The genes forming an egc were found to be predominant in S. aureus from nasal carriers. The coexistence of a seu-positive egc with tst in contrast to an egc lacking the seu gene apparently is not associated with the presence of tst and can reflect a difference between these gene groupings.

SIGNIFICANCE AND IMPACT OF THE STUDY

The egc types carried by the analysed isolates seem to have an influence on the distribution of other genes located on staphylococcal pathogenicity islands, which may modulate the repertoire of virulence factors carried by a single S. aureus strain.

Staphylococcus aureus isolates from Irish domestic refrigerators possess novel enterotoxin and enterotoxin-like genes and are clonal in nature

A previous study carried out by the National Food Centre in Dublin on bacterial contamination of Irish domestic refrigeration systems revealed that 41% were contaminated with Staphylococcus aureus. One hundred fifty-seven S. aureus isolates were screened by multiplex PCR analysis for the presence of 15 staphylococcal enterotoxin and enterotoxin-like genes (sea-see, seg-sei, selj-selo, and selq) and the toxic shock toxin superantigen tst gene. Of the refrigerator isolates, 64.3% possessed more than one staphylococcal enterotoxin or staphylococcal enterotoxin-like gene. All bar one of the 101 staphylococcal enterotoxin or staphylococcal enterotoxin-like gene-positive strains possessed the egc locus bearing the seg, sei, selm, seln, and selo genes. Twelve random amplified polymorphic DNA (RAPD) types accounted for 119 (75.8%) of the strains, two of these types accounting for 25 (RAPD type 1, 15.9%) and 52 (RAPD type 5, 33.1%), respectively. All of the RAPD type 5 isolates possessed the egc gene cluster only. The RAPD type 5 amplicon profile was identical to that of S. aureus isolates associated with osteomyelitis in broiler chickens in Northern Ireland that also possessed the egc locus only. However, the RAPD type 5 domestic refrigerator and chicken isolates differed in penicillin G sensitivity, production of Protein A and staphylokinase, and crystal violet agar growth type. These findings highlight that the average Irish household refrigerator harbors potential enterotoxin-producing S. aureus that may or may not be of animal origin and, accordingly, is a potential reservoir for staphylococcal food poisoning.

Intranasal exposure to bacterial superantigens induces airway inflammation in HLA class II transgenic mice

Staphylococcus aureus is widely prevalent in the nasopharynges of healthy individuals (carriers) but can also cause serious infections. S. aureus can elaborate a variety of superantigen exotoxins in "carrier" or "pathogenic" states. Streptococcus pyogenes can also colonize the nasopharynx and elaborate superantigens. Unlike the acute effects of superantigen exotoxins absorbed through the gut or vaginal mucosa, little is known regarding the pathogenesis of superantigens entering through the intranasal route. In the current study, we evaluated the local and systemic effects of staphylococcal enterotoxin B (SEB) and streptococcal pyrogenic exotoxin A (SPEA) delivered through the intranasal route. Superantigens were administered intranasally on multiple occasions, and experimental animals were sacrificed on day 8 for experimental analyses. SEB-induced airway inflammation was more pronounced for HLA-DR3 transgenic mice than for BALB/c mice, consistent with bacterial superantigens binding more efficiently to human than murine major histocompatibility complex class II. The nature of the airway inflammation in HLA-DR3 mice was determined by the concentration of SEB applied intranasally. Low concentrations (20 ng) induced eosinophilic airway inflammation as well as eosinophil degranulation, whereas intranasal exposure to higher concentrations (2,000 ng) resulted in neutrophilic airway inflammation, permanent airway destruction, toxic shock, and mortality. SEB-induced eosinophilic inflammatory response was enhanced in signal transducer and activator of transcription (STAT)-4-deficient HLA-DQ8 transgenic mice with defective interleukin-12 signaling. Intranasal administration of SPEA induced airway inflammation and systemic immune activation in HLA-DQ8 transgenic mice. In conclusion, repeated chronic intranasal exposure to bacterial superantigens causes airway inflammation and systemic immune activation.