Exotoxins of Staphylococcus aureus. Clin Microbiol Rev. 2000;13:16-34, table of contents. [PMID: 10627489 DOI: 10.1128/cmr.13.1.16-34.2000]

Immunotherapeutic approaches against Staphylococcus aureus

Staphylococcus aureus is a major cause of life-threatening infections such as bacteremia and endocarditis. Unfortunately, many strains of this bacterial species have become resistant to certain antibiotics, including methicillin and amoxicillin. These strains are known as methicillin-resistant S. aureus (MRSA). Therefore, the prophylactic and therapeutic potential of antistaphylococcal vaccines is currently being explored with priority. In animal models, (passive) immunization with (antibodies directed against) certain S. aureus surface components, staphylococcal toxins and capsular polysaccharides protects against S. aureus colonization or infection. However, immunization studies performed in humans show less promising results. So far, not a single antistaphylococcal vaccine successfully passed clinical trials. This article focuses on the results that were obtained with immunotherapeutic approaches directed against S. aureus in animal and human studies. In addition, it is discussed whether effective immunization approaches against S. aureus are feasible in humans.

Staphylococcal α-toxin induces a higher T cell proliferation and interleukin-31 in atopic dermatitis

BACKGROUND

Patients with atopic dermatitis (AD) are frequently colonized with α-toxin-producing Staphylococcus aureus which is in turn positively correlated with the severity of eczema.

METHODS

IN this study we addressed T cell proliferation and T cell as well as monocyte cytokine secretion upon α-toxin stimulation in peripheral blood mononuclear cells (PBMCs) from AD patients compared to healthy controls.

RESULTS

We found that α-toxin stimulation of PBMCs markedly enhanced T cell proliferation both in patients with AD and healthy controls and was significantly increased in AD patients compared to healthy controls. PBMCs of AD patients secreted significantly more IL-31 compared to those of healthy controls upon α-toxin and SEB stimulation. Moreover, α-toxin stimulation yielded an increase in T cell (IL-2, IL-9, IL-10 and IFN-γ) as well as monocyte (IL-1β and TNF-α) cytokine secretion.

CONCLUSION

Our results could partly explain how skin colonization and infection with S. aureus can contribute to chronic skin inflammation and pruritus in AD.

Roles of Toll-like receptor 2 (TLR2) and superantigens on adaptive immune responses during CNS staphylococcal infection

Staphylococcus aureus is a common etiologic agent of brain abscesses and possesses numerous virulence factors that manipulate host immunity. One example is superantigens (SAG) that clonally expand T cell subsets bearing specific Vβ receptors. Toll-like receptor 2 (TLR2) is one receptor implicated in S. aureus recognition. However, the interplay between TLR2, SAG, and adaptive immunity during brain abscess formation has not yet been investigated and could reveal novel insights into host-pathogen interactions for regulating protective immunity. A comprehensive analysis of abscess-associated T cell populations in TLR2 KO and WT mice was performed following infection with a S. aureus clinical isolate. Both natural killer T (NKT) and γδ T cell infiltrates were increased in brain abscesses of TLR2 KO mice and produced more IL-17 and IFN-γ compared to WT populations, which could have resulted from elevated bacterial burdens observed in these animals. Analysis of SAG-reactive T cells revealed a predominant Vβ(8.1,8.2) infiltrate reactive with staphylococcal enterotoxin B (SEB), whereas SEA-reactive Vβ(11) T cells were less numerous. Brain abscesses of TLR2 KO mice had fewer Vβ(8.1,8.2) and Vβ(11) T cells and produced less TNF-α and IFN-γ compared to WT animals. Treatment of primary microglia with purified SEB augmented TNF-α production in response to the TLR2 ligand Pam3Cys, which may serve to amplify proinflammatory cascades during CNS S. aureus infection. Collectively, these studies demonstrate that TLR2 impacts adaptive immunity to S. aureus infection and modulates SAG responses.

Factors influencing the clinical outcome of methicillin-resistant Staphylococcus aureus bacteraemia

Methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia is associated with high mortality due to a combination of host, pathogen and therapy related factors. This was a retrospective exploratory study to evaluate host, pathogen and therapy related factors influencing the clinical outcome of MRSA bacteraemia in a UK teaching hospital setting. Of the 38 consecutive episodes of MRSA bacteraemia over a 1-year period, 16 of 38 (40%) patients died at 1 month and 21/38 (55%) died at 6 months. Univariate analysis revealed age (p < 0.006), mean serum vancomycin level (p < 0.035), agr group I (p < 0.036) and set4-var2_11 gene (p < 0.036) at 1 month; and age (p < 0.004) and set4-var2_11 gene (p < 0.002) at 6 months as significant factors. However, there was no association between first trough vancomycin concentration and outcome at 1 month. Multivariate survival analysis from time of admission showed, for each one year increase in age, a patient is 1.121 (95% CI 1.006-1.250, p < 0.007) times more likely to die at any particular point in time, and patients with a mean serum vancomycin level of <10 mg/L, the odds ratio of adverse outcome is 16.129 (95% CI 2.398-111.111) compared to patients with a mean serum level >10 mg/L. A variety of host, pathogen, and therapy related factors influence the clinical outcome of MRSA bacteraemia.

Genotypic characterization by polymerase chain reaction of Staphylococcus aureus isolates associated with bovine mastitis

Staphylococcus aureus is recognized worldwide as a pathogen causing many serious diseases in humans and animals, and is the most common aetiological agent of clinical and subclinical bovine mastitis. The importance of evaluating the combination of S. aureus virulence factors has been emphasized both in human and veterinary medicine, and knowledge about the genetic variability within different S. aureus populations would help in the design of efficient treatments. The aim of the present study was to determine the genetic profiles of S. aureus strains isolated from milk of cows suffering from clinical and subclinical mastitis in Belgium. The presence of about forty virulence-associated genes was investigated by specific polymerase chain reaction (PCR) amplification. A high number of genotypic subtypes were observed, demonstrating further the large variation in the presence of virulence genes in S. aureus isolates and the considerable diversity of strains populations that are able to cause mastitis in cows. In accordance with other studies, we showed that some genes are associated with mastitis-causing S. aureus isolates, whereas others are absent or rarely present. We also further highlighted the presence of conserved gene combinations, namely the enterotoxigenic egc-cluster and the bovine pathogenicity island SaPIbov. Importantly, the presence of isolates carrying genes coding for toxins involved in important human infections makes the milk of cows with mastitis a potential reservoir for these toxins, and therefore a potential danger in human health, which strengthens the importance to consider raw milk consumption and its processing very carefully.

Gram-positive bacterial superantigen outside-in signaling causes toxic shock syndrome

Staphylococcus aureus and Streptococcus pyogenes (group A streptococci) are Gram-positive pathogens capable of producing a variety of bacterial exotoxins known as superantigens. Superantigens interact with antigen-presenting cells (APCs) and T cells to induce T cell proliferation and massive cytokine production, which leads to fever, rash, capillary leak and subsequent hypotension, the major symptoms of toxic shock syndrome. Both S. aureus and group A streptococci colonize mucosal surfaces, including the anterior nares and vagina for S. aureus, and the oropharynx and less commonly the vagina for group A streptococci. However, due to their abilities to secrete a variety of virulence factors, the organisms can also cause illnesses from the mucosa. This review provides an updated discussion of the biochemical and structural features of one group of secreted virulence factors, the staphylococcal and group A streptococcal superantigens, and their abilities to cause toxic shock syndrome from a mucosal surface. The main focus of this review, however, is the abilities of superantigens to induce cytokines and chemokines from epithelial cells, which has been linked to a dodecapeptide region that is relatively conserved among all superantigens and is distinct from the binding sites required for interactions with APCs and T cells. This phenomenon, termed outside-in signaling, acts to recruit adaptive immune cells to the submucosa, where the superantigens can then interact with those cells to initiate the final cytokine cascades that lead to toxic shock syndrome.

Genomic and proteomic characterization of Staphylococcus aureus mastitis isolates of bovine origin

Staphylococcus aureus colonizes and infects humans as well as animals. In the present study, 17 S. aureus strains isolated from cows suffering from mastitis were characterized. The well-established multilocus sequence typing (MLST) technique and a diagnostic microarray covering 185 S. aureus virulence and resistance genes were used for genetic and epidemiological analyses. Virulence gene expression studies were performed by analyzing the extracellular protein pattern of each isolate on 2-D gels. By this way, a pronounced heterogeneity of the extracellular proteome between the bovine isolates has been observed which was attributed to genome plasticity and variation of gene expression. Merely 12 proteins were expressed in at least 80% of the isolates, i.e. Atl, Aur, GlpQ, Hla, LtaS, Nuc, PdhB, SAB0846, SAB2176, SAB0566, SspA, and SspB forming the core exoproteome. Fifteen extracellular proteins were highly variably expressed and only present in less than 20% of the isolates. This includes the serine proteases SplB, C, and F, and the superantigens SEC-bov, SEL and TSST-1. Compared to human isolates we identified at least six proteins with significantly different expression frequencies. While SAB0846 was expressed more frequently in bovine isolates, LytM, EbpS, Spa, Geh, and LukL1 were seen less frequently in these isolates.

The SaeR/S gene regulatory system induces a pro-inflammatory cytokine response during Staphylococcus aureus infection

Community-associated methicillin-resistant Staphylococcus aureus accounts for a large portion of the increased staphylococcal disease incidence and can cause illness ranging from mild skin infections to rapidly fatal sepsis syndromes. Currently, we have limited understanding of S. aureus-derived mechanisms contributing to bacterial pathogenesis and host inflammation during staphylococcal disease. Herein, we characterize an influential role for the saeR/S two-component gene regulatory system in mediating cytokine induction using mouse models of S. aureus pathogenesis. Invasive S. aureus infection induced the production of localized and systemic pro-inflammatory cytokines, including tumor necrosis factor alpha (TNF-α), interferon gamma (IFN-γ), interleukin (IL)-6 and IL-2. In contrast, mice infected with an isogenic saeR/S deletion mutant demonstrated significantly reduced pro-inflammatory cytokine levels. Additionally, secreted factors influenced by saeR/S elicited pro-inflammatory cytokines in human blood ex vivo. Our study further demonstrated robust saeR/S-mediated IFN-γ production during both invasive and subcutaneous skin infections. Results also indicated a critical role for saeR/S in promoting bacterial survival and enhancing host mortality during S. aureus peritonitis. Taken together, this study provides insight into specific mechanisms used by S. aureus during staphylococcal disease and characterizes a relationship between a bacterial global regulator of virulence and the production of pro-inflammatory mediators.

An engineered superantigen SEC2 exhibits promising antitumor activity and low toxicity

Recent studies suggested that the histidine residues at 118 and 122 play an important role for the toxicity of staphylococcal enterotoxin C subtype 2 (SEC2), and the substitutions of both histidines with alanine can severely impair the fever activity of SEC2. We hypothesized that promising SEC2 antitumor agent with low toxicity and enhanced superantigen activity can be constructed by introducing related mutations at protein functional sites of SEC2. We showed that the SEC2 mutants H122A and H118A/H122A exhibited improved superantigen activity after introducing the point mutations at Thr20 and Gly22. A resultant mutant, named as SAM-3, has considerable abilities to inhibit the growth of H22 and Hepa1-6 tumor cells in vitro and colon 26 solid tumor in vivo. Furthermore, SAM-3 also exhibits significantly reduced toxicity compared with native SEC2. The study provides a novel strategy for designing promising superantigen immunotherapeutic agent. The constructed SEC2 mutant SAM-3 can be used as a powerful candidate for cancer immunotherapy and could compensate the deficiency caused by toxicity of native SEC2 in clinic.

Anti-Legionella activity of staphylococcal hemolytic peptides

A collection of various Staphylococci was screened for their anti-Legionella activity. Nine of the tested strains were found to secrete anti-Legionella compounds. The culture supernatants of the strains, described in the literature to produce hemolytic peptides, were successfully submitted to a two step purification process. All the purified compounds, except one, corresponded to previously described hemolytic peptides and were not known for their anti-Legionella activity. By comparison of the minimal inhibitory concentrations, minimal permeabilization concentrations, decrease in the number of cultivable bacteria, hemolytic activity and selectivity, the purified peptides could be separated in two groups. First group, with warnericin RK as a leader, corresponds to the more hemolytic and bactericidal peptides. The peptides of the second group, represented by the PSMα from Staphylococcus epidermidis, appeared bacteriostatic and poorly hemolytic.

Structure of a mutant β toxin from Staphylococcus aureus reveals domain swapping and conformational flexibility

The 3.35 Å resolution crystal structure of a mutant form of the staphylococcal sphingomyelinase β toxin in which a conserved hydrophobic β-hairpin has been deleted is reported. It is shown that this mutation induces domain swapping of a C-terminal β-strand, leading to the formation of dimers linked by a conformationally flexible hinge region. Eight dimers are seen in the asymmetric unit, exhibiting a broad spectrum of conformations trapped in place by intermolecular contacts within the crystal lattice. Furthermore, the 16 monomers within each asymmetric unit exhibit a remarkable heterogeneity in thermal factors, which can be accounted for by the varying degrees to which each monomer interacts with other molecules in the crystal. This structure provides a unique example of the challenges associated with crystallographic study of flexible proteins.

High prevalence of edin-C encoding RhoA-targeting toxin in clinical isolates of Staphylococcus aureus

Staphylococcus aureus, a major causative agent of human infection, produces a large array of virulence factors, including various toxins. Among them, the host RhoA GTPase ADP-ribosylating EDIN toxins are considered as potential virulence factors. Using the polymerase chain reaction (PCR) assay, we analyzed the virulence profile of 256 S. aureus isolates from various clinical sites of infections. We developed specific primers to detect the three isoforms of edin-encoding genes. We found a prevalence of 14% (36 bacteria) of edin-encoding genes among these clinical isolates. Strikingly, we found that 90% of all edin-bearing S. aureus isolates carried the type-C allele. Both the spa types and the profile of virulence factors of these edin-positive isolates are highly variable. Notably, we show for the first time that edin-C-positive isolates were more frequently recovered from deep-seated infections than other types of infections. Our present work, thus, strongly suggests that the presence of edin-C is a risk factor of S. aureus dissemination in tissues and, thus, represents a predictive marker for a pejorative evolution of staphylococcal infections.

Molecular screening of staphylococcal enterotoxin B gene in clinical isolates

OBJECTIVE

The role of staphylococcal enterotoxin B (SEB) in food poisoning is well known, however its role in other diseases remains to be explored. The aim of this study is the molecular screening and characterization of the SEB gene in clinically isolated strains.

MATERIALS AND METHODS

In this experimentally study, 300 Staphylococcus aureus (S. aureus) strains isolated from clinical samples were assayed. The isolated strains were confirmed by conventional bacteriological methods. Polymerase chain reaction (PCR) was used to determine the enterotoxin B (ent B) gene. Assessment of toxin production in all strains that contained the ent B gene was then performed. Finally, using specific antibody against SEB, a Western-blot was applied to confirm detection of enterotoxin B production.

RESULTS

RESULTS indicated that only 5% of the 300 clinically isolated S. aureus contained the ent B gene. All strains which contained the ent B gene produced a proteinous enterotoxin B. The results of sequence determination of the PCR product were compared with the gene bank database and 98% similarity was achieved. The results of the Western-blot confirmed that enterotoxin B was produced in strains that contained the ent B gene.

CONCLUSION

The results of this study indicate that 5% of clinically isolated S. aureus strains produce enterotoxin B. Considering that the enterotoxin B is an important superantigen, it is possible that a delay in diagnosis and lack of early proper treatment can cause an incidence of late complications, particularly in staphylococcal chronic infections. For this reason, it is suggested that in addition to detecting bacteria, an enterotoxin B detection test should be performed to control its toxigenicity.

Generation and characterization of high affinity human monoclonal antibodies that neutralize staphylococcal enterotoxin B

Background

Staphylococcal enterotoxins are considered potential biowarfare agents that can be spread through ingestion or inhalation. Staphylococcal enterotoxin B (SEB) is a widely studied superantigen that can directly stimulate T-cells to release a massive amount of proinflammatory cytokines by bridging the MHC II molecules on an antigen presenting cell (APC) and the Vβ chains of the T-cell receptor (TCR). This potentially can lead to toxic, debilitating and lethal effects. Currently, there are no preventative measures for SEB exposure, only supportive therapies.

Methods

To develop a potential therapeutic candidate to combat SEB exposure, we have generated three human B-cell hybridomas that produce human monoclonal antibodies (HuMAbs) to SEB. These HuMAbs were screened for specificity, affinity and the ability to block SEB activity in vitro as well as its lethal effect in vivo.

Results

The high-affinity HuMAbs, as determined by BiaCore analysis, were specific to SEB with minimal crossreactivity to related toxins by ELISA. In an immunoblotting experiment, our HuMAbs bound SEB mixed in a cell lysate and did not bind any of the lysate proteins. In an in vitro cell-based assay, these HuMAbs could inhibit SEB-induced secretion of the proinflammatory cytokines (INF-γ and TNF-α) by primary human lymphocytes with high potency. In an in vivo LPS-potentiated mouse model, our lead antibody, HuMAb-154, was capable of neutralizing up to 100 μg of SEB challenge equivalent to 500 times over the reported LD₅₀ (0.2 μg) , protecting mice from death. Extended survival was also observed when HuMAb-154 was administered after SEB challenge.

Conclusion

We have generated high-affinity SEB-specific antibodies capable of neutralizing SEB in vitro as well as in vivo in a mouse model. Taken together, these results suggest that our antibodies hold the potential as passive immunotherapies for both prophylactic and therapeutic countermeasures of SEB exposure.

Screening of molecular virulence markers in Staphylococcus aureus and Pseudomonas aeruginosa strains isolated from clinical infections

Staphylococcus (S.) aureus and Pseudomonas (Ps.) aeruginosa are two of the most frequently opportunistic pathogens isolated in nosocomial infections, responsible for severe infections in immunocompromised hosts. The frequent emergence of antibiotic-resistant S. aureus and Ps. aeruginosa strains has determined the development of new strategies in order to elucidate the different mechanisms used by these bacteria at different stages of the infectious process, providing the scientists with new procedures for preventing, or at least improving, the control of S. aureus and Ps. aeruginosa infections. The purpose of this study was to characterize the molecular markers of virulence in S. aureus and Ps. aeruginosa strains isolated from different clinical specimens. We used multiplex and uniplex PCR assays to detect the genes encoding different cell-wall associated and extracellular virulence factors, in order to evaluate potential associations between the presence of putative virulence genes and the outcome of infections caused by these bacteria. Our results demonstrate that all the studied S. aureus and Ps. aeruginosa strains synthesize the majority of the investigated virulence determinants, probably responsible for different types of infections.

Polymerase chain reaction detection of enterotoxins genes in coagulase-negative staphylococci isolated from Brazilian Minas cheese

For a long time, Staphylococcus aureus has been always thought to be the only pathogenic species among Staphylococcus, while coagulase-negative staphylococci (CNS) were classified as contaminant agents. However, molecular techniques have shown that these microorganisms also possess enterotoxin-encoding genes. The aim of this study was to analyze the frequency of genes for staphylococcal enterotoxins SEA, SEB, SEC, and SED in CNS strains isolated from Minas soft cheese and to assess the in vitro production of toxins. CNS were found in 65 (72.2%) samples of cheese: 23 were Staphylococcus saprophyticus, 16 Staphylococcus warneri, 10 Staphylococcus epidermidis, 9 Staphylococcus xylosus, 3 Staphylococcus haemolyticus, 2 Staphylococcus schleiferi subsp. schleiferi, and 1 each Staphylococcus capitis subsp. urealyticus and Staphylococcus caprae. Seventeen (26.2%) CNS strains had genes for enterotoxins, and sea was more frequently found (18.5%), followed by sec in three and seb in two strains, whereas the sed gene was not found. S. saprophyticus showed enterotoxin genes in 6 of 23 isolates, but only sea was observed. On the other hand, five strains of S. warneri showed the sea, seb, or sec gene. In spite of the presence of these enterotoxin genes, these strains did not produce enterotoxins in vitro. It is essential to understand the real role of CNS in food, and based on the presence of enterotoxin genes, CNS should not be ignored in epidemiological investigations of foodborne outbreaks.

Staphylococcal exotoxins are strong inducers of IL-22: A potential role in atopic dermatitis

BACKGROUND

Patients with atopic dermatitis (AD) and psoriasis are frequently colonized with Staphylococcus aureus that produces staphylococcal enterotoxin B (SEB) and α-toxin. In patients with AD, S aureus colonization is positively correlated with the severity of their eczema. Moreover, IL-22-producing cells have been shown to accumulate in AD skin and to correlate with disease severity.

OBJECTIVE

To assess IL-22 production in response to SEB and sublytic α-toxin stimulation in patients with AD and psoriasis compared with healthy controls.

METHODS

IL-22 induction was investigated in PBMCs, T cells, and autologous cocultures of keratinocytes and T cells on SEB and α-toxin stimulation in a time-dependent and dose-dependent manner at the mRNA and protein (ELISA and flow cytometry) level. Anti-IL-1 receptor or anti-IL-6 antibodies were used in blocking experiments.

RESULTS

Staphylococcal enterotoxin B and sublytic α-toxin concentrations induced IL-22 production in PBMCs and isolated CD4(+) T cells. IL-22 secretion was enhanced by α-toxin stimulation in autologous cocultures of keratinocytes and T cells. In T cells and PBMCs from patients with AD, IL-22 secretion was significantly enhanced on α-toxin stimulation compared with patients with psoriasis and healthy controls.

CONCLUSION

Increased IL-22 secretion induced by staphylococcal exotoxins in the skin partially explains how skin colonization and infection with S aureus can contribute to chronic skin inflammation in AD.

Toxigenic status of Staphylococcus aureus isolated from bovine raw milk and Minas frescal cheese in Brazil

A group of 291 Staphylococcus aureus isolates from mastitic cow's milk (n = 125), bulk tank milk (n = 96), and Minas frescal cheese (n = 70) were screened for staphylococcal enterotoxin (SE) genes (sea, seb, sec, sed, see, seg, seh, sei, selj, and sell) and for the tst-1 gene encoding staphylococcal toxic shock syndrome toxin 1 by PCR assay. A total of 109 (37.5%) of the isolates were positive for at least one of these 11 genes, and 23 distinct genotypes of toxin genes were observed. Of the S. aureus isolates bearing SE genes, 17 (13.6%) were from mastitic cow's milk, 41 (41.7%) were from bulk tank milk, and 51 (72.9%) were from Minas frescal cheese. The occurrence of exclusively more recently described SE genes (seg through sell) was considerably higher (87 of 109 PCR-positive strains) than that of classical SE genes (sea through see, 15 strains). The SE genes most commonly detected were seg and sei; they were found alone or in different combinations with other toxin genes, but in 60.8% of the cases they were codetected. No strain possessed see. The tst-1 gene was found in eight isolates but none from mastitic cow's milk. Macrorestriction analysis of chromosomal DNA from 89 S. aureus isolates positive for SE gene(s) was conducted with the enzyme SmaI. Fifty-five distinct pulsed-field gel electrophoresis patterns were found, demonstrating a lack of predominance of any specific clone. A second enzyme, Apa I, used for some isolates was less discriminating than Sma I. The high genotype diversity of potential toxigenic S. aureus strains found in this study, especially from Minas frescal cheese, suggests various sources of contamination. Efforts from the entire production chain are required to improve consumer safety.

Staphylococcus aureus hijacks a skin commensal to intensify its virulence: immunization targeting β-hemolysin and CAMP factor

The need for a new anti-Staphylococcus aureus therapy that can effectively cripple bacterial infection, neutralize secretory virulence factors, and lower the risk of creating bacterial resistance is undisputed. Here, we propose what is, to our knowledge, a previously unreported infectious mechanism by which S. aureus may commandeer Propionibacterium acnes, a key member of the human skin microbiome, to spread its invasion and highlight two secretory virulence factors (S. aureus β-hemolysin and P. acnes CAMP (Christie, Atkins, Munch-Peterson) factor) as potential molecular targets for immunotherapy against S. aureus infection. Our data demonstrate that the hemolysis and cytolysis by S. aureus were noticeably augmented when S. aureus was grown with P. acnes. The augmentation was significantly abrogated when the P. acnes CAMP factor was neutralized or β-hemolysin of S. aureus was mutated. In addition, the hemolysis and cytolysis of recombinant β-hemolysin were markedly enhanced by recombinant CAMP factor. Furthermore, P. acnes exacerbated S. aureus-induced skin lesions in vivo. The combination of CAMP factor neutralization and β-hemolysin immunization cooperatively suppressed the skin lesions caused by coinfection of P. acnes and S. aureus. These observations suggest a previously unreported immunotherapy targeting the interaction of S. aureus with a skin commensal.

Nasal carriage of Staphylococcus aureus in healthy humans with different levels of contact with animals in Tunisia: genetic lineages, methicillin resistance, and virulence factors

Nasal swabs of 423 healthy humans who showed different levels of contact with animals (frequent, 168; sporadic, 94; no contact, 161) were obtained in Tunisia (2008-2009), and 99 of them presented other associated risk factors. Methicillin-resistant Staphylococcus aureus (MRSA) was detected in one of these 423 samples (0.24%), retrieved from a veterinarian. The MRSA isolate was mecA-positive, typed as ST80-t203-SCCmecIVc-agrIII, and contained tet(K), ant(6)-Ia, and aph(3')-IIIa genes encoding tetracycline, streptomycin, and kanamycin resistance, respectively. This MRSA isolate also contained the lukF/lukS virulence gene encoding Panton-Valentine leukocidin. Fifty-four (12.8%) additional nasal samples contained methicillin-susceptible S. aureus (MSSA) and one isolate/sample was characterized. A high diversity of spa types (n = 43; 4 new) and pulsed-field gel electrophoresis (PFGE) types (n = 37) was detected among the 55 recovered S. aureus strains. The percentages of antimicrobial resistance/detected resistance genes were as follows: tetracycline [22%/tet(K)-tet(L)-tet(M)], erythromycin [5%/msrA], ciprofloxacin [14.5%], trimethoprim-sulfamethoxazole [2%/dfrA], streptomycin [11%/ant(6)-Ia], kanamycin [7%/aph(3')-IIIa], amikacin [5%], and chloramphenicol [2%]. Four and two isolates carried the lukF/lukS and eta and/or etb genes, respectively, and always in individuals with contact with animals. Eleven isolates carried the tst gene and were recovered from individuals with different levels of contact with animals.

Characterization of toxin genes and antimicrobial susceptibility of Staphylococcus aureus isolates from Louisiana retail meats

Staphylococcus aureus is a leading cause of food poisoning worldwide due to the production of heat-stable enterotoxins. Recently, the isolation of methicillin-resistant S. aureus (MRSA) from food animals and retail meats raised additional food safety concerns. In this study, we characterized 152 S. aureus isolates, including 22 MRSA recovered from Louisiana retail pork and beef meats, for the prevalence of nine enterotoxin and four other exotoxin genes by polymerase chain reaction and antimicrobial susceptibility testing by broth microdilution. Overall, 85% of S. aureus isolates were positive for at least one of six enterotoxin genes identified and 66% harbored two to four enterotoxin genes. The two most predominant ones were seg and sei (66% each), followed by seh (20%), sed (15%), sej (13%), and sea (1%). No isolates harbored enterotoxin genes seb, sec, or see, the toxic shock syndrome toxin 1 gene tst, or the exfoliative toxin genes eta or etb. Three MRSA isolates were the only ones harboring Panton-Valentine leucocidin. Resistances were common to penicillin (71%), ampicillin (68%), and tetracycline (67%), followed by erythromycin (30%), clindamycin (18%), oxacillin with 2% NaCl (14%), ciprofloxacin (13%), levofloxacin (13%), gentamicin (3%), quinupristin/dapfopristin (3%), chloramphenicol (2%), and moxifloxacin (1%). Multidrug resistance was commonly observed among MRSA isolates and S. aureus isolates from pork. This study demonstrated that S. aureus isolates found in Louisiana retail pork and beef meats possessed various enterotoxin genes and antimicrobial resistance profiles. Therefore, vigilant food safety practice needs to be implemented for people who handle raw meat products to prevent foodborne infections and intoxications due to S. aureus contamination.

Staphylococcal superantigen (TSST-1) mutant analysis reveals that t cell activation is required for biological effects in the rabbit including the cytokine storm

Staphylococcal superantigens (sAgs), such as toxic shock syndrome toxin 1 (TSST-1), induce massive cytokine production, which may result in toxic shock syndrome (TSS) and sepsis. Recently, we reported that in vitro studies in human peripheral blood mononuclear cells (PBMC) do not reflect the immunological situation of the host, because after exposure to superantigens (sAgs) in vivo, mononuclear cells (MNC) leave the circulation and migrate to organs, e.g., the spleen, liver and lung. Our experimental model of choice is the rabbit because it is comparable to humans in its sensitivity to sAg. T cell activation has been assessed by lymphocyte proliferation and IL-2 gene expression after in vivo challenge with TSST-1 and the mutant antigens; expression of the genes of proinflammatory cytokines were taken as indicators for the inflammatory reaction after the combined treatment with TSST-1 and LPS. The question as to whether the biological activities of TSST-1, e.g., lymphocyte extravasation, toxicity and increased sensitivity to LPS, are mediated by T cell activation or activation by MHC II-only, are unresolved and results are contradictory. We have addressed this question by studying these reactions in vivo, with two TSST-1 mutants: one mutated at the MHC binding site (G31R) with reduced MHC binding with residual activity still present, and the other at the T cell binding site (H135A) with no residual function detectable. Here, we report that the mutant G31R induced all the biological effects of the wild type sAg, while the mutant with non-functional TCR binding did not retain any of the toxic effects, proving the pivotal role of T cells in this system.

Different types of cell death induced by enterotoxins

The infection of bacterial organisms generally causes cell death to facilitate microbial invasion and immune escape, both of which are involved in the pathogenesis of infectious diseases. In addition to the intercellular infectious processes, pathogen-produced/secreted enterotoxins (mostly exotoxins) are the major weapons that kill host cells and cause diseases by inducing different types of cell death, particularly apoptosis and necrosis. Blocking these enterotoxins with synthetic drugs and vaccines is important for treating patients with infectious diseases. Studies of enterotoxin-induced apoptotic and necrotic mechanisms have helped us to create efficient strategies to use against these well-characterized cytopathic toxins. In this article, we review the induction of the different types of cell death from various bacterial enterotoxins, such as staphylococcal enterotoxin B, staphylococcal alpha-toxin, Panton-Valentine leukocidin, alpha-hemolysin of Escherichia coli, Shiga toxins, cytotoxic necrotizing factor 1, heat-labile enterotoxins, and the cholera toxin, Vibrio cholerae. In addition, necrosis caused by pore-forming toxins, apoptotic signaling through cross-talk pathways involving mitochondrial damage, endoplasmic reticulum stress, and lysosomal injury is discussed.

How should staphylococcal food poisoning outbreaks be characterized?

Staphylococcal food poisoning is one of the most common food-borne diseases and results from the ingestion of staphylococcal enterotoxins (SEs) preformed in food by enterotoxigenic strains of Staphylococcus aureus. To date, more than 20 SEs have been described: SEA to SElV. All SEs have superantigenic activity whereas only a few have been proved to be emetic, representing a potential hazard for consumers. Characterization of staphylococcal food poisoning outbreaks (SFPOs) has considerably progressed compared to 80 years ago, when staphylococci were simply enumerated and only five enterotoxins were known for qualitative detection. Today, SFPOs can be characterized by a number of approaches, such as the identification of S. aureus biovars, PCR and RT-PCR methods to identify the se genes involved, immunodetection of specific SEs, and absolute quantification by mass spectrometry. An integrated gene-to-protein approach for characterizing staphylococcal food poisoning is advocated.

Community-acquired methicillin-resistant Staphylococcus aureus: community transmission, pathogenesis, and drug resistance

Methicillin-resistant Staphylococcus aureus (MRSA) is able to persist not only in hospitals (with a high level of antimicrobial agent use) but also in the community (with a low level of antimicrobial agent use). The former is called hospital-acquired MRSA (HA-MRSA) and the latter community-acquired MRSA (CA-MRSA). It is believed MRSA clones are generated from S. aureus through insertion of the staphylococcal cassette chromosome mec (SCCmec), and outbreaks occur as they spread. Several worldwide and regional clones have been identified, and their epidemiological, clinical, and genetic characteristics have been described. CA-MRSA is likely able to survive in the community because of suitable SCCmec types (type IV or V), a clone-specific colonization/infection nature, toxin profiles (including Pantone-Valentine leucocidin, PVL), and narrow drug resistance patterns. CA-MRSA infections are generally seen in healthy children or young athletes, with unexpected cases of diseases, and also in elderly inpatients, occasionally surprising clinicians used to HA-MRSA infections. CA-MRSA spreads within families and close-contact groups or even through public transport, demonstrating transmission cores. Re-infection (including multifocal infection) frequently occurs, if the cores are not sought out and properly eradicated. Recently, attention has been given to CA-MRSA (USA300), which originated in the US, and is growing as HA-MRSA and also as a worldwide clone. CA-MRSA infection in influenza season has increasingly been noted as well. MRSA is also found in farm and companion animals, and has occasionally transferred to humans. As such, the epidemiological, clinical, and genetic behavior of CA-MRSA, a growing threat, is focused on in this study.

Interkingdom signaling between pathogenic bacteria and Caenorhabditis elegans

Investigators have recently turned to the soil nematode Caenorhabditis elegans as a small animal infection model to study infectious disease. To extrapolate findings concerning bacterial pathogenesis from non-mammals to mammals, virulence factors should be conserved in function, independent of the infection model. Emerging from these studies is the observation that bacterial virulence regulatory networks function in a conserved manner across multiple hosts, including nematodes, mice and plants. Several regulatory networks have been implicated in nematode innate immune function and are being exploited in the C. elegans infection model to develop novel chemical therapies against bacterial pathogens.

Structure of a virulence regulatory factor CvfB reveals a novel winged helix RNA binding module

CvfB is a conserved regulatory protein important for the virulence of Staphylococcus aureus. We show here that CvfB binds RNA. The crystal structure of the CvfB ortholog from Streptococcus pneumoniae at 1.4 A resolution reveals a unique RNA binding protein that is formed from a concatenation of well-known structural modules that bind nucleic acids: three consecutive S1 RNA binding domains and a winged helix (WH) domain. The third S1 and the WH domains are required for cooperative RNA binding and form a continuous surface that likely contributes to the RNA interaction. The WH domain is critical to CvfB function and contains a unique sequence motif. Thus CvfB represents a novel assembly of modules for binding RNA.

The systemic and pulmonary immune response to staphylococcal enterotoxins

In response to environmental cues the human pathogen Staphylococcus aureus synthesizes and releases proteinaceous enterotoxins. These enterotoxins are natural etiologic entities of severe food poisoning, toxic shock syndrome, and acute diseases. Staphylococcal enterotoxins are currently listed as Category B Bioterrorism Agents by the Center for Disease Control and Prevention. They are associated with respiratory illnesses, and may contribute to exacerbation of pulmonary disease. This likely stems from the ability of Staphylococcal enterotoxins to elicit powerful episodes of T cell stimulation resulting in release of pro-inflammatory cytokines. Here, we discuss the role of the immune system and potential mechanisms of disease initiation and progression.

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.

Regulation of hemolysin expression and virulence of Staphylococcus aureus by a serine/threonine kinase and phosphatase

Exotoxins, including the hemolysins known as the alpha (alpha) and beta (beta) toxins, play an important role in the pathogenesis of Staphylococcus aureus infections. A random transposon library was screened for S. aureus mutants exhibiting altered hemolysin expression compared to wild type. Transposon insertions in 72 genes resulting in increased or decreased hemolysin expression were identified. Mutations inactivating a putative cyclic di-GMP synthetase and a serine/threonine phosphatase (Stp1) were found to reduce hemolysin expression, and mutations in genes encoding a two component regulator PhoR, LysR family transcriptional regulator, purine biosynthetic enzymes and a serine/threonine kinase (Stk1) increased expression. Transcription of the hla gene encoding alpha toxin was decreased in a Deltastp1 mutant strain and increased in a Deltastk1 strain. Microarray analysis of a Deltastk1 mutant revealed increased transcription of additional exotoxins. A Deltastp1 strain is severely attenuated for virulence in mice and elicits less inflammation and IL-6 production than the Deltastk1 strain. In vivo phosphopeptide enrichment and mass spectrometric analysis revealed that threonine phosphorylated peptides corresponding to Stk1, DNA binding histone like protein (HU), serine-aspartate rich fibrinogen/bone sialoprotein binding protein (SdrE) and a hypothetical protein (NWMN_1123) were present in the wild type and not in the Deltastk1 mutant. Collectively, these studies suggest that Stk1 mediated phosphorylation of HU, SrdE and NWMN_1123 affects S. aureus gene expression and virulence.

Exfoliative toxins of Staphylococcus aureus

Staphylococcus aureus is an important pathogen of humans and livestock. It causes a diverse array of diseases, ranging from relatively harmless localized skin infections to life-threatening systemic conditions. Among multiple virulence factors, staphylococci secrete several exotoxins directly associated with particular disease symptoms. These include toxic shock syndrome toxin 1 (TSST-1), enterotoxins, and exfoliative toxins (ETs). The latter are particularly interesting as the sole agents responsible for staphylococcal scalded skin syndrome (SSSS), a disease predominantly affecting infants and characterized by the loss of superficial skin layers, dehydration, and secondary infections. The molecular basis of the clinical symptoms of SSSS is well understood. ETs are serine proteases with high substrate specificity, which selectively recognize and hydrolyze desmosomal proteins in the skin. The fascinating road leading to the discovery of ETs as the agents responsible for SSSS and the characterization of the molecular mechanism of their action, including recent advances in the field, are reviewed in this article.

Staphylococcal enterocolitis: forgotten but not gone?

PURPOSE

Staphylococcus aureus may cause antibiotic-associated diarrhea and enterocolitis, with or without preceding antibiotic use, in immunocompromised adults or infants, or individuals with predisposing conditions, but there is little appreciation of this condition clinically. CLINICAL DISEASE: The main clinical feature that helps to differentiate staphylococcal enterocolitis (SEC) from Clostridium difficile-associated diarrhea is large-volume, cholera-like diarrhea in the former case. A predominance of gram-positive cocci in clusters on gram stain of stool or biopsy specimens and the isolation of S. aureus as the dominant or sole flora support the diagnosis.

PATHOGENESIS

The pathogenesis of SEC requires the interaction of staphylococcal enterotoxins, which function as superantigens, with interstitial epithelial lymphocytes and intestinal epithelial cells (IECs).

MANAGEMENT

Most SEC represents recent S. aureus acquisition, so that improved infection prevention practices can reduce disease recurrence. Management should include aggressive fluid management and repletion and oral vancomycin.

Secreted virulence factor comparison between methicillin-resistant and methicillin-sensitive Staphylococcus aureus, and its relevance to atopic dermatitis

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strains have emerged as serious health threats in the last 15 years. They are associated with large numbers of atopic dermatitis skin and soft tissue infections, but when they originate from skin and mucous membranes, have the capacity to produce sepsis and highly fatal pulmonary infections characterized as necrotizing pneumonia, purpura fulminans, and postviral toxic shock syndrome. This review is a discussion of the emergence of 3 major CA-MRSA organisms, designated CA-MRSA USA400, followed by USA300, and most recently USA200. CA-MRSA USA300 and USA400 isolates and their methicillin-sensitive counterparts (community-associated methicillin-sensitive S aureus) typically produce highly inflammatory cytolysins alpha-toxin, gamma-toxin, delta-toxin (as representative of the phenol soluble modulin family of cytolysins), and Panton Valentine leukocidin. USA300 isolates produce the superantigens enterotoxin-like Q and a highly pyrogenic deletion variant of toxic shock syndrome toxin 1 (TSST-1), whereas USA400 isolates produce the superantigens staphylococcal enterotoxin B or staphylococcal enterotoxin C. USA200 CA-MRSA isolates produce small amounts of cytolysins but produce high levels of TSST-1. In contrast, their methicillin-sensitive S aureus counterparts produce various cytolysins, apparently in part dependent on the niche occupied in the host and levels of TSST-1 expressed. Significant differences seen in production of secreted virulence factors by CA-MRSA versus hospital-associated methicillin-resistant S aureus and community-associated methicillin-sensitive S aureus strains appear to be a result of the need to specialize as the result of energy drains from both virulence factor production and methicillin resistance.

Contribution of the flexible loop region to the function of staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB), a toxin produced by Staphylococcus aureus, causes food poisoning and other fatal diseases by inducing high levels of pro-inflammatory cytokines. These cytokines are released from CD4+ T cells and major histocompatibility complex (MHC) class II antigen-presenting cells, which are activated through binding of wild-type (WT) SEB to both the MHC class II molecule and specific T-cell receptor Vbeta chains. Here, we focused on a trypsin/cathepsin cleavage site of WT SEB, which is known to be cleaved in vivo between Lys97 and Lys98, located within the loop region. To know the function of the cleavage, an SEB mutant, in which both of these Lys residues have been changed to Ser, was examined. This mutant showed prolonged tolerance to protease cleavage at a different site between Thr107 and Asp108, and structural analyses revealed no major conformational differences between WT SEB and the mutant protein. However, differential scanning calorimetric analysis showed an increase in enthalpy upon thermal denaturation of the mutant protein, which correlated with the speed of cleavage between Thr107 and Asp108. The mutant protein also had slightly increased affinity for MHC. In the in vivo experiment, the SEB mutant showed lower proliferative response in peripheral blood mononuclear cells and had lower cytokine-induction activity, compared with WT SEB. These results highlight the importance of the flexible loop region for the functional, physical and chemical properties of WT SEB, thus providing insight into the nature of WT SEB that was unrevealed previously.

Methicillin-resistant Staphylococcus aureus (MRSA) in food production animals

Until recently, reports on methicillin-resistant Staphylococcus aureus (MRSA) in food production animals were mainly limited to occasional detections in dairy cattle mastitis. However, since 2005 a MRSA clone, CC398, has been reported colonizing pigs, veal calves and broiler chickens and infecting dairy cows. Many aspects of its prevalence in pigs remain unclear. In other livestock, colonizing capacity and reservoir status still require elucidation. MRSA CC398 has also been detected in meat, but, as for other MRSA, the risk this poses is somewhat unclear. Currently, the most worrying aspect of MRSA CC398 appears to be its capacity to spread to humans. This might complicate MRSA control measures in human healthcare, urging research into risk factors and transmission routes. Although infections with MRSA CC398 are much less reported than carriage, more investigation into its pathogenic potential is required. Moreover, the origin and evolution of this clone remain unknown.