Epidemiology, variable genetic organization and regulation of the EDIN-B toxin in Staphylococcus aureus from bacteraemic patients

Staphylococcus aureus-derived virulent phenol-soluble modulin α triggers alarmin release to drive IL-36-dependent corneal inflammation

Methicillin-resistant Staphylococcus aureus (MRSA) isolated from patients with keratitis produces substantial amounts of phenol-soluble modulin α (PSMα). However, the role of PSMα in S. aureus keratitis remains unclear. We observed that PSMα-producing and PSMα-deficient strains could infect the cornea in our experimental mouse keratitis model; however, only the PSMα-producing strain delayed epithelial wound healing and induced stromal inflammation. PSMα induced damage to the epithelium, the release of alarmins IL-1α and IL-36α, and the expression of inflammatory chemokines by resident corneal cells in the mouse corneal organ culture. The IL-36 (but not IL-1) receptor antagonist attenuated mouse keratitis induced by PSMα-containing bacterial culture supernatants, as well as by infection with PSMα-producing S. aureus, suggesting that the corneal inflammations were dependent on IL-36. Recombinant PSMα elicited IL-36-dependent corneal inflammation in mice. Thus, PSMα and the subsequently released IL-36 are critical factors triggering inflammation during S. aureus keratitis.

Adaptation of Staphylococcus aureus in a Medium Mimicking a Diabetic Foot Environment

Staphylococcus aureus is the most prevalent pathogen isolated from diabetic foot infections (DFIs). The purpose of this study was to evaluate its behavior in an in vitro model mimicking the conditions encountered in DFI. Four clinical S. aureus strains were cultivated for 16 weeks in a specific environment based on the wound-like medium biofilm model. The adaptation of isolates was evaluated as follows: by Caenorhabditis elegans model (to evaluate virulence); by quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) (to evaluate expression of the main virulence genes); and by Biofilm Ring test^® (to assess the biofilm formation). After 16 weeks, the four S. aureus had adapted their metabolism, with the development of small colony variants and the loss of β-hemolysin expression. The in vivo nematode model suggested a decrease of virulence, confirmed by qRT-PCRs, showing a significant decrease of expression of the main staphylococcal virulence genes tested, notably the toxin-encoding genes. An increased expression of genes involved in adhesion and biofilm was noted. Our data based on an in vitro model confirm the impact of environment on the adaptation switch of S. aureus to prolonged stress environmental conditions. These results contribute to explore and characterize the virulence of S. aureus in chronic wounds.

Molecular analysis of accessory gene regulator functionality and virulence genes in Staphylococcus aureus derived from pediatric wound infections

Staphylococcus aureus is a major human pathogen causing infections with high morbidity and mortality in both healthcare and community settings. The accessory gene regulator (Agr) is a key genetic element controlling the expression of numerous virulence factors in S. aureus. The significance of a functional Agr system in clinical S. aureus isolates derived from pediatric wound infections is still unclear. Therefore, the present study was conducted to identify virulence genes and determine Agr functionality from this cohort of patients. A total of 48 S. aureus wound isolates were collected from patients referred to Tehran Children's Medical Center Hospital from April 2017 to April 2018. In addition, in vitro antimicrobial susceptibility of the isolates was assessed using the disk diffusion and E-test methods. Conventional PCR was performed for the detection of toxins (tsst-1, hla, hlb, hld, eta, etb, etd, edin-A, edin-B, edin-C) and Agr typing (agrI, agrII, agrIII, agrIV). Agr functionality was assessed by quantitative reverse transcriptase real-time PCR (qRT-PCR). All S. aureus isolates were found to be susceptible to linezolid and vancomycin. The most frequently detected toxin gene was eta (100%), and the most prevalent Agr type was agrIII (56.3%). Importantly, qRT-PCR revealed that Agr was functional in 28 (58%) of wound isolates. Consequently, our data suggests that a functional Agr system may not be required for the development of S. aureus wound infections.

Staphylococcus aureus Toxins and Diabetic Foot Ulcers: Role in Pathogenesis and Interest in Diagnosis

Infection of foot ulcers is a common, often severe and costly complication in diabetes. Diabetic foot infections (DFI) are mainly polymicrobial, and Staphylococcus aureus is the most frequent pathogen isolated. The numerous virulence factors and toxins produced by S. aureus during an infection are well characterized. However, some particular features could be observed in DFI. The aim of this review is to describe the role of S. aureus in DFI and the implication of its toxins in the establishment of the infection. Studies on this issue have helped to distinguish two S. aureus populations in DFI: toxinogenic S. aureus strains (harboring exfoliatin-, EDIN-, PVL- or TSST-encoding genes) and non-toxinogenic strains. Toxinogenic strains are often present in infections with a more severe grade and systemic impact, whereas non-toxinogenic strains seem to remain localized in deep structures and bone involving diabetic foot osteomyelitis. Testing the virulence profile of bacteria seems to be a promising way to predict the behavior of S. aureus in the chronic wounds.

EDIN-B Promotes the Translocation of Staphylococcus aureus to the Bloodstream in the Course of Pneumonia

It is crucial to define risk factors that contribute to host invasion by Staphylococcusaureus. Here, we demonstrate that the chromosomally encoded EDIN-B isoform from S. aureus contributes to the onset of bacteremia during the course of pneumonia. Deletion of edinB in a European lineage community-acquired methicillin resistant S. aureus (CA-MRSA) strain (ST80-MRSA-IV) dramatically decreased the frequency and magnitude of bacteremia in mice suffering from pneumonia. This deletion had no effect on the bacterial burden in both blood circulation and lung tissues. Re-expression of wild-type EDIN-B, unlike the catalytically inactive mutant EDIN-R185E, restored the invasive characteristics of ST80-MRSA-IV.

Subpopulations of Staphylococcus aureus clonal complex 121 are associated with distinct clinical entities

We investigated the population structure of Staphylococcus aureus clonal complex CC121 by mutation discovery at 115 genetic housekeeping loci from each of 154 isolates, sampled on five continents between 1953 and 2009. In addition, we pyro-sequenced the genomes from ten representative isolates. The genome-wide SNPs that were ascertained revealed the evolutionary history of CC121, indicating at least six major clades (A to F) within the clonal complex and dating its most recent common ancestor to the pre-antibiotic era. The toxin gene complement of CC121 isolates was correlated with their SNP-based phylogeny. Moreover, we found a highly significant association of clinical phenotypes with phylogenetic affiliations, which is unusual for S. aureus. All isolates evidently sampled from superficial infections (including staphylococcal scalded skin syndrome, bullous impetigo, exfoliative dermatitis, conjunctivitis) clustered in clade F, which included the European epidemic fusidic-acid resistant impetigo clone (EEFIC). In comparison, isolates from deep-seated infections (abscess, furuncle, pyomyositis, necrotizing pneumonia) were disseminated in several clades, but not in clade F. Our results demonstrate that phylogenetic lineages with distinct clinical properties exist within an S. aureus clonal complex, and that SNPs serve as powerful discriminatory markers, able to identify these lineages. All CC121 genomes harboured a 41-kilobase prophage that was dissimilar to S. aureus phages sequenced previously. Community-associated MRSA and MSSA from Cambodia were extremely closely related, suggesting this MRSA arose in the region.

Distribution of edin in Staphylococcus aureus isolated from diabetic foot ulcers

Staphylococcus aureus is both a common colonizer of human skin and the most frequently isolated pathogen in diabetes foot infections (DFIs). The spread of DFI to soft tissue and bony structures is a major causal factor for lower-limb amputation. It is therefore of great importance to differentiate colonizing from infecting strains of S. aureus. Epidermal cell differentiation inhibitors known as EDIN and EDIN-like factors, a group of toxins targeting RhoA master regulator of the actin cytoskeleton, may confer virulence properties on S. aureus. In this study, for the first time, analysis of S. aureus strains, recovered in DFIs at an initial stage and during the follow-up, showed that 71.4% of edin-positive strains were associated with moderate-to-severe infections (grades 3 and 4 of the IDSA/IWGDF classification) compared with 28.6% of edin-positive strains associated with low-grade infections. Most of these strains were edin-B positive (86.7%) and belonged to CC25/28-MSSA (n = 10). One edin-B-positive ST152-MSSA strain was negative for the two highly prevalent predictive markers of infecting strains (lukDE and hlgv). Collectively, this points towards the edin-B encoding gene as a bonafide subsidiary predictive risk marker of DFI.

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.

How does Staphylococcus aureus escape the bloodstream?

Staphylococcus aureus is a major cause of bacteraemia, which frequently leads to infective endocarditis, osteomyelitis, septic arthritis and metastatic abscess formation. The development of these secondary infections is due to bacterial dissemination from the blood into surrounding tissues and is associated with significantly increased morbidity and mortality. Despite the importance of S. aureus extravasation in disease progression, there is relatively little understanding of the molecular mechanisms by which this pathogen crosses the endothelial barrier and establishes new sites of infection. Recent work has identified a number of putative routes by which S. aureus can escape the bloodstream. In this article we review these new developments and set them in the context of strategies used by other established pathogens to traverse cellular barriers.

The Staphylococcus aureus epidermal cell differentiation inhibitor toxin promotes formation of infection foci in a mouse model of bacteremia

Inactivation of the host GTPase RhoA by staphylococcal epidermal cell differentiation inhibitor (EDIN) exotoxins triggers the formation of large transcellular tunnels, named macroapertures, in endothelial cells. We used bioluminescent strains of Staphylococcus aureus to monitor the formation of infection foci during the first 24 h of hematogenous bacterial dissemination. Clinically derived EDIN-expressing S. aureus strains S25 and Xen36 produced many disseminated foci. EDIN had no detectable impact on infection foci in terms of histopathology or the intensity of emitted light. Moreover, EDIN did not modify the course of bacterial clearance from the bloodstream. In contrast, we show that EDIN expression promotes a 5-fold increase in the number of infection foci produced by Xen36. This virulence activity of EDIN requires RhoA ADP-ribosyltranferase activity. These results suggest that EDIN is a risk factor for S. aureus dissemination through the vasculature by virtue of its ability to promote the formation of infection foci in deep-seated tissues.