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

YAP promotes cell-autonomous immune responses to tackle intracellular Staphylococcus aureus in vitro

Transcriptional cofactors YAP/TAZ have recently been found to support autophagy and inflammation, which are part of cell-autonomous immunity and are critical in antibacterial defense. Here, we studied the role of YAP against Staphylococcus aureus using CRISPR/Cas9-mutated HEK293 cells and a primary cell-based organoid model. We found that S. aureus infection increases YAP transcriptional activity, which is required to reduce intracellular S. aureus replication. A 770-gene targeted transcriptomic analysis revealed that YAP upregulates genes involved in autophagy/lysosome and inflammation pathways in both infected and uninfected conditions. The YAP-TEAD transcriptional activity promotes autophagic flux and lysosomal acidification, which are then important for defense against intracellular S. aureus. Furthermore, the staphylococcal toxin C3 exoenzyme EDIN-B was found effective in preventing YAP-mediated cell-autonomous immune response. This study provides key insights on the anti-S. aureus activity of YAP, which could be conserved for defense against other intracellular bacteria.

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.

Short communication: Detection of antibiotic resistance, mecA, and virulence genes in coagulase-negative Staphylococcus spp. from buffalo milk and the milking environment

The aim of this study was to determinate whether coagulase-negative staphylococci (CNS) from buffalo milk or the milking environment possess virulence factors that are associated with intramammary infections or antimicrobial resistance. Milk samples (n = 320) from 80 lactating buffalo were evaluated for clinical and subclinical mastitis by physical examination, the strip cup test, California Mastitis Test (CMT), and somatic cell count (SCC) over a 4-mo period. In addition, swabs were obtained from the hands of consenting milkers (16), liners (64), and from the mouths (15) and nostrils (15) of buffalo calves. No clinical cases of mastitis were observed; however, CMT together with SCC results indicated that 8 animals had subclinical mastitis. Eighty-four CNS isolates were identified by MALDI-TOF MS and cydB real-time PCR (qPCR) and then evaluated by qPCR for presence of the eta, etb, sea, sec, cna, seb, sei, seq, sem, seg, see, and tst toxin genes, adhesion- and biofilm-associated genes (eno, ebps, fib, fnbA, coa), and the methicillin resistance gene (mecA). Resistance to antibiotics commonly used for mastitis treatment in Brazil was determined using the Kirby-Bauer test. Two strains were positive for the see and eta toxin genes; and mecA (1), eno (27), ebps (10), fnbA (10), and coa (5) genes were also detected. A notable number of isolates were resistant to erythromycin (30), penicillin (26), and cotrimoxazole (18); importantly, 10 vancomycin-resistant isolates were also detected. A smaller number of isolates were resistant to rifampicin (8), oxacillin (7), clindamycin (5), cefepime (4), tetracycline (3), ciprofloxacin (2), and chloramphenicol (1), and none were resistant to gentamicin or ciprofloxacin. Isolates with resistance to 2 (13 isolates), 3 (3), 4 (3), 5 (1), and 6 (1) antibiotics were detected. Taken together, our findings suggest that CNS isolates may not be a significant cause of clinical or even subclinical mastitis in buffaloes, but they may be a reservoir of virulence and antibiotic resistance genes.

High Nasal Carriage Rate of Staphylococcus aureus Containing Panton-Valentine leukocidin- and EDIN-Encoding Genes in Community and Hospital Settings in Burkina Faso

The objectives of the present study were to investigate the rate of S.aureus nasal carriage and molecular characteristics in hospital and community settings in Bobo Dioulasso, Burkina Faso. Nasal samples (n = 219) were collected from 116 healthy volunteers and 103 hospitalized patients in July and August 2014. Samples were first screened using CHROMagar Staph aureus chromogenic agar plates, and S. aureus strains were identified by mass spectrometry. Antibiotic susceptibility was tested using the disk diffusion method on Müller-Hinton agar. All S. aureus isolates were genotyped using DNA microarray. Overall, the rate of S. aureus nasal carriage was 32.9% (72/219) with 29% in healthy volunteers and 37% in hospital patients. Among the S. aureus isolates, only four methicillin-resistant S. aureus (MRSA) strains were identified and all in hospital patients (3.9%). The 72 S. aureus isolates from nasal samples belonged to 16 different clonal complexes, particularly to CC 152-MSSA (22 clones) and CC1-MSSA (nine clones). Two clones were significantly associated with community settings: CC1-MSSA and CC45-MSSA. The MRSA strains belonged to the ST88-MRSA-IV or the CC8-MRSA-V complex. A very high prevalence of toxinogenic strains 52.2% (36/69), containing Panton-Valentine leucocidin- and EDIN-encoding genes, was identified among the S. aureus isolates in community and hospital settings. This study provides the first characterization of S. aureus clones and their genetic characteristics in Burkina Faso. Altogether, it highlights the low prevalence of antimicrobial resistance, high diversity of methicillin-sensitive S. aureus clones and high frequency of toxinogenic S. aureus strains.

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.

Clostridial C3 Toxins Target Monocytes/Macrophages and Modulate Their Functions

The C3 enzymes from Clostridium (C.) botulinum (C3bot) and Clostridium limosum (C3lim) are single chain protein toxins of about 25 kDa that mono-ADP-ribosylate Rho-A, -B, and -C in the cytosol of mammalian cells. We discovered that both C3 proteins are selectively internalized into the cytosol of monocytes and macrophages by an endocytotic mechanism, comparable to bacterial AB-type toxins, while they are not efficiently taken up into the cytosol of other cell types including epithelial cells and fibroblasts. C3-treatment results in disturbed macrophage functions, such as migration and phagocytosis, suggesting a novel function of clostridial C3 toxins as virulence factors, which selectively interfere with these immune cells. Moreover, enzymatic inactive C3 protein serves as a transport system to selectively deliver pharmacologically active molecules into the cytosol of monocytes/macrophages without damaging these cells. This review addresses also the generation of C3-based molecular tools for experimental macrophage pharmacology and cell biology as well as the exploitation of C3 for development of novel therapeutic strategies against monocyte/macrophage-associated diseases.

Virulence Plasmids of Nonsporulating Gram-Positive Pathogens

Gram-positive bacteria are leading causes of many types of human infection, including pneumonia, skin and nasopharyngeal infections, as well as urinary tract and surgical wound infections among hospitalized patients. These infections have become particularly problematic because many of the species causing them have become highly resistant to antibiotics. The role of mobile genetic elements, such as plasmids, in the dissemination of antibiotic resistance among Gram-positive bacteria has been well studied; less well understood is the role of mobile elements in the evolution and spread of virulence traits among these pathogens. While these organisms are leading agents of infection, they are also prominent members of the human commensal ecology. It appears that these bacteria are able to take advantage of the intimate association between host and commensal, via virulence traits that exacerbate infection and cause disease. However, evolution into an obligate pathogen has not occurred, presumably because it would lead to rejection of pathogenic organisms from the host ecology. Instead, in organisms that exist as both commensal and pathogen, selection has favored the development of mechanisms for variability. As a result, many virulence traits are localized on mobile genetic elements, such as virulence plasmids and pathogenicity islands. Virulence traits may occur within a minority of isolates of a given species, but these minority populations have nonetheless emerged as a leading problem in infectious disease. This chapter reviews virulence plasmids in nonsporulating Gram-positive bacteria, and examines their contribution to disease pathogenesis.

Bacterial factors exploit eukaryotic Rho GTPase signaling cascades to promote invasion and proliferation within their host

Actin cytoskeleton is a main target of many bacterial pathogens. Among the multiple regulation steps of the actin cytoskeleton, bacterial factors interact preferentially with RhoGTPases. Pathogens secrete either toxins which diffuse in the surrounding environment, or directly inject virulence factors into target cells. Bacterial toxins, which interfere with RhoGTPases, and to some extent with RasGTPases, catalyze a covalent modification (ADPribosylation, glucosylation, deamidation, adenylation, proteolysis) blocking these molecules in their active or inactive state, resulting in alteration of epithelial and/or endothelial barriers, which contributes to dissemination of bacteria in the host. Injected bacterial virulence factors preferentially manipulate the RhoGTPase signaling cascade by mimicry of eukaryotic regulatory proteins leading to local actin cytoskeleton rearrangement, which mediates bacterial entry into host cells or in contrast escape to phagocytosis and immune defense. Invasive bacteria can also manipulate RhoGTPase signaling through recognition and stimulation of cell surface receptor(s). Changes in RhoGTPase activation state is sensed by the innate immunity pathways and allows the host cell to adapt an appropriate defense response.

Transcellular tunnel dynamics: Control of cellular dewetting by actomyosin contractility and I-BAR proteins

Dewetting is the spontaneous withdrawal of a liquid film from a non-wettable surface by nucleation and growth of dry patches. Two recent reports now propose that the principles of dewetting explain the physical phenomena underpinning the opening of transendothelial cell macroaperture (TEM) tunnels, referred to as cellular dewetting. This was discovered by studying a group of bacterial toxins endowed with the property of corrupting actomyosin cytoskeleton contractility. For both liquid and cellular dewetting, the growth of holes is governed by a competition between surface forces and line tension. We also discuss how the dynamics of TEM opening and closure represent remarkable systems to investigate actin cytoskeleton regulation by sensors of plasma membrane curvature and investigate the impact on membrane tension and the role of TEM in vascular dysfunctions.

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.

cAMP signaling by anthrax edema toxin induces transendothelial cell tunnels, which are resealed by MIM via Arp2/3-driven actin polymerization

RhoA-inhibitory bacterial toxins, such as Staphylococcus aureus EDIN toxin, induce large transendothelial cell macroaperture (TEM) tunnels that rupture the host endothelium barrier and promote bacterial dissemination. Host cells repair these tunnels by extending actin-rich membrane waves from the TEM edges. We reveal that cyclic-AMP signaling produced by Bacillus anthracis edema toxin (ET) also induces TEM formation, which correlates with increased vascular permeability. We show that ET-induced TEM formation resembles liquid dewetting, a physical process of nucleation and growth of holes within a thin liquid film. We also identify the cellular mechanisms of tunnel closure and reveal that the I-BAR domain protein Missing in Metastasis (MIM) senses de novo membrane curvature generated by the TEM, accumulates at the TEM edge, and triggers Arp2/3-dependent actin polymerization, which induces actin-rich membrane waves that close the TEM. Thus, the balance between ET-induced TEM formation and resealing likely determines the integrity of the host endothelium barrier.