Faculty Bibliography

Infections caused by the Gram-positive bacterium Staphylococcus aureus remain a significant health threat globally. The production of bicomponent pore-forming leukocidins plays an important role in S. aureus pathogenesis. Transcriptionally, these toxins are primarily regulated by the Sae and Agr regulatory systems. However, the posttranslational regulation of these toxins is largely unexplored. In particular, one of the leukocidins, LukAB, has been shown to be both secreted into the extracellular milieu and associated with the bacterial cell envelope. Here, we report that a major cell wall hydrolase, autolysin (Atl), controls the sorting of LukAB from the cell envelope to the extracellular milieu, an effect independent of transcriptional regulation. By influencing the sorting of LukAB, Atl modulates S. aureus cytotoxicity toward primary human neutrophils. Mechanistically, we found that the reduction in peptidoglycan cleavage and increased LukAB secretion in the atl mutant can be reversed through the supplementation of exogenous mutanolysin. Altogether, our study revealed that the cell wall hydrolase activity of Atl and the cleavage of peptidoglycan play an important role in controlling the sorting of S. aureus toxins during secretion.

Vaccines against Staphylococcus aureus have eluded researchers for >3 decades while the burden of staphylococcal diseases has increased. Early vaccine attempts mainly used rodents to characterize preclinical efficacy, and all subsequently failed in human clinical efficacy trials. More recently, leukocidin AB (LukAB) has gained interest as a vaccine antigen. We developed a minipig deep surgical wound infection model offering 3 independent efficacy readouts: bacterial load at the superficial and at the deep-seated surgical site, and dissemination of bacteria. Due to similarities with humans, minipigs are an attractive option to study novel vaccine candidates. With this model, we characterized the efficacy of a LukAB toxoid as vaccine candidate. Compared to control animals, a 3-log reduction of bacteria at the deep-seated surgical site was observed in LukAB-treated minipigs and dissemination of bacteria was dramatically reduced. Therefore, LukAB toxoids may be a useful addition to S. aureus vaccines and warrant further study.

Pore-forming toxins (PFTs) are commonly produced by pathogenic bacteria, and understanding them is key to the development of virulence-targeted therapies. Streptococcus agalactiae, or group B Streptococcus (GBS), produces several factors that enhance its pathogenicity, including the PFT β-hemolysin/cytolysin (βhc). Little is understood about the cellular factors involved in βhc pore formation. We conducted a whole-genome CRISPR-Cas9 forward genetic screen to identify host genes that might contribute to βhc pore formation and cell death. While the screen identified the established receptor, CD59, in control experiments using the toxin intermedilysin (ILY), no clear candidate genes were identified that were required for βhc-mediated lethality. Of the top targets from the screen, two genes involved in membrane remodeling and repair represented candidates that might modulate the kinetics of βhc-induced cell death. Upon attempted validation of the results using monoclonal cell lines with targeted disruption of these genes, no effect on βhc-mediated cell lysis was observed. The CRISPR-Cas9 screen results are consistent with the hypothesis that βhc does not require a single nonessential host factor to mediate target cell death. IMPORTANCE CRISPR-Cas9 forward genetic screens have been used to identify host cell targets required by bacterial toxins. They have been used successfully to both verify known targets and elucidate novel host factors required by toxins. Here, we show that this approach fails to identify host factors required for cell death due to βhc, a toxin required for GBS virulence. These data suggest that βhc may not require a host cell receptor for toxin function or may require a host receptor that is an essential gene and would not be identified using this screening strategy.

Methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infections are associated with significant morbidity and mortality. MRSA secretes a number of virulence factors and pore-forming toxins that enable tissue invasion. Prior studies have found associations between decreased toxin production and poor outcomes in invasive MRSA infection, particularly in pneumonia. In this retrospective observational cohort study of MRSA bacteremia in adult patients 2007-2015, we examined whether cytotoxicity was associated with 30-day mortality. Isolates were obtained from 776 patients and screened for cytotoxicity in a human HL-60 cell model, antimicrobial susceptibility and spa type, and clinical data were abstracted from charts. We did not find an association between low cytotoxic activity and 30-day mortality in univariate logistic regression analyses. There was a difference in distribution of the genotypes across cytotoxicity phenotypes, with spa-CC008 accounting for a larger proportion of isolates in the high cytotoxicity group. Isolates with a skin and soft tissue primary infective site had a higher median cytotoxicity. There was no association between cytotoxicity and host factors such as age or comorbidity burden. The isolates in our study came from heterogeneous primary sites of infection and were predominantly from spa-CC002 and spa-CC008 lineages, so it is possible that findings in prior studies reflect a different distribution in genotypes and clinical syndromes. Overall, in this large study of cytotoxicity of MRSA bloodstream isolates, we did not find the low cytotoxicity phenotype to be predictive of poor outcomes in MRSA bacteremia.

Mutations in the rifampicin (Rif)-binding site of RNA polymerase (RNAP) confer antibiotic resistance and often have global effects on transcription that compromise fitness and stress tolerance of resistant mutants. We suggested that the non-essential genome, through its impact on the bacterial transcription cycle, may represent an untapped source of targets for combination antimicrobial therapies. Using transposon sequencing, we carried out a genome-wide analysis of fitness cost in a clinically common rpoB H526Y mutant. We find that genes whose products enable increased transcription elongation rates compound the fitness costs of resistance whereas genes whose products function in cell wall synthesis and division mitigate it. We validate our findings by showing that the cell wall synthesis and division defects of rpoB H526Y result from an increased transcription elongation rate that is further exacerbated by the activity of the uracil salvage pathway and unresponsiveness of the mutant RNAP to the alarmone ppGpp. We applied our findings to identify drugs that inhibit more readily rpoB H526Y and other Rif^R alleles from the same phenotypic class. Thus, genome-wide analysis of fitness cost of antibiotic-resistant mutants should expedite the discovery of new combination therapies and delineate cellular pathways that underlie the molecular mechanisms of cost.

Staphylococcus aureus bi-component pore-forming leukocidins are secreted toxins that directly target and lyse immune cells. Intriguingly, one of the leukocidins, Leukocidin AB (LukAB), is found associated with the bacterial cell envelope in addition to secreted into the extracellular milieu. Here, we report that retention of LukAB on the bacterial cells provides S. aureus with a pre-synthesized active toxin that kills immune cells. On the bacteria, LukAB is distributed as discrete foci in two distinct compartments: membrane-proximal and surface-exposed. Through genetic screens, we show that a membrane lipid, lysyl-phosphatidylglycerol (LPG), and lipoteichoic acid (LTA) contribute to LukAB deposition and release. Furthermore, by studying non-covalently surface-bound proteins we discovered that the sorting of additional exoproteins, such as IsaB, Hel, ScaH, and Geh, are also controlled by LPG and LTA. Collectively, our study reveals a multistep secretion system that controls exoprotein storage and protein translocation across the S. aureus cell wall.

The microbial populations in the gut microbiome have recently been associated with COVID-19 disease severity. However, a causal impact of the gut microbiome on COVID-19 patient health has not been established. Here we provide evidence that gut microbiome dysbiosis is associated with translocation of bacteria into the blood during COVID-19, causing life-threatening secondary infections. Antibiotics and other treatments during COVID-19 can potentially confound microbiome associations. We therefore first demonstrate that the gut microbiome is directly affected by SARS-CoV-2 infection in a dose-dependent manner in a mouse model, causally linking viral infection and gut microbiome dysbiosis. Comparison with stool samples collected from 97 COVID-19 patients at two different clinical sites also revealed substantial gut microbiome dysbiosis, paralleling our observations in the animal model. Specifically, we observed blooms of opportunistic pathogenic bacterial genera known to include antimicrobial-resistant species in hospitalized COVID-19 patients. Analysis of blood culture results testing for secondary microbial bloodstream infections with paired microbiome data obtained from these patients suggest that bacteria translocate from the gut into the systemic circulation of COVID-19 patients. These results are consistent with a direct role for gut microbiome dysbiosis in enabling dangerous secondary infections during COVID 19.

The generation of oxidative stress is a host strategy used to control Staphylococcus aureus infections. Sulfur containing amino acids, cysteine and methionine, are particularly susceptible to oxidation because of the inherent reactivity of sulfur. Due to the constant threat of protein oxidation, many systems evolved to protect S. aureus from protein oxidation or to repair protein oxidation after it occurs. The S. aureus peptide methionine sulfoxide reductase (Msr) system reduces methionine sulfoxide to methionine. Staphylococci have four Msr enzymes, which all perform this reaction. Deleting all four msr genes in USA300 LAC (Δmsr) sensitizes S. aureus to hypochlorous acid (HOCl) killing, however, Δmsr does not exhibit increased sensitivity to H₂O₂ stress or superoxide anion stress generated by paraquat or pyocyanin. Consistent with increased susceptibility to HOCl killing, Δmsr is slower to recover following co-culture with both murine and human neutrophils than USA300 wildtype. Δmsr is attenuated for dissemination to the spleen following murine intraperitoneal infection and exhibits reduced bacterial burdens in a murine skin infection model. Notably, no differences in bacterial burdens were observed in any organ following murine intravenous infection. Consistent with these observations, USA300 wildtype and Δmsr have similar survival phenotypes when incubated with murine whole blood. However, Δmsr is killed more efficiently by human whole blood. These findings indicate that species-specific immune cell composition of the blood may influence the importance of Msr enzymes during S. aureus infection of the human host.IMPORTANCEOxidative stress is a host defense strategy to control bacterial infections, and bacteria have evolved systems to counteract this innate immune defense. Here we investigate the peptide methionine sulfoxide reductase system in Staphylococcus aureus that repairs oxidized methionine residues in proteins, preventing the need to resynthesize damaged proteins de novo Most organisms have an Msr system, and in S. aureus these enzymes are protective against HOCl killing, the major oxidant produced by neutrophils. The S. aureus Msr system does not have a significant contribution to pathogenesis in bacteremia murine infection models but does protect S. aureus in both skin and intraperitoneal infection models. Strains lacking Msr activity are killed equivalently to wildtype by murine whole blood, and Δmsr is more sensitive to killing by human whole blood than the wildtype strain. These data identify the Msr enzymes as important and potentially specific factors for S. aureus pathogenesis in the human host.

Staphylococcus aureus has evolved into diverse lineages, known as clonal complexes (CCs), which exhibit differences in the coding sequences of core virulence factors. Whether these alterations affect functionality is poorly understood. Here, we studied the highly polymorphic pore-forming toxin LukAB. We discovered that the LukAB toxin variants produced by S. aureus CC30 and CC45 kill human phagocytes regardless of whether CD11b, the previously established LukAB receptor, is present, and instead target the human hydrogen voltage-gated channel 1 (HVCN1). Biochemical studies identified the domain within human HVCN1 that drives LukAB species specificity, enabling the generation of humanized HVCN1 mice with enhanced susceptibility to CC30 LukAB and to bloodstream infection caused by CC30 S. aureus strains. Together, this work advances our understanding of an important S. aureus toxin and underscores the importance of considering genetic variation in characterizing virulence factors and understanding the tug of war between pathogens and the host.

Leukocidin ED (LukED) is a pore-forming toxin produced by Staphylococcus aureus, which lyses host cells and promotes virulence of the bacteria. LukED enables S. aureus to acquire iron by lysing erythrocytes, which depends on targeting the host receptor Duffy antigen receptor for chemokines (DARC). The toxin also targets DARC on the endothelium, contributing to the lethality observed during bloodstream infection in mice. LukED is comprised of two monomers: LukE and LukD. LukE binds to DARC and facilitates hemolysis, but the closely related Panton-Valentine leukocidin S (LukS-PV) does not bind to DARC and is not hemolytic. The interaction of LukE with DARC and the role this plays in hemolysis are incompletely characterized. To determine the domain(s) of LukE that are critical for DARC binding, we studied the hemolytic function of LukE-LukS-PV chimeras, in which areas of sequence divergence (divergence regions, or DRs) were swapped between the toxins. We found that two regions of LukE's rim domain contribute to hemolysis, namely residues 57-75 (DR1) and residues 182-196 (DR4). Interestingly, LukE DR1 is sufficient to render LukS-PV capable of DARC binding and hemolysis. Further, LukE, by binding DARC through DR1, promotes the recruitment of LukD to erythrocytes, likely by facilitating LukED oligomer formation. Finally, we show that LukE targets murine Darc through DR1 in vivo to cause host lethality. These findings expand our biochemical understanding of the LukE-DARC interaction and the role that this toxin-receptor pair plays in S. aureus pathophysiology.