Bacterial virulence is a multifaceted trait where the interactions between pathogen and host factors affect the severity and outcome of the infection. Toxin secretion is central to the biology of many bacterial pathogens and is widely accepted as playing a crucial role in disease pathology. To understand the relationship between toxicity and bacterial virulence in greater depth, we studied two sequenced collections of the major human pathogen Staphylococcus aureus and found an unexpected inverse correlation between bacterial toxicity and disease severity. By applying a functional genomics approach, we identified several novel toxicity-affecting loci responsible for the wide range in toxic phenotypes observed within these collections. To understand the apparent higher propensity of low toxicity isolates to cause bacteraemia, we performed several functional assays, and our findings suggest that within-host fitness differences between high- and low-toxicity isolates in human serum is a contributing factor. As invasive infections, such as bacteraemia, limit the opportunities for onward transmission, highly toxic strains could gain an additional between-host fitness advantage, potentially contributing to the maintenance of toxicity at the population level. Our results clearly demonstrate how evolutionary trade-offs between toxicity, relative fitness, and transmissibility are critical for understanding the multifaceted nature of bacterial virulence.

Background Understanding the changing epidemiology of Staphylococcus aureus bacteremia, as well as the variables associated with poor outcomes, can yield insight into potential interventions. Methods This study was a retrospective, observational cohort study of adult patients at an academic medical center in New York City who had S. aureus bloodstream infections between 1 January 2007 and 31 December 2015. Participants were divided into 3 periods: group 1 (2007–2009), group 2 (2010–2012), and group 3 (2013–2015) for trend analysis. All clinical strains were genotyped (spa.). The main outcome was 30-day all-cause mortality. Results There were 1264 episodes of methicillin-susceptible S. aureus (MSSA) and 875 episodes of methicillin-resistant S. aureus (MRSA) bacteremia, with a rising proportion due to MSSA (55% group 1; 59% group 2; 63% group 3; P = .03.) There were no significant changes in average age, gender, Charlson score, and distribution of strain genotypes. Mortality in MRSA infection was unchanged (25% group 1; 25% group 2; 26% group 3), while mortality in MSSA infection significantly declined (18% group 1; 18% group 2; 13% group 3). The average time to antistaphylococcal therapy (AST) in MSSA infection declined during the study (3.7 days group 1; 3.5 group 2; 2.2 group 3). In multivariate analysis, AST within 7 days of initial positive MSSA culture was associated with survival. Conclusions Mortality in MSSA bloodstream infection is declining, associated with a decrease in time to targeted therapy. These results emphasize the potential for rapid diagnostics and early optimization of treatment to impact outcomes in MSSA bacteremia.

Few studies have focused on the risks of peripheral intravenous catheters (PIVs) as sources for Staphylococcus aureus bacteremia (SAB), a life-threatening complication. We identified 34 PIV-related infections (7.6%) in a cohort of 445 patients with SAB. Peripheral intravenous catheter-related SAB was associated with significantly longer bacteremia duration and thrombophlebitis at old PIV sites rather than current PIVs.

Methicillin-susceptible Staphylococcus aureus (MSSA) bloodstream infections (BSIs) often lead to severe complications despite the availability of effective antibiotics. It remains unclear whether elevated vancomycin MICs are associated with worse outcomes. We conducted a 2-year retrospective cohort study (n ϭ 252) of patients with MSSA BSIs at a tertiary care hospital. We defined reduced vancomycin susceptibility (RVS) as a Microscan MIC of 2 mg/liter. All strains were genotyped (spa) and assessed for agr functionality. Multivariable logistic regression models were used to examine the impact of RVS phenotype and strain genotype on 30-day allcause mortality and complicated bacteremia (metastatic spread, endovascular infection, or duration Ն3 days). One-third of patients (84/252) were infected with RVS isolates. RVS Infections were more frequently associated with metastatic or embolic sites of infection (36% versus 17%, P Ͻ 0.001), and endovascular infection (26% versus 12%, P ϭ 0.004). These infections occurred more often in patients with fewer underlying comorbidities (Charlson comorbidity index of Ն3 [73% versus 88%, P ϭ 0.002]). Genotyping identified 127 spa-types and 14 Spa-clonal complexes (Spa-CCs). Spa-CC002 and Spa-CC008 were more likely to exhibit the RVS phenotype versus other Spa-CCs (OR ϭ 2.2, P Ͻ 0.01). The RVS phenotype was not significantly associated with 30-day mortality; however, it was associated with complicated bacteremia (adjusted odds ratio of 2.35 [range, 1.26 to 4.37]; P ϭ 0.007) in adjusted analyses. The association of RVS strains with complicated infection and fewer underlying comorbidities suggests the phenotype as a potential marker of strain virulence in MSSA BSIs. The RVS phenotype itself was not a significant predictor of mortality in this patient cohort. Further studies are necessary to explore this host-pathogen relationship.KEYWORDS MSSA bacteremia, reduced vancomycin susceptibility S taphylococcus aureus is a dynamic pathogen causing a broad range of clinical syndromes, from localized skin-and-soft tissue infections to invasive disease (1). S. aureus bloodstream infections (SAB) often result in endovascular seeding and are frequently associated with poor outcomes, including 30-day mortality estimated at 20 to 40% (2-6). While bloodstream infections (BSIs) caused by methicillin-resistant S. aureus (MRSA) have historically attracted the most attention, methicillin-sensitive S. aureus (MSSA) is responsible for the majority of S. aureus BSIs (7,8).Morbidity and mortality in patients with S. aureus BSI are due to a complex interplay between timely and effective treatment, host-pathogen interactions, underlying pa-

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.