The impact of Panton-Valentine leukocidin (PVL) on the outcome in Staphylococcus aureus pneumonia is controversial. We genotyped S. aureus isolates from patients with hospital-acquired pneumonia (HAP) enrolled in two registrational multinational clinical trials for the genetic elements carrying pvl and 30 other virulence genes. A total of 287 isolates (173 methicillin-resistant S. aureus [MRSA] and 114 methicillin-susceptible S. aureus [MSSA] isolates) from patients from 127 centers in 34 countries for whom clinical outcomes of cure or failure were available underwent genotyping. Of these, pvl was detected by PCR and its product confirmed in 23 isolates (8.0%) (MRSA, 18/173 isolates [10.4%]; MSSA, 5/114 isolates [4.4%]). The presence of pvl was not associated with a higher risk for clinical failure (4/23 [17.4%] versus 48/264 [18.2%]; P = 1.00) or mortality. These findings persisted after adjustment for multiple potential confounding variables. No significant associations between clinical outcome and (i) presence of any of the 30 other virulence genes tested, (ii) presence of specific bacterial clone, (iii) levels of alpha-hemolysin, or (iv) delta-hemolysin production were identified. This study suggests that neither pvl presence nor in vitro level of alpha-hemolysin production is the primary determinant of outcome among patients with HAP caused by S. aureus.

The impact of Panton-Valentine leukocidin (PVL) on the severity of complicated skin and skin structure infections (cSSSI) caused by Staphylococcus aureus is controversial. We evaluated potential associations between clinical outcome and PVL presence in both methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) isolates from patients enrolled in two large, multinational phase three clinical trials assessing ceftaroline fosamil for the treatment of cSSSI (the CANVAS 1 and 2 programs). Isolates from all microbiologically evaluable patients with monomicrobial MRSA or MSSA infections (n = 473) were genotyped by PCR for pvl and underwent pulsed-field gel electrophoresis (PFGE). Genes encoding pvl were present in 266/473 (56.2%) isolates. Infections caused by pvl-positive S. aureus were associated with younger patient age, North American acquisition, and presence of major abscesses (P<0.001 for each). Cure rates of patients infected with pvl-positive and pvl-negative S. aureus were similar overall (93.6% versus 92.8%; P = 0.72), and within MRSA-infected (94.5% vs. 93.1%; P = 0.67) and MSSA-infected patients (92.2% vs. 92.7%; P = 1.00). This finding persisted after adjustment for multiple patient characteristics. Outcomes were also similar when USA300 PVL+ and non-USA300 PVL+ infections were compared. The results of this contemporary, international study suggest that pvl presence was not the primary determinant of outcome in patients with cSSSI due to either MRSA or MSSA.

Although it has recently been shown that A/J mice are highly susceptible to Staphylococcus aureus sepsis as compared to C57BL/6J, the specific genes responsible for this differential phenotype are unknown. Using chromosome substitution strains (CSS), we found that loci on chromosomes 8, 11, and 18 influence susceptibility to S. aureus sepsis in A/J mice. We then used two candidate gene selection strategies to identify genes on these three chromosomes associated with S. aureus susceptibility, and targeted genes identified by both gene selection strategies. First, we used whole genome transcription profiling to identify 191 (56 on chr. 8, 100 on chr. 11, and 35 on chr. 18) genes on our three chromosomes of interest that are differentially expressed between S. aureus-infected A/J and C57BL/6J. Second, we identified two significant quantitative trait loci (QTL) for survival post-infection on chr. 18 using N2 backcross mice (F1 [C18A]×C57BL/6J). Ten genes on chr. 18 (March3, Cep120, Chmp1b, Dcp2, Dtwd2, Isoc1, Lman1, Spire1, Tnfaip8, and Seh1l) mapped to the two significant QTL regions and were also identified by the expression array selection strategy. Using real-time PCR, 6 of these 10 genes (Chmp1b, Dtwd2, Isoc1, Lman1, Tnfaip8, and Seh1l) showed significantly different expression levels between S. aureus-infected A/J and C57BL/6J. For two (Tnfaip8 and Seh1l) of these 6 genes, siRNA-mediated knockdown of gene expression in S. aureus–challenged RAW264.7 macrophages induced significant changes in the cytokine response (IL-1 β and GM-CSF) compared to negative controls. These cytokine response changes were consistent with those seen in S. aureus-challenged peritoneal macrophages from CSS 18 mice (which contain A/J chromosome 18 but are otherwise C57BL/6J), but not C57BL/6J mice. These findings suggest that two genes, Tnfaip8 and Seh1l, may contribute to susceptibility to S. aureus in A/J mice, and represent promising candidates for human genetic susceptibility studies.

Author Summary

Staphylococcus aureus has a wide spectrum of human infection, ranging from asymptomatic nasal carriage to overwhelming sepsis and death. Mouse models offer an attractive strategy for investigating complex diseases such as S. aureus infections. A/J mice are highly susceptible to S. aureus infection compared with C57BL/6J mice. We showed that genes on chromosomes 8, 11, and 18 in A/J are responsible for susceptibility to S. aureus by using chromosome substitution strains (CSS). From the ∼4200 genes on these three chromosomes, we identified 191 which were differentially expressed between A/J and C57BL/6J when challenged with S. aureus. Next, we identified two significant QTLs on chromosome 18 that are associated with susceptibility to S. aureus infection in N2 backcross mice. Ten genes (March3, Cep120, Chmp1b, Dcp2, Dtwd2, Isoc1, Lman1, Spire1, Tnfaip8, and Seh1l) mapped to the two significant QTLs and were differentially expressed between A/J and C57BL/6J. One gene on each QTL, Tnfaip8 and Seh1l, affected expression of cytokines in mouse macrophages exposed to S. aureus. These cytokine response patterns were consistent with those seen in S. aureus-challenged peritoneal macrophages from CSS 18, but not C57BL/6J. Tnfaip8 and Seh1l are strong candidates for genes influencing susceptibility to S. aureus of A/J mice.