Fig. 1. Platelets are essential for blood immunity against Staphylococcus aureus (SA) bacteremia and SA α-toxin induces thrombocytopenia to evade platelet-mediated microbicidal activity.

(A) Correlation of circulating platelet counts with leukocyte counts and duration of bacteremia in 49 consecutive patients with SA bacteremia from a tertiary medical center. Spearman’s rank correlation coefficient compared variables. (B) Mortality in patient cohort associated with different white blood cell (WBC) and platelet (PLTS) count cutoffs; Chi-square without Yates correction. (C) Washed isolated human platelets and neutrophils exposed to SA or Streptococcus pneumoniae (SPN) at MOI = 0.01 for 2 h. Samples were sonicated, serial diluted, and plated on THA plates for enumeration of bacterial colony forming units (CFU). (D) Reduction in platelet count 2 h after intravenous infection of mice with SA (n = 8) vs. non-infected littermate controls (n = 4). Two biological replicates were performed and data pooled; data represented as mean ± SEM. (E) Ex vivo killing of SA upon 2 h co-incubation with blood collected from mice 16 h after treatment with anti-CD41 antibody (n= 9) or IgG control (n= 12). (F) Mice treated with platelet depleting anti-CD41 antibody (n = 5) or IgG control (n = 4) for 16 h prior to intravenous SA infection. Organs harvested and CFU enumerated 2 h post-infection in triplicate for each sample. (G) Assessment of α-toxin production by the infecting SA isolate in 49 consecutive bacteremia cases and its association with patient platelet counts and (H) WBC counts. (I-J) Platelet counts (I) and CFUs (J) in outbred CD-1 mice intravenously challenged with wild-type SA (n = 4) or isogenic ΔHla mutant (n = 4). Blood harvested by cardiac puncture, complete blood count performed and colony forming units (CFUs) enumerated 4 h post-infection. (K) Ex vivo killing of SA by freshly isolated human platelets (2 h co-incubation) vs. isogenic ΔHla mutant. All data represented as mean ± SEM and representative of at least 3 independent experiments. Statistical significance was determined by unpaired two-tailed Student’s t-test (C-F, I, J, K) or one way analysis of variance (ANOVA) with Bonferroni’s multiple comparisons test (G,H). *P < 0.05. For floating bar graphs, + denotes the mean, whiskers represent min. to max, and floating box represents 25th to 75th percentile.