Figure 2.

Methicillin-resistant Staphylococcus aureus (MRSA) coinfection elicited a prominent anti-α-toxin response relative to methicillin-susceptible S. aureus (MSSA) coinfection. A, α-Toxin–specific antibody level displayed as a function of study day and by coinfection with MRSA, MSSA, or neither. B, Ratio of highest convalescent titer vs acute titers for each patient when serial specimens were available, classified by coinfection status. C, α-Toxin–specific antibody assessed by functional neutralization of recombinant α-toxin in a rabbit erythrocyte lysis assay. “IC₅₀” indicates the inhibitory concentration of diluted serum that results in 50% inhibition of 2 nM recombinant α-toxin. D, Ratio of highest neutralization titer in convalescent sera vs acute sera for each patient when serial specimens were available, classified by coinfection status. Linear mixed-effects model analysis on log-transformed data revealed that the time profiles for MSRA are significantly different from those for MSSA and no S. aureus for α-toxin–binding immunoglobulin G (IgG) and α-toxin–neutralizing IC₅₀, as indicated above. Black symbols indicate patients who died during admission, while open symbols indicate patients with influenza-like illness who had an alternate viral diagnosis. Bars indicate geometric means. *P < .05, **P < .01, and ***P < .001.