Are "PSSA" coming back?

On the way to Lac St. Francois a couple weeks ago, we drove near St. Albans, Vermont, the location of the hospital outbreak mystery described in one of Mike’s recent posts (you may remember, the “case of the circumstantial evidence of a complicated conspiracy of hospital personnel using laboratory control strains to intentionally infect patients in order to ruin a doctor who claimed to have uncovered an illegal kickback scheme involving radiology services…”, yeah, that’s the one!).

In expert testimony from this case, the statement is made that penicillin-susceptible Staphylococcus aureus (PSSA) are extremely rare (“less than 2% of all S. aureus isolates”). This widely-held assumption (that PSSA are basically “extinct”) is incorrect, and in some centers there appears to have been a substantial increase in PSSA over the past few years. See this report from John Crane that 15% of all S. aureus from ICU patients in Buffalo, NY, are now PSSA, and this report from IDWeek 2012 that PSSA accounted for 20% of all MSSA (and 13% of all S. aureus) from positive blood cultures in the Kaiser Permanente system (regional reference laboratory in LA). We plan to examine this issue as well in the next round of our nationwide S. aureus resistance surveillance.

The perception that PSSA no longer exist persists in part because many labs don’t test or report the drug (to detect inducible beta-lactamases, labs have to perform a beta-lactamase test on any S. aureus that tests susceptible, before reporting it).

Why is this important? It is another indication of how complex is the epidemiology of S. aureus, demonstrating that emergence and virulence are not necessarily tied to resistance, and that the “loss” of a drug may not be the end of the story—and also, of course, as a reminder that the drug of choice for PSSA is…..penicillin.