Methods: All S. aureus isolated from blood cultures in southwestern Skåne county (population approx. 500,000), Sweden in 2014 were subjected to susceptibility testing using E-tests for bensylpenicillin and disk diffusion with PcG (1U), and finally the chromogenic nitrocephin test for betalactamase. All isolates with a negative nitrocephin test and that were determined as susceptible according to EUCAST were subjected to a PCR detecting the S. aureus penicillinase as well as to specific S. aureus protein A (spa) typing.
Results: During 2014, 275 S. aureus isolates were isolated from blood in our catchment area. A total of 85 (31%) of the isolates were likely penicillin-susceptible according to aforementioned criteria. In these isolates, a penicillinase was detected in only a few cases, leaving the true incidence of penicillin-susceptible isolates just below 30%. Spa-typing suggested that while penicillin-susceptible isolates are not clonal, they are limited to a restricted number of spa types.
Conclusion: Penicillin-susceptible S. aureus are more common in Sweden than previously reported, and testing for penicillin susceptibility should be performed more often in clinical practice. The current EUCAST methodology for detection of susceptibility is a good, but not perfect, predictor of true penicillin-susceptibility.
J. Ahl, None
F. Månsson, None
K. Riesbeck, None