Possible misinterpretation of penicillin susceptibility in Staphylococcus aureus blood isolate due to in vitro loss of the blaZ gene

We describe a case of recurrent catheter-related blood stream infections (BSI) with Staphylococcusaureus, in which the first isolate tested susceptible to penicillin, while subsequent isolates were resistant. Phenotypic susceptibility correlated with the absence/presence of the blaZ gene. The in vitro stability of penicillin resistance was investigated by subculturing single colonies. In two out of five colonies, phenotypical resistance was lost after a single subculture, which correlated with loss of the blaZ gene. This in vitro phenomenon probably resulted in a very major error in the microbiology report of the first BSI, where penicillin had been recommended as treatment.

Automated summary

This case report describes recurrent catheter-related bloodstream infections caused by Staphylococcus aureus, initially susceptible to penicillin but later developing resistance. Experimental findings showed that the loss of penicillin resistance correlated with the absence of the blaZ gene.