Fitness cost and impaired survival in penicillin-resistant Streptococcus gordonii isolates selected in the laboratory.

Cycling of Streptococcus gordonii (115 times) with penicillin resulted in a MIC increase of more than 100-fold, from 0.008 to 2 microg/ml. The 2-microg/ml MIC maximum was already reached after 36 passages but resulted in impaired fitness. Although the MIC did not increase further, fitness was partially recovered during the 79 additional cycles.