Penicillin minimum inhibitory concentration drift in identical sequential Streptococcus pneumoniae isolates from colonized healthy infants

We monitored the timing of acquisition of nasopharyngeal colonization of St reptococcus pneumoniae in 125 healthy infants during their first 2 years of life. S. pneumoniae was isolated at least once from 59 (47%) of 125 infant s aged between 2 and 18 months. Twenty-four infants (19%) were colonized wi th penicillin-resistant S, pneumoniae at some time during the study. During the course of this investigation, we identified sequential pneumococcal is olates of the same serotype from 5 infants, in which the penicillin minimum inhibitory concentration (MIC) increased over time. For 4 of the 5 infants , sequential isolates were identical, as determined by pulsed-field gel ele ctrophoresis, Sequential S, pneumoniae nasopharyngeal isolates from some he althy infants demonstrated drift in penicillin MIC values over time, from p enicillin-susceptible to penicillin-resistant.