Growth properties of mecillinam-resistant bacterial variants in urine.

Resistant variants are often formed when susceptible Enterobacteriaceae are grown in media containing mecillinam. These variants have not yet been detected in patients or identified as a cause of treatment failure in limited clinical trials. Mecillinam-resistant organisms were formed by 5 out of 13 urinary isolates of Enterobacteriaceae incubated in urine containing mecillinam. The mean generation time of these five variants in urine containing therapeutic concentrations of mecillinam was 3.2 times that of normal organisms in antibiotic-free urine. When three of these five resistant variants were subcultured in antibiotic-free urine, the generation time, morphology, and antibiotic susceptibility returned to normal. On antibiotic-free agar medium, all five mecillinam-resistant variants readily reverted to the "susceptible" form and were therefore more likely to be phenotypic rather than genotypic mutants. In a second series of experiments, the flushing effect of the bladder on the clearance of organisms was partly simulated by frequent subculture in urine. Under these conditions, the cell density of cultures of all bacteria remained high during 10 subcultures over 52 h. However, bacterial populations progressively decreased in urine containing mecillinam until none could be detected at 28 h or thereafter. The slow growth rate of mecillinam-resistant variants may explain why detectable numbers of these organisms fail to colonize the urinary tract during treatment. Other factors may reinforce the postulated effect of the reduced growth rate in vivo.