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Growth properties of mecillinam-resistant bacterial variants in urine
Abstract:
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.
Insights
Mecillinam-resistant Enterobacteriaceae variants show slower growth and readily revert to susceptible forms. This reduced growth may prevent colonization during urinary tract infections, explaining their absence in clinical settings.
Area of Science:
- Microbiology
- Pharmacology
- Infectious Diseases
Background:
- Enterobacteriaceae can develop mecillinam resistance in vitro.
- Mecillinam-resistant variants have not been clinically detected or linked to treatment failures.
- Urinary tract infections (UTIs) are common, and antibiotic resistance is a growing concern.
Purpose of the Study:
- To investigate the characteristics of mecillinam-resistant Enterobacteriaceae variants.
- To determine if these variants can colonize the urinary tract under simulated conditions.
- To understand the implications of mecillinam resistance for UTI treatment.
Main Methods:
- Incubation of 13 urinary Enterobacteriaceae isolates in mecillinam-containing urine.
- Measurement of generation times for resistant variants in vitro.
- Subculturing of resistant variants in antibiotic-free media and urine.
- Simulation of bladder flushing via frequent subculturing in urine.
Main Results:
- Five of 13 isolates developed mecillinam resistance.
- Resistant variants exhibited a 3.2-fold longer mean generation time in mecillinam-containing urine.
- Resistant variants reverted to susceptible phenotypes in antibiotic-free conditions.
- Bacterial populations decreased in mecillinam-containing urine over 52 hours, with no detection after 28 hours.
Conclusions:
- Mecillinam resistance in Enterobacteriaceae appears to be primarily phenotypic, characterized by reduced growth rates.
- The slow growth rate of these variants likely inhibits their colonization of the urinary tract during mecillinam treatment.
- Further research is needed to explore other factors influencing in vivo colonization.