The biological cost of antibiotic resistance in urinary isolates of Escherichia coli

Abstract number: P2034

Sundqvist M., Sjölund M., Cederbrandt G., Cars O., Andersson D.I., Kahlmeter G.

Objectives: The development and stability of a resistant bacterial population is dependent on the biological cost associated with resistance. A previous study showed that fosfomycin resistance in Escherichia coli confers a biological cost in vitro. To investigate whether this holds true for other resistance determinants in E. coli we have experimentally measured the biological cost of 12 different antibiotic resistance phenotypes in clinical isolates of E. coli.

Methods: UTI isolates of E. coli resistant to ampicillin, trimethoprim, nalidixic acid and/or mecillinam, alone or in combination (n= 11–54 isolates per phenotype) and E. coli susceptible to all ten tested antimicrobial agents, wildtype, (n = 50) were collected at the Dept of Clin Microbiology, Växjö. Susceptibility testing was performed as recommended by the Swedish Reference Group for Antibiotics. Isolates resistant to nitrofurantoin or cephalosporines were excluded. To estimate the biological cost of each phenotype, the growth rate was determined by using BioScreenC, a turbidometric reader which monitors bacterial growth continuously. Each isolate was inoculated to a final density of 105 cfu/ml in a well with 0.4 mL LB. Growth was recorded during 8 h at 37°C as an increase in optical density at 540 nm. Isolates were run in triplicate. Viable count was performed to verify the results for the reference strain (E. coli ATCC 25922) and isolates with isolated mecillinam resistance. Student's t-test was used for statistics. p < 0.001 was considered significant. Results are presented as means with 95% CI given in parenthesis.

Results: The mean generation times for mecillinam resistant isolates, as single, 26.9 min (25.4–28.5), and as part of a combination with other resistance determinants, 18.7 min (17.2–20.2), were significantly longer than in wildtype isolates, 16.1 min (15.5–16.6). Generation times for other resistance phenotypes did not differ significantly from wildtype.

Conclusions: Based on growth rate measurements in vitro, resistance to ampicillin, trimethoprim and nalidixic acid alone or in combination imposed no measurable fitness cost on clinical isolates of E. coli. This implies that once these resistance determinants are established, they are likely to persist despite decreases in selective pressure. In contrast mecillinam resistance conferred significantly reduced fitness. This finding provides a potential explanation to why mecillinam resistance is uncommon among clinical isolates.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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