Antibiotic Selection of Escherichia coli Sequence Type 131 in a Mouse Intestinal Colonization Model

Abstract

ABSTRACTThe ability of different antibiotics to select for extended-spectrum β-lactamase (ESBL)-producingEscherichia coliremains a topic of discussion. In a mouse intestinal colonization model, we evaluated the selective abilities of nine common antimicrobials (cefotaxime, cefuroxime, dicloxacillin, clindamycin, penicillin, ampicillin, meropenem, ciprofloxacin, and amdinocillin) against a CTX-M-15-producingE. colisequence type 131 (ST131) isolate with a fluoroquinolone resistance phenotype. Mice (8 per group) were orogastrically administered 0.25 ml saline with 108CFU/mlE. coliST131. On that same day, antibiotic treatment was initiated and given subcutaneously once a day for three consecutive days. CFU ofE. coliST131,Bacteroides, and Gram-positive aerobic bacteria in fecal samples were studied, with intervals, until day 8.Bacteroideswas used as an indicator organism for impact on the Gram-negative anaerobic population. For three antibiotics, prolonged colonization was investigated with additional fecal CFU counts determined on days 10 and 14 (cefotaxime, dicloxacillin, and clindamycin). Three antibiotics (cefotaxime, dicloxacillin, and clindamycin) promoted overgrowth ofE. coliST131 (P< 0.05). Of these, only clindamycin suppressedBacteroides, while the remaining two antibiotics had no negative impact onBacteroidesor Gram-positive organisms. Only clindamycin treatment resulted in prolonged colonization. The remaining six antibiotics, including ciprofloxacin, did not promote overgrowth ofE. coliST131 (P> 0.95), nor did they suppressBacteroidesor Gram-positive organisms. The results showed that antimicrobials both with and without an impact on Gram-negative anaerobes can select for ESBL-producingE. coli, indicating that not only Gram-negative anaerobes have a role in upholding colonization resistance. Other, so-far-unknown bacterial populations must be of importance for preventing colonization by incomingE. coli.