Colonization dynamics of ampicillin-resistant Escherichia coli in the infantile colonic microbiota

Objectives: To compare the colonization dynamics of ampicillin-resistant and ampicillin-susceptible Escherichia coli strains in the infantile intestinal microbiota. Methods: We followed 128 infants over the first year of life with regular quantitative faecal cultures and recordings of antibiotic treatment. E. coli strains were quantified, and their resistance pattern and car-riage of b-lactamase genes (TEM, SHV and OXA), phylogenetic group (A, B1, B2 or D), virulence gene profile (fimA, papC, sfaD/E, kfiC neuB, hlyA and iutA) and time of persistence in the microbiota were determined. Results: Twelve percent (n5 32) of the E. coli strains were resistant to ampicillin, as they carried the blaTEM (84%) or blaSHV genes. Ampicillin-resistant strains belonged mostly to phylogenetic group D and carried pap genes (P 5 0.023) significantly more often than ampicillin-susceptible strains due to a strong association between carriage of pap and blaSHV. In 31 of 32 cases, colonization by ampicillin-resistant strains occurred in infants not previously treated with b-lactam antibiotics. Ampicillin-resistant strains were equally capable as susceptible ones of persisting in the intestinal microbiota and did not have lower faecal population counts. Genes encoding b-lactamases were in most cases retained during the entire colonization period. Conclusions: The results suggest that ampicillin-resistant E. coli strains are not hampered in their colonizing capacity, and b-lactamase genes, therefore, may only slowly be eliminated from the com-mensal E. coli strain pool