Tetracycline resistance in Escherichia coli and persistence in the infantile colonic microbiota.

The ecological impact of antibiotic resistance in the absence of selective pressure has been poorly studied. We assessed the carriage of tetracycline resistance genes, persistence in the microbiota, fecal population counts and virulence factor genes in 309 commensal, intestinal Escherichia coli strains obtained from 128 Swedish infants followed during the first year of life with regular quantitative fecal cultures. No infant was given tetracycline, but 25% received other antibiotics. Tetracycline resistance was identified in 12% of strains, all of which carried either tet(A) (49%) or tet(B) (51%) genes. Resistance to other antibiotics occurred in 50% of tet(A)-positive strains, 42% of tet(B)-positive strains and 13% of tetracycline-sensitive strains. However, colonization with tetracycline-resistant strains was unrelated to treatment with antibiotics. Strains that were tet(B)- or tet(A)-positive carried the genes for P fimbriae and aerobactin, respectively, more often than susceptible strains. Tetracycline-resistant and -susceptible strains were equally likely to persist among the intestinal microbiota for > or = 3 weeks and had similar population numbers. However, when a resistant strain and a susceptible strain colonized a child simultaneously, the resistant variety showed lower counts (P = 0.03). In cases of long-term colonization by initially tetracycline-resistant E. coli strains, loss of tet genes occurred in 3 of 13 cases with variable effects on population counts. The results indicate that there is limited pressure against the carriage of tet genes in the infantile gut microbiota even in the absence of antibiotics. Resistant strains may possess colonization factors that balance the cost of producing resistance elements.