Changes in the aerobic faecal flora of patients treated with antibiotics for acute intra-abdominal infection.

Background: An open observational study was performed to investigate changes in the rectal flora and antibiotic susceptibility among faecal bacteria in patients treated with antibiotics for acute intra-abdominal infection.

Methods: One hundred and forty patients with acute intra-abdominal infection requiring antibiotic treatment and hospitalization were included. Eight surgical units from the southern part of Sweden participated, between January 2006 and November 2007. Antibiotic treatments were according to local guidelines. Rectal swabs were obtained on admission (sample 1) and 2-14 days after the end of antibiotic treatment (sample 2). Aerobic bacteria and yeasts were analysed. The material was divided into 2 groups: 1 group with Enterobacteriaceae and 1 group with non-fermentative Gram-negative bacteria. The susceptibility to antibiotics in each group was compared between samples 1 and 2.

Results: The main finding of this study on patients with severe intra-abdominal infections was a shift in the aerobic faecal flora following antibiotic treatment, from Escherichia coli to other more resistant Enterobacteriaceae, Enterococcus faecium, and yeasts. The susceptibility to cephalosporins and piperacillin-tazobactam decreased in Enterobacteriaceae.

Conclusions: Following antibiotic treatment, a shift in the aerobic rectal flora to species with intrinsic antibiotic resistance was observed. This indicates that the emergence of resistance is not due to new mutations, but rather to selection of more resistant species. This should be taken into account when designing treatments for secondary intra-abdominal infections.