Aim: Rifaximin is a non-absorbed antibiotic relative of rifampicin. The location of effect and staphylococcal resistance are two recent potential concerns with rifaximin. In this study we evaluate the location of effect of rifaximin as well as the development of staphylococcal rifampicin resistance.
Methods: Rats were divided into three groups. Group 1 gavaged for 10 days with PBS, group 2 gavaged with rifaximin for 10 days, and group 3 gavaged with rifaximin for 10 days and housed for 30 days. In each group, stool was collected daily for quantitative culture of Staphylococcus spp. and coliforms. After euthanasia luminal bacterial counts were determined at multiple gut locations by qPCR. Rifampicin susceptibility was tested on Staphylococcus pre and post rifaximin.
Results: At baseline, rats had a median of 2.90 × 10(6) cfu/ml Staphylococcus spp. in stool. After 10 days of rifaximin, this dropped to 1.20 × 10(5) cfu/ml (P < 0.01). With coliform counts, rats had a median of 1.86 × 10(4) cfu/ml at baseline which dropped to 2.2 × 10(3) cfu/ml (P < 0.01) after rifaximin. After cessation of rifaximin, coliform counts recovered within 3 days. When examining the total bacterial counts by qPCR, rifaximin reduced small bowel bacterial levels, but not colon. This reduction was sustained for 30 days. No colonies of Staphylococcus became resistant and only one colony was intermediate. The mean inhibitory concentration for rifampicin was not different before and after rifaximin.
Conclusion: Staphylococcal spp. fail to demonstrate resistance to rifampicin after rifaximin. The transient reductions in stool coliform counts recover while rifaximin appears to produce durable reductions in duodenal bacteria.