Persistence of a vancomycin-resistant Enterococcus faecium in an anaerobic continuous-flow culture of porcine microflora in the presence of subtherapeutic concentrations of vancomycin

Recombined porcine continuous-flow culture (RPCF) maintained in a continuous-flow fermentation system is effective in protecting neonatal and weaned pigs against infection by enteropathogens.(2,13,14) In the current study, we demonstrate the effect of RPCF on vancomycin-resistant enterococci (VRE) in the presence and absence of subtherapeutic levels of vancomycin. Also examined was the ability of VRE to transfer vancomycin resistance to endogenous Enterococcus faecalis 137.1. When RPCF was challenged with VRE, the rate of VRE clearance was dependent on the method of challenge. In the control experiment, RPCF was challenged with 7.0 log(10)/CFU/ml VRE. Clearance of VRE from the culture was observed within 7 days at a rate of 1.44 log(10)/day. RPCF containing 0.001 mug/ml vancomycin cleared VRE at a slightly lower rate of 0.94 log(10)/day. RPCF containing 0.01 mug/ml or 0.1 mug/ml vancomycin reduced the level of VRE from 7.0 log(10)/CFU/ml to 2.0 log(10)/CFU/ml within 9 days, but failed to clear the VRE after 24 days. During the period of decline, the VRE clearance rate for the 0.01 mug/ml and 0.1 mug/ml vancomycin-treated cultures was 0.52 log(10)/day, and 0.53 log(10)/day, respectively. E. faecalis 137.1 endogenous to RPCF did not acquire the vancomycin resistance genes throughout the experiment as evidenced by direct selection, ribotyping, and pulsed-field gel electrophoresis.