Resistance to cefotaxime and peptidoglycan composition in Enterococcus faecalis are influenced by exogenous sodium chloride.

The influence of NaCl on the susceptibility of Enterococcus faecalis to cefotaxime was tested with JH2-2, a laboratory strain, and 20 clinical strains grown on tryptic soy agar supplemented with 5% horse blood. Growth with 3% NaCl in the medium resulted in an increase in cefotaxime resistance and the appearance of a heterogeneous resistance phenotype: for the majority of the strains, the MICs of cefotaxime increased from 4 to 512 micrograms ml-1. By a competition assay using cefotaxime and [3H]benzylpenicillin, it was shown for strain JH2-2 that at the MIC penicillin-binding protein (PBP) 2 and PBP3 were the apparent essential PBPs in medium without NaCl, whilst the low-affinity PBPs 4 and 1 were the apparent essential PBPs for cell growth in medium containing 3% NaCl. Analysis of JH2-2 peptidoglycan by HPLC and MS after growth in the presence of 3% NaCl showed a relative increase in unsubstituted monomers and a relative decrease in alanine- and dialanine-substituted monomers. It is therefore hypothesized that modification of the number of alanine-substituted precursors in the presence of NaCl could interfere with the functions of the different PBPs and thus play a role in cefotaxime resistance in E. faecalis.