Importance of amino acid alterations and expression of penicillin-binding protein 5 to ampicillin resistance of Enterococcus faecium in Taiwan

The importance of the amino acid sequence in the C-terminal domain of penicillin-binding protein 5 (PBP5) and the levels of PBP5 expression to ampicillin resistance of Taiwan clinical isolates of Enterococcus faecium were studied. Sequence data revealed the existence of 12 amino acid sequence variants within the C-terminal domain of PBP5 in the 33 tested isolates (ampicillin minimum inhibitory concentrations (MICs) 1 mg/L to >256 mg/L). Western blot analyses of the levels of PBP5 showed that, with few exceptions, lower amounts of PBP5 were present in the susceptible strains than in the resistant strains. More importantly, a significant correlation (P = 0.004, Fisher's exact test) between the expression of PBP5 and ampicillin resistance was detected. Point mutations in PBP5, including addition of aspartic acid or serine after position 466 and change of methionine to alanine or threonine at position 485, alanine or isoleucine to threonine at position 499 and glutamate to valine at position 629, were found to be significantly associated with ampicillin resistance. A significant correlation was obtained for the combined mutation (alleles 10 and 11), suggesting that combined mutation of PBP5 can be a marker for ampicillin resistance of E. faecium.