Analysis of PBP5 of early U.S. isolates of Enterococcus faecium: sequence variation alone does not explain increasing ampicillin resistance over time

Abstract

Recent studies have shown that ampicillin resistance has increased steadily over the past 3 decades within U.S. Enterococcus faecium isolates. Analysis of the predicted PBP5 protein of 41 isolates showed a consensus PBP5 pattern for the 9 isolates with MICs of <4 μg/ml that is distinctly different from the PBP5 consensus of the 32 isolates with MICs of >4 μg/ml with ∼5% difference between these; however, there were no consistent amino acid changes that correlated with specific increases in the MICs of ampicillin within the latter group. Analysis of three other genes encoding cell wall/surface proteins also showed that there are two distinct evolutionary groups for each gene, but with occasional mixing of genes, consistent with a species that evolves by recombination.

Fig. 1.

PBP5 phylogeny coincides with MIC distribution. An UPGMA tree of PBP5 was constructed using the amino acid sequences from all strains analyzed in this study. PBP5 splits isolates into two groups using the amino acid sequence, and these two groups coincide with the MIC distribution. Isolates with an ampicillin MIC of <4 μg/ml all group together in the lower branch of the tree (pbp5-S group) (except for TX2050, which has an MIC of 4 μg/ml), while all isolates with an MIC of >4 μg/ml group together in the top branch of the tree (pbp5-R group). The MIC in micrograms per milliliter and the multilocus sequence type are listed next to the strain name. STND designates sequence typing that was not done.

Fig. 2.

UPGMA trees of four E. faecium genes show that isolates generally fall into two distinct lineages. UPGMA trees of pbp5 (A), gls20 (B), pbp2 (C), and wlcA (D) are depicted using the sequenced U.S. isolates and available on the Broad Institute or NCBI website. In order to determine if other genes showed the same two groupings as pbp5 (designated pbp5-S and pbp5-R) (A), UPGMA trees of gls20 (B), pbp2 (C), and wlcA (D) were made. The boxes indicate isolates that share the ampicillin pbp5-S (MIC of <4 μg/ml) consensus. The sequence for wlcA of Efm 1231502 was unavailable.