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Correlation between phenotypic characteristics and DNA relatedness within Enterococcus faecium strains
L M Teixeira1, R R Facklam, A G Steigerwalt
1Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA.
Abstract:
We noted that a number of enterococcal strains isolated from human clinical specimens resembled Enterococcus faecium but were able to produce acid from glycerol, raffinose, and/or sorbitol, while others failed to form acid from mannitol. An additional concern was that many of these strains with atypical phenotypic characteristics also appeared to acquire vancomycin resistance. In order to determine if such atypical strains were variants of E. faecium or new Enterococcus species, 35 E. faecium or E. faecium-like strains (grouped into 10 phenotypes on the basis of the results of the following tests: capacity to form acid from glycerol, mannitol, raffinose, or sorbitol and susceptibility to vancomycin) and four strains of Enterococcus faecalis were taken from our culture collection, analyzed for their whole-cell protein profiles by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and identified to the species level by DNA-DNA reassociation experiments. All E. faecium-like strains, including four mannitol-negative variants, conformed to at least two of three DNA-DNA relatedness criteria: they were 70% or more related to the type strain of E. faecium at optimal conditions, they had less than 5% divergence within the related sequences, and they had a relatedness of 60% or greater under stringent conditions. The protein profiles of atypical strains were similar to those of typical strains and were easily distinguishable from those of E. faecalis and other enterococcal species. The five E. faecalis strains were 12 to 16% related to the E. faecium type strain. These results indicate that the phenotypic description of E. faecium should include all of these variable characteristics.
Insights
Atypical Enterococcus faecium strains with variable sugar fermentation and vancomycin resistance were confirmed as E. faecium variants. DNA-DNA reassociation and protein profiling support expanding the phenotypic description of E. faecium.
Area of Science:
- Microbiology
- Bacterial Taxonomy
- Clinical Microbiology
Background:
- Enterococcal strains from clinical specimens exhibit variable phenotypic characteristics.
- Some atypical strains resemble Enterococcus faecium but show unusual acid production and vancomycin resistance.
- Distinguishing between variants and new species is crucial for accurate identification and treatment.
Purpose of the Study:
- To determine if atypical enterococcal strains with variable phenotypes are variants of Enterococcus faecium or distinct species.
- To clarify the phenotypic spectrum of Enterococcus faecium.
- To assess the utility of molecular methods in differentiating these strains.
Main Methods:
- Phenotypic characterization including acid production from glycerol, mannitol, raffinose, and sorbitol, and vancomycin susceptibility testing.
- Whole-cell protein profiling using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE).
- DNA-DNA reassociation experiments for species identification.
Main Results:
- Atypical strains, including mannitol-negative variants, showed high DNA-DNA relatedness (≥70% at optimal, ≥60% under stringent conditions) to the E. faecium type strain.
- Protein profiles of atypical strains were similar to typical E. faecium and distinct from Enterococcus faecalis.
- Enterococcus faecalis strains exhibited low DNA-DNA relatedness to the E. faecium type strain (12-16%).
Conclusions:
- Atypical enterococcal strains with variable fermentation patterns and vancomycin resistance are confirmed as variants of Enterococcus faecium.
- The phenotypic description of Enterococcus faecium should encompass these observed variations.
- Molecular methods like DNA-DNA hybridization and protein profiling are reliable for accurate enterococcal species identification.