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Comparison of two beta-lactamase-producing strains of Streptococcus faecalis
Abstract:
A second strain of enterococcus (PA) producing beta-lactamase (Bla+ phenotype) was compared with the previously reported Bla+ enterococcus, strain HH22. As with the original strain, there was a marked inoculum effect when PA was tested with penicillin, ampicillin, and piperacillin; no difference was noted with methicillin, cephalothin, imipenem, or vancomycin; the difference with ticarcillin was intermediate. High-level gentamicin resistance (Gmr) transferred from PA to an enterococcal recipient strain at a frequency approximately 100-fold lower than for HH22; all Gmr transconjugants from both strains were Bla+, but only PA showed linkage of Gmr and Bla+ with transfer of resistance to streptomycin, tetracycline, and chloramphenicol. EcoRI digestion of plasmid DNA from Gmr Bla+ transconjugants showed no similarities between the two strains. A 5.1-kilobase EcoRI Bla+-encoding fragment derived from HH22 was cloned into an Escherichia coli cloning vector and shown to hybridize to a 10.2-kilobase EcoRI fragment derived from PA; both fragments hybridized to an 840-base-pair staphylococcal Bla+ gene probe. These data indicate that the penicillinases are similar but encoded on different or differently arranged plasmids. The fact that both are transferable emphasizes the potential for this new streptococcal resistance determinant to disseminate.
Insights
A new enterococcus strain (PA) exhibits beta-lactamase production, similar to strain HH22. Both strains show transferable resistance, highlighting the potential spread of this new determinant.
Area of Science:
- Microbiology
- Molecular Biology
- Antimicrobial Resistance
Background:
- Enterococcus strains can acquire beta-lactamase (Bla+) production, conferring resistance to penicillin-class antibiotics.
- Previous reports identified a Bla+ enterococcus strain (HH22); this study investigates a second Bla+ strain (PA).
Purpose of the Study:
- To compare the characteristics of a new beta-lactamase-producing enterococcus strain (PA) with a previously identified strain (HH22).
- To investigate the genetic basis and transferability of beta-lactamase and high-level gentamicin resistance (Gmr) in these strains.
Main Methods:
- Phenotypic testing of antibiotic resistance, including inoculum effect analysis.
- Plasmid DNA analysis using EcoRI digestion and Southern hybridization with a staphylococcal beta-lactamase gene probe.
- Conjugation experiments to assess the transfer of antibiotic resistance markers.
Main Results:
- Strain PA exhibited a marked inoculum effect with penicillin, ampicillin, and piperacillin, similar to HH22.
- High-level gentamicin resistance (Gmr) transfer from PA was less frequent than from HH22.
- While both strains' Gmr transconjugants were Bla+, only PA showed linked transfer of Gmr and Bla+ with other resistance markers.
- Plasmid DNA analysis revealed distinct genetic elements encoding beta-lactamase in PA and HH22, despite similarity to a staphylococcal probe.
Conclusions:
- The beta-lactamases in strains PA and HH22 are similar but encoded on different plasmids or differently arranged genetic elements.
- The transferability of these resistance determinants in enterococci poses a significant risk for dissemination within clinical settings.