Evidence of Nosocomial Infection in Japan Caused by High-Level Gentamicin-Resistant Enterococcus faecalis and Identification of the Pheromone-Responsive Conjugative Plasmid Encoding Gentamicin Resistance |
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Authors: | Xinghua Ma Michiaki Kudo Ayako Takahashi Koichi Tanimoto Yasuyoshi Ike |
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Affiliation: | Department of Microbiology,1. Laboratory of Bacterial Drug Resistance,3. and Department of Laboratory Medicine and Clinical Laboratory Center,2. Gunma University School of Medicine, Maebashi, Gunma, Japan |
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Abstract: | A total of 1,799 Enterococcus faecalis isolates were isolated from inpatients of Gunma University Hospital, Gunma, Japan, between 1992 and 1996. Four hundred thirty-two (22.3%) of the 1,799 isolates had high-level gentamicin resistance. Eighty-one of the 432 isolates were classified and were placed into four groups (group A through group D) with respect to the EcoRI restriction endonuclease profiles of the plasmid DNAs isolated from these strains. The 81 isolates were isolated from 36 patients. For 35 of the 36 patients, the same gentamicin-resistant isolates were isolated from the same or different specimens isolated from the same patient at different times during the hospitalization. For one other patient, two different groups of the isolates were isolated from the same specimen. Groups A, B, C, and D were isolated from 5, 14, 12, and 6 patients, respectively. The strains had multiple-drug resistance. The restriction endonuclease digestion patterns of the E. faecalis chromosomal DNAs isolated from isolates in the same group were also identical. The patients who had been infected with the gentamicin-resistant isolates from each group were geographically clustered on a ward(s). These results suggest that the isolates in each group were derived from a common source and had spread in the ward. The gentamicin-resistant isolates exhibited a clumping response upon exposure to pheromone (E. faecalis FA2-2 culture filtrate). The gentamicin resistance transferred at a high frequency to the recipient E. faecalis isolates by broth mating, and the pheromone-responsive plasmids encoding the gentamicin resistance were identified in these isolates.Enterococcus strains have become a significant cause of nosocomial infections (15, 17, 18, 22, 27). Of the members of the genus Enterococcus, E. faecalis and E. faecium are commonly isolated from humans. These two organisms account for 85 to 95 and 5 to 10% of the strains isolated from clinical infections, respectively. The Enterococcus strains isolated from clinical infections have multiple-drug resistance. The multiple-drug resistance of the enterococci provides these organisms with a selective advantage in the hospital environment. Outbreaks of nosocomial infections caused by enterococcal strains resistant to various drugs have been reported previously (9, 10, 16–18, 23, 28, 29).In a study of clinical isolates from patients in Gunma University Hospital in Gunma, Japan, enterococci were found to be the second most common among the gram-positive bacteria, after Staphylococcus aureus (unpublished data). Of the clinical E. faecalis isolates, most (about 80%) were resistant to tetracycline. Between 30 and 40% of the isolates were resistant to gentamicin or erythromycin. Ampicillin- or vancomycin-resistant strains were not isolated (14, 24). Certain E. faecalis conjugative plasmids confer a mating response to the small sex pheromones secreted by potential recipient cells (1–4, 8, 11). This mating signal induces the synthesis of a surface aggregation substance that facilitates the formation of mating aggregates and plasmid transfer (2–4, 7, 11, 25). Most (60%) of the drug-resistant strains exhibit a clumping response with a culture filtrate of a plasmid-free E. faecalis recipient strain (24), suggesting that the strains harbor a pheromone-responding plasmid.To our knowledge, there is no report concerning nosocomial infection caused by enterococci in Japan. In this report, we describe nosocomial infections in Gunma University Hospital caused by high-level gentamicin-resistant isolates of E. faecalis and isolation of the pheromone-responsive plasmids from the isolates. |
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