Plasmid-mediated resistance to antibiotic synergism in enterococci.

Abstract

A B S T R A C T Mating experiments have shown thathigh-level resistance (minimal inhibitory concentration >2,000 ,uglml) to streptomycin and kanamycin, and resistance to penicillin-streptomycin and penicillinkanamycin synergism are transferable by conjugation from resistant clinical isolates of enterococci to a sensitive recipient strain. Cesium chloride-ethidium bromide ultracentrifugation revealed a satellite (plasmid) band in resistant clinical isolates and the transconjugant strains but not in the sensitive recipient. Examination of these satellite bands by agarose gel electrophoresis and electron microscopy demonstrated a common plasmid with a weight of 45 megadaltons.Novobiocin treatment of a resistant clinical isolate produced simultaneous loss of high-level resistance to streptomycin and kanamycin, and of resistance to penicillin-aminoglycoside synergism.These results suggest that (a) high-level resistance to streptomycin and kanamycin among some clinical isolates of enterococci is associated with a 45 megadalton plasmid, and (b) the same plasmid is also responsible for the resistance to penicillin-aminoglycoside synergism observed in these strains.

Antibiotic-induced lysis of enterococci.

“…Conversely, cells grown in media that permitted less rapid penicillin lysis were less susceptible to spontaneous lysis (Table III). Our studies have demonstrated more rapid and extensive penicillin lysis and killing of S. faecalis than those of most previous investigators (3)(4)(5)(35)(36). We believe that the reasons for these differences include (a) strain variation and (b) differences in the specific testing conditions used.…”

Antibiotic combinations: should they be tested?

“…Although ribosomal resistance to streptomycin may account for high-level resistance (MIC, >2,000 pLg/ml) to the drug in a small proportion of clinical isolates (51), in the vast majority of cases highlevel aminoglycoside resistance and subsequent lack of penicillin-aminoglycoside synergism is due to the presence of aminoglycoside-modifying enzymes. Plasmid-mediated enzymes conferring high-level resistance to streptomycin and kanamycin have been recognized for more than a decade (107); by the late 1970s, approximately 50% of enterococcal blood isolates recovered at the Massachusetts General Hospital were highly resistant to streptomycin or kanamycin or both (28) and thus evaded killing by these agents in combination with penicillin.…”

Aminoglycoside-Inactivating Enzymes in Clinical Isolates of Streptococcus Faecalis

“…A 45-Mdalton plasmid has been identified in five resistant isolates, which was not present in strains without high-level aminoglycoside resistance; this plasmid was also found in transconjugants of the sensitive recipient strain, after they had acquired high-level aminoglycoside resistance from resistant clinical isolates (8).…”

“…Because previous work has demonstrated that highlevel resistance and resistance to synergism were transferable by conjugation into a sensitive recipient strain (8), we have examined a series of Streptococcus fae-calis strains for aminoglycoside-inactivating enzymes: 10 clinical isolates with high-level resistance to streptomycin and kanamycin, a transconjugant strain produced by mating a sensitive recipient with one of the resistant strains, the sensitive recipient, and 5 sensitive clinical isolates were studied. A 45-Mdalton plasmid has been identified in five resistant isolates, which was not present in strains without high-level aminoglycoside resistance; this plasmid was also found in transconjugants of the sensitive recipient strain, after they had acquired high-level aminoglycoside resistance from resistant clinical isolates (8).…”