Mechanism of Resistance to Antibiotic Synergism in Enterococci

Abstract

Enterococci exhibit two types of resistance to streptomycin. Moderately highlevel resistance is observed in most naturally occurring strains and can be overcome by simultaneous exposure to penicillin. In addition, very high-level resistance is found in those strains against which penicillin plus streptomycin fail to produce synergism in vitro. To study the mechanism of streptomycin resistance in enterococci, ribosomes from a wild-type strain and from a highly streptomycin-resistant mutant were isolated, characterized, and studied in an in vitro amino acid incorporation system. The ribosomes from the organism with moderately high-level streptomycin resistance were sensitive to streptomycin in vitro, suggesting that this type of resistance is caused by failure of streptomycin to reach the ribosomes. Very highlevel resistance (and lack of penicillin-streptomycin synergism), on the other hand, appears to be due to ribosomally mediated streptomycin resistance. Most strains of enterococci are resistant to streptomycin by conventional antibiotic sensitivity testing (22). The addition of penicillin overcomes this resistance (i.e., produces synergism with streptomycin) in many but not all strains of

Effects of medium and inoculum variations on screening for high-level aminoglycoside resistance in Enterococcus faecalis

“…Effect of penicillin and each aminoglycoside, tested alone and in combination, on the growth of a E. faecalis strain that was susceptible to synergy between penicillin and each aminoglycoside tested (A) and a E. faecalis strain that was resistant to synergy between penicillin and each aminoglycoside tested (B). The following antibiotics were tested: penicillin (PC), 10 U/ml; streptomycin (ST), 25 ,ug/ml; amikacin (AK), 20 ,ug/ml; kanamycin (KN), 20 ,ug/ml; gentamicin (GM), 5 ,ug/ml; tobramycin (TB), 5 kg/ml; control (C), no antibiotics. All time-kill studies were conducted by using DPB.…”

“…Among these drugs, only streptomycin, gentamicin, and occasionally, kanamycin should be considered for synergy screening. Regardless of whether E. faecalis resistance to streptomycin is mediated by ribosomal mutation (4,25) or enzymatic inactivation (13), only streptomycin can be used to screen for streptomycin-penicillin synergy. Because the same bifunctional enzyme (i.e., 2"-phosphotransferase-6'-acetyltransferase) that mediates high-level gentamicin resistance also mediates tobramycin and netilmicin resistance (3), a gentamicin screen alone currently is sufficient to reliably predict synergy between penicillin and gentamicin, tobramycin, or netilmicin.…”

“…Because high-level aminoglycoside resistance is most frequently mediated by constitutively produced aminoglycoside-modifying enzymes that are encoded on transferable plasmids, the incidence of enterococci that are refractory to combination therapy is expected to increase (3,10,13,15,25). Indeed, reports by Zervos et al (24) have indicated that as much as 55% of their enterococcal isolates demonstrated high-level gentamicin resistance.…”

Antibiotic combinations: should they be tested?

“…Of interest was the fact that penicillin also enhanced the uptake of streptomycin in a laboratory mutant strain of Streptococcus faecalis which was highly resistant (minimal inhibitory concentration [MIC], >2,000 ,ug/ml) to streptomycin and which was not synergistically killed by this combination. High-level streptomycin resistance in this mutant was shown to be due to insensitivity of the bacterial ribosome to the drug (194). 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.…”

“…Gram-negative bacilli. In 1962, Plotz and Davis (194) examined interactions between penicillin G (at high concentrations) and streptomycin against Escherichia coli. Prior penicillin exposure enhanced the bactericidal activity of streptomycin and augmented the uptake of [14C]streptomycin by bacterial cells.…”