Resistance to antibiotic synergism in Streptococcus faecalis: further studies with amikacin and with a new amikacin derivative, 4'-deoxy, 6'-N-methylamikacin

Abstract

Streptococcus faecalis strains may resist penicillin-aminoglycoside synergy by the production of plasmid-mediated aminoglycoside-modifying enzymes. One of these enzymes, aminoglycoside 3'-phosphotransferase, has been shown to have a broad range of substrate specificity, including amikacin. We have studied a derivative of amikacin, 4'-deoxy, 6'-N-methylamikacin (BB-K311), against 11 clinical blood isolates of S. faecalis. Minimal inhibitory concentrations of BB-K311 were quite similar to those of amikacin, ranging from 125 to 1,000 micrograms/ml. In assays for antibiotic synergy, penicillin and amikacin produced enhanced killing compared with the penicillin alone only against those three strains which lacked the phosphotransferase enzyme. The other eight enzyme-positive strains actually demonstrated significant antagonism between penicillin against all 11 strains, regardless of enzyme production. Analysis of substrate profiles with crude preparations of the aminoglycoside 3'-phosphotransferase enzyme confirmed that BB-K311 was a very poor substrate for modification, as expected from the synergy studies. Use of other aminoglycoside analogs confirmed the 3'-OH site of modification. These findings suggest that removing the 4'-OH group in amikacin effectively blocks 3'-phosphorylation by S. faecalis enzyme.

Antibiotic combinations: should they be tested?

“…However, because amikacin is not a good substrate for this enzyme, combinations of penicillin plus amikacin do exhibit synergism, unless the strain also exhibits high-level kanamycin resistance which reflects the presence of a 3'-phosphotransferase-modifying enzyme as well (181). Strains of Streptococcus faecalis possessing the latter enzymatic activity likewise resist penicillin-amikacin synergism despite amikacin MICs as low as 250 jig/ml (29). In fact, against strains producing this enzyme, combinations of penicillin with amikacin are antagonistic.…”

“…Therefore, for practical purposes, screening for highlevel resistance to streptomycin or gentamicin is preferable to detailed time-kill studies except in unusual circumstances such as in assessing causes of treatment failure. Amikacin represents an exception to this rule in that resistance to synergism is usually not manifest by extraordinarily high MICs; however, high-level (MIC, >2,000 jig/ml) resistance to kanamycin is predictive of the presence of a 3'-APH (aminoglycoside-phosphorylating) enzyme which renders amikacin inactive in terms of synergism (29). Also, the presence of 6'-AAC (acetylating) enzymes in most isolates of Streptococcus faecium renders tobramycin, netilmicin, and other aminoglycosides susceptible to modification at that position unable to participate synergistically with penicillin, even in the absence of high-level resistance (127,181).…”

“…Antagonism is often defined as a 100-fold reduction in killing by the combination compared with that seen with the most active drug alone (108). However, in some circumstances, adverse influence of one antibiotic on the activity of another of smaller (<2-log1o CFU/ml) magnitudes occurs in a highly reproducible manner, convincingly suggesting antagonistic interactions (29,169).…”

The life and times of the Enterococcus

“…This enzyme also has activity against amikacin. Although strains do not display HLR to amikacin, Krogstad (29,112 (232). HLR to gentamicin has also been reported in E. faecium and in community-acquired E. faecalis (58,154).…”

“…When such strains cause endocarditis, adding an aminoglycoside adds nothing to the effect of the cell-wall-active agent alone but will expose patients to increased toxicity (29,30,97,112,135). Optimal therapy for these patients is unknown.…”