Interactions between glycopeptides and beta-lactams against isogenic pairs of teicoplanin-susceptible and -resistant strains of Staphylococcus haemolyticus

Abstract

Four isogenic derivatives with stably increased glycopeptide MICs (all become resistant to teicoplanin) were obtained from four glycopeptide-susceptible clinical isolates of Staphylococcus haemolyticus. All strains were extensively analyzed and compared for a number of distinctive features. In particular, the results provided insights into the puzzling issue of antistaphylococcal interactions between glycopeptides and beta-lactams, especially the paradox of double zones around beta-lactam disks and the relationships between autolysis rate and type of interaction.

FIG. 1.

Combination effects, resulting from broth microdilution checkerboard tests, of teicoplanin plus oxacillin against test strains Sh3R and Sh4R (A), teicoplanin plus ampicillin against test strains Sh3 and Sh4 (B), and teicoplanin plus penicillin against test strains Sh3 and Sh4 (C). FIC indices are given at appropriate points.

FIG. 2.

Autolysis rates of strain Sh3 at different times (expressed as percent decrease in optical density at 620 nm [OD₆₂₀]) in the presence of 1 μg/ml teicoplanin and different penicillin concentrations: ▤, 0.06 μg/ml; ░⃞, 0.25 μg/ml; ▨, 1 μg/ml; and ▪, 8 μg/ml. □, control (antibiotic-free medium).

FIG. 3.

Diffusion tests using Etest strips. Zones of inhibition around teicoplanin strips (A to D) on Mueller-Hinton agar unsupplemented or supplemented with 1 μg/ml penicillin and inoculated with strain Sh3 or strain Sh3R. Incomplete growth inhibition around penicillin (E) and oxacillin (F) strips, with a clear double zone around the former and a barely distinguishable one around the latter, on Mueller-Hinton agar supplemented with 1 μg/ml teicoplanin and inoculated with strain Sh3R.

FIG. 4.

Diffusion tests using commercial disks. Zones of inhibition around oxacillin, penicillin, and ampicillin disks for selected test strains, using Mueller-Hinton agar, supplemented with 2% NaCl (A), 2% NaCl and 1 μg/ml teicoplanin (B), 1 μg/ml teicoplanin (C), or 0.25 μg/ml vancomycin (D), as the test medium.