In vitro activity of teicoplanin and vancomycin against Grampositive bacteria from human clinical and veterinary sources

The minimum inhibitory concentration (MIC) of teicoplanin and vancomycin was determined by the agar dilution method for 186 Gram‐positive bacteria from human clinical and veterinary sources. Teicoplanin MIC values were ≤4 μg/ml for 94% of staphylococci (group A, n = 52) and ≤2 μg/ml for all streptococci, enterococci, aerococci and pediococci (group B, n = 75). Seventy‐eight percent of Gram‐positive rods, Rhodococcus and Leuconostoc spp. (group C, n = 59) were inhibited by 4 ug/ml. Teicoplanin resistance (MIC ≥ 16 μg/ml) was demonstrated for all Nocardia strains and for some strains of Lactobacillus, E. rhusiopathiae, Leuconostoc , and S. haemolyticus. Cross‐resistance between teicoplanin and vancomycin was observed for all Nocardia strains and for some strains of Lactobacillus, E. rhusiopathiae , and Leuconostoc. Three methicillin‐resistant S. haemolyticus strains were either resistant or intermediately susceptible to teicoplanin and susceptible to vancomycin. Eight strains (motile enterococci four, E. rhusiopathiae three and Leuconostoc sp. one) were susceptible to teicoplanin and resistant to vancomycin. Teicoplanin disc diffusion on Danish Blood Agar with NeoSensitabs® (Rosco), PDM® AB Biodisc and locally prepared discs revealed a wide range of zone diameters in groups B and C. The relation between MIC values and zone diameters for teicoplanin was analysed by the error‐rate bounded method. Zone size interpretive criteria as suggested by the manufacturers (≥ 15 mm) produced 2.7% (95% confidence limits 0.9‐6.2%) and 1.6% (95% confidence limits 0.3–4.6%) very major errors for NeoSensitabs and PDM‐disc, respectively. Using a zone size breakpoint for susceptibility of ≥ 25 mm for NeoSensitabs and ≥20 mm for PDM‐disc, the proportions of very major errors were 0.5% (95% confidence limits 0.0–3.0%) at the expense of 5.9% (95% confidence limits 3.0–10.3%) indeterminate strains that belonged to E. rhusiopathiae, Leuconostoc, Lactobacillus and S. haemolyticus. However, using these zone size breakpoints five major errors (β‐haemolytic streptococci, group B three, S. aureus one, Leuconostoc sp. one) were observed for NeoSensitabs and two major errors (β‐haemolytic streptococcus, group B one, Leuconostoc sp. one) were observed for PDM‐disc. Susceptibility testing against teicoplanin among these taxa should therefore include a determination of MIC.