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Antibiotic Susceptibility and Resistance of Staphylococcus Chromogenes from Bovine Mastitis
© The Author(s); licensee BioMed Central Ltd. 2003
- Published: 31 March 2003
- Susceptibility Pattern
- mecA Gene
- Subclinical Mastitis
An unselected series of seventy-six strains isolated from subclinical mastitis from forty-four dairy herds in Flanders, Belgium, tentatively identified as Staphylococcus chromogenes was examined for confirmation of identification and antibiotic susceptibility patterns.
The presumptive identification was only based upon complete absence of hemolysis and a typically weakly positive DNase test, usually with yellow pigmentation. A confirmatory identification was performed by tRNA intergenic spacer PCR as described by Maes et al. (1997, J. Clin. Microbiol. 35, 2477–2481). Antibiotic resistance profiles to several antibiotics were determined as MIC's (Minimal Inhibitory Concentrations) using agar plate dilution on Mueller-Hinton II medium (BBL) according to NCCLS guidelines. The presence of β-lactamase was tested by visual inspection of the margins of penicillin disk diffusion zones (positive when heaped up with fully grown colonies), and the nitro-cefin test (Oxoid). mecA Gene determination with PCR was carried out only on penicillin-resistant strains in order to detect methicillin-type resistance (resistance to β-lactamase-resistant penicillins and cefalosporins).
Susceptibility of S. chromogenes from bovine mastitis (n = 62).
Twenty-four strains of S. chromogenes were β-lactamase-positive and the presence of β-lactamase was associated with pigmentation (Fisher Exact test: P = 0.002). Of the fifty-one yellow-pigmented strains only fifteen were b-lactamase positive, while nine out of eleven white colonies were β-lacta-mase positive. The results of visual inspection of the margins of penicillin inhibition zones completely corresponded to those of nitrocefin tests. The mecA gene, indicating methicilline resistance, was found in two strains. One of these strains was not pigmented, DNase negative, non-hemolytic, tube coagulase-negative, clumping factor-negative, lipase-negative, lecithinase-negative and novo-biocin-resistant and proved to be a highly atypical S. aureus. This was confirmed by tRNA intergenic spacer PCR, femB gene determination, and acriflavine (8 μg/ml) resistance. A second strain was identified as S. sciuri.
The dilution method, with the NCCLS recommended low number of cells (105), is not suitable to determine susceptibility and resistance to penicillins and cephalosporins in staphylococci. In comparison with reports of the susceptibility patterns of S. aureus, S. chromogenes appeared to have a similar antibiotic resistance profile, though the resistance rates may be somewhat lower.
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