Detection of different Staphylococcus aureus strains in bovine milk from subclinical mastitis using PCR and routine techniques

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Abstract
Contamination of fresh milk with Staphylococcus aureus was assessed comparatively through routine phenotypic (coagulase tube test and coagulase slide test) and genotypic (PCR) screening of 128 S. aureus strains isolated from 555 milk samples. These samples were collected from 362 cows with subclinical mastitis, hosted in different dairy herds at various locations of the Northern and Northeastern rural areas of the State of Rio de Janeiro, 39.7% of which were CMT-positive. All S. aureus isolates tested positive for the presence of the coagulase gene by PCR and the isolates could be grouped into four distinct classes according to the size of the PCR product. The strains also yielded variable results when assayed with coagulase test. Taken together, these data indicate the existence of extensive polymorphism at the coagulase gene locus in the genus Staphylococcus and exemplifies the extent of molecular and phenotypic heterogeneity associated with the strains circulating in rural herds. Quinhentas e cinqüenta e cinco amostras de leite, provenientes de 362 vacas com mastite subclínica em diferentes propriedades rurais do Estado do Rio de Janeiro, Brasil, de 1995 a 1997, foram submetidas ao teste "Califórnia Mastitis Test" (CMT). 39,7% das amostras foram positivas, das quais foram isoladas 128 cepas de Staphylococcus aureus. Todas as cepas isoladas foram positivas para o gene da coagulase utilizando a técnica de PCR, todavia, resultados de coagulase através das técnicas em tubo e "coagulase slide test" foram variáveis. Após a amplificação do gen de coagulase através da técnica de PCR utilizando iniciadores específicos para o referido gen, fragmentos com diferentes pesos moleculares foram vistos através de análise em gel de agarose, sugerindo a ocorrência de polimorfismo genético. O estudo também sugere a ocorrência de diferentes cepas da bactéria atuando nos rebanhos leiteiros causando mastite bovina

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