Performance evaluation of oxacillin-resistant Staphylococcus aureus genotypes and taxa on human and animal blood agar culture media

The performance characteristics of growth, morphological aspects and hemolytic activities of oxacillin-resistant S. aureus (ORSA) strains were studied on twelve types of blood agar (BA) culture media (sheep, bovine, horse, rabbit and human). ORSA isolates were also previously characterized by isoenzymes genotyping and genetic and grouping analysis. Variations in the diameter of the colonies were detected among seven sets of BA media. In terms of morphology, 99, 53 and 98% presented shiny, yellow, and glossy colonies, respectively, regardless of the type of BAs. The rabbit BA favored the expression of hemolysins for most isolates (74%), followed by the human BA and other animal BAs. Certain BA media promoted the expression of hemolysins; however, the expression was correlated with a deficit in the colonial growth potential and vice-versa. The data point to the existence of two or more isolates genetically identical or highly related: (i) that either share or do not share the same wild species-specific phenotypes related to appearance without any influence from the external environment, and that are (ii) potentially virulent depending on the external environment. This study also suggestes the use of rabbit BA for the phenotypical characterization of S. aureus.   Key words: Colonial morphology, hemolysis, human and animal blood agar, oxacillin-resistant Staphylococcus aureus, clinical microbiology.