that only 40% of them were positive for both genotypic and phenotypic characters. This confirms that some strains had “silent” genes when the presence of the determinant using PCR was observed but this was not phenotypically expressed. Moreover, one of the isolates was a gelatinase producer, but did not show the gelE gene. Comparison of b-hemolysis and detection of the cyl gene revealed that cytolysin determinants behave as “silent” genes in two non-hemolytic isolates.