Detection of two phospholipases C by means of plate tests for the rapid identification of pathogenic Listeria monocytogenes

Two simple plate tests that detected the presence of phosphatidylinositol-specific phospholipase C (PI-PLC) and phosphatidylcholine-phospholipase C (PC-PLC) were used to assess their suitability as a reliable and rapid means of differentiating the pathogenic from non pathogenic strains of Listeria monocytogenes, as an alternative to mouse test. Eighty-seven strains of Listeria monocytogenes belonging to serovars 1 and 4 and two untypable strains, from food and human samples, were examined. Thirty-nine strains did not produce PI-PLC and four of these were also negative for PC-PLC. Moreover, these strains showed a weak haemolytic activity on blood agar plates. The strains that did not exhibit activity were subjected to PCR amplification to detect the related genes (plcA and plcB), the gene encoding haemolysin (hlyA) and their genetic regulator (prfA). The presence of the four genes was demonstrated in all the strains. However, both the Listeria monocytogenes strains which lacked the expression of the two phospholipases and those which produced the two enzymes were positive in the mouse test, which thus remains the most reliable test for differentiating pathogenic strains from non-pathogenic strains of Listeria monocytogenes.