Correlation between the presence of virulence-associated genes as determined by PCR and actual virulence to mice in various strains of Listeria spp.

Five chromosomal genes, prfA, plcA, hlyA, mpl and plcB, are implicated in the virulence of Listeria monocytogenes and some of these genes have been used for the identification of bacteria by polymerase chain reaction (PCR). Using 6 strains of L. monocytogenes and 3 L. innocua strains, the relationship was examined between the presence of five virulence-associated genes and actual virulence to mice in terms of 50% lethal dose (LD50), bacterial viability in the organ of infected mice and the intracellular growth in cultured macrophages. None of the five genes could be amplified by PCR in all the L. innocua strains and they were actually avirulent to mice. All L. monocytogenes strains were shown to be virulent and to have intact virulence-associated genes except for the strain ATCC15313. This particular strain was revealed to be avirulent and defective in hlyA and plcA in PCR amplification. It was suggested that PCR detection of genes prfA, mpl, or plcB may not be sufficient to detect virulent strains of L. monocytogenes. It appeared that the ability to produce listeriolysin O (LLO), which is encoded by hlyA, was critical for the expression of virulence regardless of the amount of LLO produced.