Identification and enumeration of virulent Listeria strains

Recent outbreaks and a continuous increase in cases of listeriosis underscore the need for rapid, sensitive and reliable techniques to detect Listeria. Of the species of Listeria, only L. monocytogenes has been found to be associated with human infections. One factor which definitely contributes to its pathogenicity is the presence of hemolysins, although L. ivanovii and L. seeligeri also elaborate hemolysins. Based upon cloned hemolysin genes, we have developed DNA probes specifically for detecting L. monocytogenes. The technique combines growth of bacterial colonies on selective agar plates and DNA hybridization of these colonies on a solid matrix. This technique permits identification and enumeration, and the entire procedure can be completed in 3-4 days. Our method was found to be suitable for identifying and enumerating this organism in various foods, the main vehicle of human infection. Advantages and disadvantages of this technique are discussed and compared with other existing techniques.