Molecular typing of Listeria monocytogenes isolated in Japan by pulsed-field gel electrophoresis.

A total of 40 strains of Listeria monocytogenes which have been demonstrated to be serovar 1/2a, 1/2b, and 4b were genotyped by pulsed-field gel electrophoresis (PFGE) after separate digestion with Apa I, Asc I, Sma I, and Sse 8387 I. Twenty-seven unrelated strains including four representative strains showed distinctly different genotypes according to their PFGE profiles. Then nine strains isolated from shredded cheese of different lots and four strains isolated from the cheese-processing environment were shown to display the same genotype. Therefore, it is suggested that the Listeria was spread in cheese by cross-contamination from the cheese-processing environment. Thus, PFGE analysis has a good typeability and excellent discriminatory power, and has provided a useful tool for investigation of the source of Listeria contamination.