Abstract
Twenty-three strains of the seven recognized Listeria species were studied by using DNA-DNA optical hybridization. The level of error in the data was low. Our results supported the results of Rocourt et al. (J. Rocourt, F. Grimont, P. A. D. Grimont, and H. P. R. Seeliger, Curr. Microbiol. 7:383-388, 1982), although there was some overlap between Listeria monocytogenes and Listeria innocua. We suggest that there may be more than one cluster in the species L. monocytogenes or the species may form a large spectrum of relatedness. The level of intraspecies homology in L. monocytogenes is very broad, as determined in both this study and other studies.