Fitness costs associated with class IIa bacteriocin resistance in Listeria monocytogenes B73.

In order to assess the potential for the spread of class IIa bacteriocin resistance in natural populations of Listeria monocytogenes, the fitness costs associated with resistance to leucocins A, B and E and sakacin A in L. monocytogenes B73 in the absence of bacteriocin were examined. The resistant phenotype had a lower growth rate (and thus relative fitness) than the sensitive phenotype in monoculture experiments. Furthermore, resistant phenotypes were unable to invade populations of the sensitive strain, even at frequencies of 10(-1) or higher, when grown in co-culture. These results held true for resistant strains that had been exposed to bacteriocin for 25 successive growth cycles. It was concluded that the class IIa bacteriocin-resistant phenotype of L. monocytogenes B73 is unlikely to become stable in natural populations based on this evidence. Due to the possibility of variations in the frequencies of spontaneous mutation and fitness among Listeria strains, however, the extrapolation of these results to the species as a whole should not be made.