FITNESS COSTS ASSOCIATED WITH CLASS IIA BACTERIOCIN RESISTANCE IN LISTERIA-MONOCYTOGENES B73

In order to assess the potential for the spread of class IIa bacterioc in resistance in natural populations of Listeria monocytogenes, the fi tness costs associated with resistance to leucocins A, B and E and sak acin A in L. monocytogenes B73 in the absence of bacteriocin were exam ined. The resistant phenotype had a lower growth rate (and thus relati ve fitness) than the sensitive phenotype in monoculture experiments. F urthermore, resistant phenotypes were unable to invade populations of the sensitive strain, even at frequencies of 10(-1) or higher, when gr own in co-culture. These results held true for resistant strains that had been exposed to bacteriocin for 25 successive growth cycles. It wa s concluded that the class IIa bacteriocin-resistant phenotype of L. m onocytogenes B73 is unlikely to become stable in natural populations b ased on this evidence. Due to the possibility of variations in the fre quencies of spontaneous mutation and fitness among Listeria strains, h owever, the extrapolation of these results to the species as a whole s hould not be made.