Susceptibility of Listeria monocytogenes and Listeria innocua strains isolated from short-ripened cheeses to some antibiotics and heavy metal salts

Twenty-eight isolates of Listeria monocytogenes and 18 of L. innocua obtain ed from short-ripened cheeses manufactured in Asturias (northern Spain) wer e screened for antibiotic and heavy metal salt susceptibility patterns. No remarkable differences were detected between L. monocytogenes and L, innocu a or between clusters previously defined in each species by pulsed-field ge l electrophoresis analysis of the chromosomal DNA. beta -lactams, tetracycl ine, erythromycin and rifampicin were the most effective antibiotics with M IC (minimum inhibitory concentration) variations within the species not exc eeding one order of dilution. Chloramphenicol, streptomycin, kanamycin, gen tamicin and neomycin displayed poor activity and their MICs showed a wider range of values than the other antibiotics. Significant rates of low-level resistance to streptomycin, kanamycin and gentamicin were also recorded Fos fomycin was not effective against Listeria as the isolates were resistant t o high levels of this antibiotic. All strains displayed high susceptibility to mercuric chloride and were resistant to high lead nitrate concentration s. Susceptibility to ferric citrate, cupric, zinc and cadmium sulfate was i ntermediate between that of mercuric chloride and lead nitrate. All isolate s had similar susceptibility patterns against mercury, iron, zinc and lead salts. However, one L. monocytogenes strain showed lower tolerance to cupri c sulfate than the remaining population and the MICs for cadmium sulfate va ried in a wider range of concentrations than for the other salts. Clearly d ifferentiated resistant strains with respect to a more sensitive population were not detected for any antimicrobial agent These data suggest a moderat e selective pressure exerted by aminoglycosides, cadmium and cupric sulfate but not by other antimicrobials in dairy farm environments of Listeria iso lates tested (C) 2001 Academic Press.