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 obtained from short- ripened cheeses manufactured in Asturias (northern Spain) were screened for antibiotic and heavy metal salt susceptibility patterns. No remarkable differences were detected between L. monocytogenes and L. innocua or between clusters previously defined in each species by pulsed-field gel electrophoresis analysis of the chromosomal DNA. β -lactams, tetracycline, erythromycin and rifampicin were the most effective antibiotics with MIC (minimum inhibitory concentration) variations within the species not exceeding one order of dilution. Chloramphenicol, streptomycin, kanamycin, gentamicin 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. Fosfomycin was not effective againstListeria as the isolates were resistant to high levels of this antibiotic. All strains displayed high susceptibility to mercuric chloride and were resistant to high lead nitrate concentrations. Susceptibility to ferric citrate, cupric, zinc and cadmium sulfate was intermediate between that of mercuric chloride and lead nitrate. All isolates had similar susceptibility patterns against mercury, iron, zinc and lead salts. However, one L. monocytogenes strain showed lower tolerance to cupric sulfate than the remaining population and the MICs for cadmium sulfate varied in a wider range of concentrations than for the other salts. Clearly differentiated resistant strains with respect to a more sensitive population were not detected for any antimicrobial agent. These data suggest a moderate selective pressure exerted by aminoglycosides, cadmium and cupric sulfate but not by other antimicrobials in dairy farm environments of Listeria isolates tested.