In vitro susceptibility of Listeria monocytogenes: comparison of the E test with the agar dilution test

In vitro susceptibility testing was performed on 66 strains of Listeria monocytogenes. Recently obtained clinical isolates from Austria, France, Norway and Switzerland and 17 reference strains from three type culture collections were tested against ampicillin, gentamicin, trimethoprim-sulfamethoxazole, ampicillin-sulbactam, meropenem, and first-, second-, third- and fourth-generation cephalosporins. All isolates were susceptible to ampicillin, gentamicin, and trimethoprim-sulfamethoxazole, ampicillin-sulbactam, and meropenem and more than 86% of the minimal inhibitory concentration E test values were within plus/minus one dilution step of the agar dilution test. In vitro susceptibility of L. monocytogenes has not changed markedly during the last decades; the level of susceptibility of old reference strains did not differ from that of recently encountered clinical isolates. The E test seems to be a suitable method for Listeria.