Table 1

L. monocytogenes strains used in this study

Table 1
  • a The EC designation was determined with the DNA-DNA hybridization results for previously described genetic markers (30) and confirmed by multilocus genotyping (MLGT) (28).

  • b For cadmium (Cd), black and gray boxes indicate MICs of ≥140 μg/ml (growth at 70 μg/ml) and 70 μg/ml (growth at 35 μg/ml but not at 70 μg/ml), respectively. Tolerance levels were determined based on growth in the presence of the indicated amount of cadmium chloride on Iso-Sensitest agar, as described previously (23). For arsenic (Ars), black and white boxes indicate resistance and susceptibility, respectively, based on the presence or absence of growth in the presence of sodium (meta)arsenite (500 μg/ml) on Iso-Sensitest agar, as described previously (23).

  • c Black and white squares represent the presence and absence, respectively, of signal in the hybridization experiment using the probe targeting the designated resistance gene or of the amplicon derived from the primers targeting the relevant DNA probe.

  • d Black and white squares signify the presence and absence, respectively, of the amplicon when PCR was conducted with primers P4F and P4R (Fig. 1 and Table 2). The extension time was set at 1 min.

  • e MLGT was conducted as described previously (28).

  • f MICs for cadmium (Cd), arsenic (Ars, arsenite), and arsenate (another chemical form of arsenic, which is reduced to arsenite and then extruded by arsenic resistance cassettes [25]) were determined as previously described (23), using Iso-Sensitest agar plates containing different heavy metal concentrations: 10, 35, 70, 140, and 180 μg/ml of cadmium chloride (Fischer Scientific, Pittsburg, PA); 50, 125, 250, 500, 750, 1,250, 2,500, and 5,000 μg/ml of sodium (meta)arsenite (Sigma, St. Louis, MO); and 50, 125, 250, 500, 750, 1,250, 2,500, 5,000, 10,000, 12,000, 15,000, and 30,000 μg/ml of sodium arsenate dibasic heptahydrate (Sigma).