Tellurite Resistance in Shiga Toxin-Producing Escherichia coli

Potassium tellurite (K 2 TeO 3 ) is an effective selective agent for O157:H7 Shiga toxin-producing Escherichia coli (STEC), whereas tellurite resistance in non-O157 STEC is variable with information on O45 minimal. High-level K 2 TeO 3 resistance in STEC is attributable to the ter gene cluster with terD an indicator of the cluster's presence. Polymerase chain reactions for terD and K 2 TeO 3 minimum inhibitory concentration (MIC) determinations in broth cultures were conducted on 70 STEC and 40 non-STEC control organisms. Sixty-six STEC strains (94.3%) were terD+ compared to 28 control organisms (70.0%; P < 0.001). The prevalence of terD in O103 STEC strains was 70%, whereas in all other serogroups it was ≥ 90%. The K 2 TeO 3 geometric mean MIC ranking for STEC serogroups from highest to lowest was O111 > O26 > O145 > O157 > O103 > O121 = O45. The K 2 TeO 3 geometric mean MIC was significantly higher in terD+ than in terD- STEC, but not in terD+ versus terD- control strains. Resistance to K 2 TeO 3 (MIC ≥ 25 mg/L) was exhibited by 65/66 terD+ and 0/4 terD- STEC strains, compared to 12/28 terD+ and 8/12 terD- control strains. These results confirm previous studies showing the significantly higher prevalence of the ter gene cluster in STEC strains, and the relationship between these genes and K 2 TeO 3 resistance in STEC and especially intimin (eae)-positive STEC, in contrast to non-STEC organisms. O45 and O121 STEC, although frequently terD positive, on average had significantly lower levels of K 2 TeO 3 resistance than O26, O111, and O145 STEC.