2002 Volume 19 Issue 2 Pages 57-63
Modified EC broth (mEC) with novobiocin (20 mg/L) is widely used for the isolation of Escherichia coli O157: H7 from foods. The mEC with novobiocin at 20 mg/L (mEC+20n) is usually prepared by autoclaving mEC without novobiocin at 121°C for 15 min and then adding a filter sterilized aqueous solution of sodium novobiocin after cooling to produce a final concentration of 20 mg/L. In Japan, mEC containing 25 mg/L novobiocin [25n mECA (Eiken) and 25n mEC-B (Kyokuto)] autoclaved at 121°C for 15 min is used for the isolation of E. coli O157: H7 from foods.
We compared bacterial growth in 10 concentrations of novobiocin and mEC with inoculums of 10 cfu and 105 cfu using 6 strains of E. coli O157: H7 at 42°C for 18 hr. At the 10 cfu inoculum level growth was inhibited in 25n mEC-A and mEC with novobiocin 25 mg/L (added after autoclaving).
We then compared 4 concentrations of novobiocin and mEC at recovering E. coli O157: H7 from food samples with both direct plating and IMS plating (immunomagnetic separation method). The eighteen food samples inoculated with E. coli O157: H7 (0.1-23.3 cfu/g or ml) were incubated at 42°C for 18 hr. E. coli O157: H7 was isolated from 18 (100%) with 20n mEC, 16 (89%) with mEC + 20n, 15 (83%) with 25n mEC-A, and 16 (89%) with 25n mEC-B using the IMS plating.
The stability after 3 years storage of 20n mEC at 40°C, 30°C, room temperature, 10°C, and 4°C was examined for growth of E. coli O157: H7, for inhibition of other bacteria, for pH, and for moisture (%). The growth of E. coli O157: H 7 and the inhibition of Grampositive bacteria and Serratia marcescens on mEC with novobiocin plate were the same as the control medium. The pH and moisture after 3 years storage for 20n mEC were almost the same as the original medium under room temperature condition.
We have found 20n mEC is less selective than mEC + 20n and more selective than mEC. The 20 n mEC is easy to prepare without adding a filter-sterilized aqueous solution of sodium novobiocin to produce a final concentration of 20 mg/ L after the medium cooled. The 20n mEC is a practical medium for screening large numbers of food samples as well as samples involved in disease outbreaks.