Comparison of the Recovery of Escherichia coli from Frozen Foods and Nutmeats by Confirmatory Incubation in EC

FISHBEIN, MORRIS (Food and Drug Administration, Washington, D.C.), AND BERNARD F. SURKIEWICZ. Comparison of the recovery of Escherichia coli from frozen foods and nutmeats by confirmatory incubation in EC medium at 44.5 and 45.5 C. Appl. Microbiol. 12:127-131. 1964.-The productivity of confirmatory EC broth for the isolation of fecal Escherichia coli was determined at 44.5 and 45.5 C. A variety of frozen precooked foods and an assortment of nutmeats were examined after primary incubation in Lauryl Sulfate Tryptose (LST) broth. In 85.3% of the cases, the parallel tubes of EC broth incubated for 24 hr at 44.5 and 45.5 C gave rise to identical E. coli responses of positive, false positive, and negative. The remaining 14.7% of the reactions represent the qualitative difference between the two temperatures. The EC test at 45.5 C was more specific for E. coli, since twoto threefold fewer false positives were produced at this temperature than at 44.5 C. However, fecal E. coli recoveries were slightly higher (4%Vc) at the lower temperature. Incubating the EC tubes from the interval of 24 to 48 hr gave rise to an additional 4.3% of E. coli recovery, but this was accompanied by an excessive production of false positives (75.9%), representing a 3.5-fold decrease in specificity. It is recommended that, in the confirmatory use of EC broth in the examination of frozen foods and nutmeats for the recovery of fecal E. coli, the test be made at 45.5 C in a water bath and limited to 24 hr of incubation only, to insure optimal specificity. During the study, a "fixed" productivity ratio was noted; E. coli+/LST+ equaled approximately one-fourth or 25%. The significance of this ratio is discussed. The utilization of the coliform group as an indicator of fecal pollution is historically based on the development of sanitary public water supplies and the problems inherently associated with such supplies. After an experience of well over half a century, this index of pollution remains unaltered. A system of bacterial methodology has been devised, elaborated, and refined through the years to seek out this group of indicator organisms in our water supplies (American Public Health Association, 1960). As a result the public water supply, although handled in massive volumes and consumed by nman in enormous quantities, is a truly safe substance bacteriologically. The application of the identical methodology to solid foodstuffs is another matter. The coliform group is still the indicator group, or more specifically, Escherichia coli may be sought for, but the problem of isolation and identification becomes more complicated. This is because the foodstuff itself is not inert but may affect the physicobiochemical environment in which the organism-medium reaction occurs; the associated bacterial flora may be of a nature to compete more successfully with the indicator organism; and finally the handling and storage of the foodstuffs may debilitate the indicator organism. MacConkey (1901) and Eijkman (1904) used elevated temperatures for the recovery of coliforms, and indeed the British have had a continuing experience with MacConkey's broth in the approximate range of 44 C (Wilson et al., 1935; Bardsley, 1938; Sherwood and Clegg, 1942; Taylor, 1945; Taylor, Jones, and Franklin, 1951: Thomas et al., 1955). In the United States there has been a similar experience with incubation at elevated temperatures for the fecal indicator organism (Williams, Weaver, and Scherago, 1933; Levin, Epstein, and Vaughn, 1934; Ostrolenk and Hunter, 1940; Stuart et al., 1942; Hajna and Perry, 1943; Geldreich et al., 1958). However, the experience has been varied both as to the temperature and the medium employed. In more recent years, the application of EC Medium appears to have become somewhat stabilized (Geldreich et al., 1958; Tennant, Reid, and Rockwell, 1959; Shelton et al., 1961; Fishbein, 1962). Nevertheless, the matter of the proper conditions and temperature of incubation of EC Medium is still subject to wide variation in America. Incubation of EC iMedium has been by either water bath or air incubator, at temperatures of 44.5, 45.0, or 45.5 C. Substances examined have been water, shellfish, and sewage (Perry and Hajna, 1944); water (Wattie, 1948; Geldreich et al., 1958); seawater (Tennant et al., 1959); precooked frozen foods (Shelton et al., 1961); and seafoods (Raj and Liston, 1961). There may well be more than one answer, depending upon the foodstuff