Enterobacteriaceae in ground meats

Abstract

Presumptive Escherichia coli counts for 312 samples of non-frozen ground beef were determined and compared with proposed Canadian standards. Results for frozen pork sausages, packaged at manufacturer level, indicated little difference in distribution of presumptive E. coli loads compared with retail ground beef. Use of solid media and direct inoculation of EC broth at 45 °C did not give alternative, rapid methods of estimating E. coli loads in ground beef. Counts on violet red bile agar (VRBA) within 18–24 h incubation at 35 °C gave reliable estimates of coliform bacteria (bile-precipitating colonies) and Enterobacteriaceae (total count), with only 1.3 and 10.7% false positives, respectively. Bile-precipitating isolates from VRBA were primarily E. coli, also Serratia liquefaciens, aerogenic Enterobacter agglomerans, Enterobacter cloacae, Citrobacter freundii, and Klebsiella pneumoniae. Non-bile-precipitating colonies were primarily aerogenic E. agglomerans and S. liquefaciens; however, in the most probable number technique E. agglomerans was screened out. In addition to E. coli, E. agglomerans and S. liquefaciens were the principal types of Enterobacteriaceae in these samples. Enterobacter agglomerans gave a variety of IMViC reactions, including the type I (++−−) reaction, whereas S. liquefaciens were predominantly IMViC type −−++. Incubating EC broth at 45.5 °C, as opposed to 44.5 °C, reduced the number of false positives.