This paper presents a historical review of antimicrobial use in food animals, the causes of residues in meat and milk, the types of residues found, their regulation in Canada, tests used for their detection, and test performance parameters, with an emphasis on immunoassay techniques. The development of residue detection methods began shortly after the introduction of antimicrobials to food animal production in the late 1940s. From initial technical concerns expressed by the dairy industry to the present public health and international trade implications, there has been an ongoing need for reliable, sensitive, and economical methods for the detection of antimicrobial residues in food animal products such as milk and meat. Initially there were microbial growth inhibition tests, followed by more sensitive and specific methods based on receptor binding, immunochemical, and chromatographic principle. An understanding of basic test performance parameters and their implications is essential when choosing an analytical strategy for residue testing. While each test format has its own attributes, none test will meet all the required analytical needs. Therefore the use of a tiered or integrated system employing assays designated for screening and confirmation is necessary to ensure that foods containing violative residues are not introduced into the food chain.

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The microbiological quality of ready-to-use (RTU) vegetables, including chopped lettuce, salad mix, carrot sticks, cauliflower florets, sliced celery, coleslaw mix, broccoli florets, and sliced green peppers was determined before and after processing. Microbial profiles were obtained 24 h after processing and on days 4, 7, and 11 after storage at 4 and 10°C to simulate temperature abuse. In addition, the microbial profiles of four RTU vegetables, coleslaw mix, salad mix, cauliflower florets, and sliced green peppers were determined 7 days after distribution to a select group of Ontario hospitals. RTU vegetables, with the exception of green peppers, showed up to a 1-log decrease in aerobic colony counts after processing. These counts increased to preprocessing levels after 4 days of storage at both 4 and 10°C. RTU vegetables stored at temperature abuse conditions (10°C)had significantly higher counts (P < 0.001) on days 4 to 11 as compared to those stored at 4°C. Green peppers had the highest bacterial counts while cauliflower and chopped lettuce had the lowest counts at both storage temperatures (P < 0.05). Increased levels of Listeria monocytogenes in RTU vegetables were associated with temperature abuse. Levels of >100 MPN/g for L. monocytogenes were detected in 8 of 120 (6.7%) samples stored at 10°C but not in 175 samples stored at 4°C after 7 days (P < 0.05). Overall, L. monocytogenes was detected in 13 of 120 (10.8%) RTU vegetables stored for up to 11 days at 10°C and 5 of 176 (2.8%) samples stored at 4°C (P < 0.05). E. coli was detected in 2 of the 120 (1.7%) processed RTU vegetables after day 7 of storage at 10°C and 1 of the 65 (1.5%) unprocessed vegetables from the same batches of vegetables used for processing. This indicator organism was not detected in RTU vegetable samples stored at 4°C or in any of the RTU vegetable samples obtained from hospital coolers. Other pathogenic bacteria, such as Salmonella spp., Campylobacter spp., Yersinia enterocolitica (serotype O:3) and verocytotoxigenic E. coli (VTEC) were not detected in any of the RTU vegetables tested, Recommendations regarding processing, distribution, and storage of these products are presented.

Escherickhia coli H.I.8, an 0128 infant diarrhea isolate, produces low titers of a unique Shiga-like toxin (SLT), called SLT-IIva, which is a variant of SLT-II. We investigated induction of toxin synthesis and the putative association of a bacteriophage with toxin synthesis. Induction of broth cultures of strain H.I.8 with mitomycin yielded a 3,000-fold increase in SLT-IIva, production of a colicin, and appearance of a bacteriophage. Southern hybridization demonstrated that the genes for SLT-IIva were not carried by the bacteriophage.

Governments regulate antimicrobial residues in slaughtered animals with surveillance programs for detecting drugs in food-producing animals. Although initial screening bioassay systems are recognized for their sensitivities to antimicrobial drug groups, none are sensitive to sulfonamides at or near the maximum residue levels (MRLs) in the Codex Alimentarious. We have developed a sulfonamide-sensitive rapid assay using Bacillus stearothermophilus inoculated PM indicator agar containing bromcresol purple and trimethoprim, where the end point is a combination of color change in the agar and zone of microbial growth inhibition around the sampling disk. Five sulfonamides, plus 16 other antimicrobial drugs were tested in standard concentrations in water, bovine kidney, and ground beef. Sulfonamides were detected at concentrations near the MRLs, and they were presumptively identified using para-aminobenzoic acid. The rapid assay was extremely sensitive to beta-lactams that were presumptively identified using penase. The system also was sensitive to tetracyclines, aminoglycosides, and macrolides, of which tetracyclines and gentamicin were identified using enzyme-linked immunosorbent assay (ELISA). In trials on slaughterhouse tissues submitted for testing in Ontario's meat surveillance program, the rapid assay identified twofold the number of positive kidneys and threefold the number of positive diaphragm samples compared to a standard microbiological inhibition test (MIT) currently approved. Fifty-three of 471 carcasses were sulfonamide positive with the rapid assay, while no sulfonamides were detected with the MIT. ELISA and thin-layer chromatography were used on selected samples to confirm the rapid assay sulfonamide presumptive results.