Validation of Various Antimicrobial Interventions for Use in a Bone Dust Cabinet in a Commercial Beef Harvest Facility

ObjectivesThe objective of this study was to evaluate the efficacy of 3 antimicrobial spray interventions (peroxyacetic acid, PAA; lactic acid, LA; lactic/citric acid blend, LCA) in reducing inoculated populations of Shiga toxin-producing Escherichia coli (STEC) on pre-rigor beef tissue. A secondary objective was to validate E. coli biotype I to serve as surrogates for STEC.Materials and MethodsThe efficacy of each intervention was assessed using a 14-strain mixture of rifampicin-resistant STEC, comprised of 2 strains of E. coli O157:H7 and 2 strains each of E. coli serogroups O26, O45, O103, O111, O121, and O145. In addition, this study served to validate the utility for a non-pathogenic 5-strain mixture of rifampicin-resistant E. coli biotype I to serve as surrogates for the aforementioned STEC mixture. For 3 sampling days, 90 tissue samples from pre-rigor plate subprimals were obtained from beef carcasses immediately following slaughter. The tissue samples were evenly split into 2 inoculation groups (n = 45 samples/group, 15/group/d): i) STEC, or ii) surrogate. Within each inoculation group, tissue samples were randomly assigned to 1 of 9 treatments: i) 200 ppm PAA; ii) 1% LCA; iii) 1.5% LCA; iv) 2.5% LCA; v) 5% LA; vi) 8% LA; vii) 10% LA; viii) potable water; or ix) untreated control. The external fat surface of pre-rigor tissues samples was spot inoculated (5 to 6 log CFU/cm2) with 100 μL of the STEC or surrogate inoculum and was spread over a 50 cm2 area using a sterile plastic spreader. Within each inoculation group, treatments were applied using a custom-built, laboratory-scale spray cabinet (0.53 lpm, 137.9 kPA over 8 floodjet spray nozzles). Tissue surfaces were sampled approximately 10 min after spray-treatment application, using sponges hydrated with D/E neutralizing broth, and analyzed for surviving STEC and surrogate populations on tryptic soy agar supplemented with rifampicin (100 µg/ml). This experiment was conducted as a randomized complete block design. Data were evaluated using the MIXED procedure of SAS (SAS Inst. Inc., Cary, NC). To compare surviving populations of the surrogates and STEC, data were analyzed using the MIXED Procedure in SAS with microbial population of the untreated control samples used as a covariate to adjust least-squares means to a common pre-treatment inocluated plate count.ResultsWhen applied as a spray treatment to pre-rigor beef tissue, LA applied as a 10% solution was more (P < 0.05) effective at reducing STEC and surrogate populations than water, PAA, 1, 1.5 or 2.5% LCA, and 5 and 8% LA (Table 1). Additionally, the 5, 8, and 10% LA treatments were more (P < 0.05) effective at reducing both inoculum types than water, PAA, or 1, 1.5 or 2.5% LCA. No differences (P ≥ 0.05) in surviving STEC populations were observed for tissue samples treated with PAA, 1.5% LCA or 2.5% LCA. Pairwise comparisons indicated surviving STEC and surrogate populations did not differ (P ≥ 0.05).ConclusionWhen all treatments were compared, LA, at 10%, was found to have the greatest effect against STEC populations. As evidenced by similar surviving populations of STEC and surrogate populations, the 5-strain, non-pathogenic E. coli would effectively serve as a surrogate inoculum for the 14-strain STEC cocktail used in this study.