Abstract

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Food preservation processes are usually applied with the aim of slowing down or preventing spoilage and pathogenic bacteria in food materials. This involves the application of one or more environmental stresses (or hurdles) in the form of extremes of temperature, pH, and the manipulation of other optimal growth conditions.Environmental stress that may result from the use of hurdles, has however been shown to play a role in the emergence of antimicrobial resistance. This study was conducted with the aim of studying the combined effects of pH, low storage temperature and storage period on the antibacterial susceptibility of enterotoxigenic Bacillus cereus isolates from pasteurized cow-milk to selected antibacterial agents. Mcfarland standard 1 (about 8.5 log units) of a confirmed B. cereus isolate from raw milk was inoculated into sterile pasteurized cow-milk samples at varying pH levels (6.3.6.4,6.7,6.8), and stored at refrigeration temperature (4-100C) for a period of 72 h. Isolates were assayed for the B. cereus diarrhoel enterotoxin haemolysin BL (HBL) production using B. cereus enterotoxin reversed passive agglutination (BCET-RPLA) toxin detection kits (OXOID) and then subjected to antibacterial susceptibility tests using the Kirby-Bauer method against 10 antimicrobial agents (OXOID). Antibacterial agents included Ampicillin (10μg), Cephalothin (30μg), Amoxicillin-clavulanic acid(20/10μg), Cefpodoxime (10μg) Ceftriaxone (30μg), Erythromycin (30μg), Amikacin (30g), Tetracycline (30μg)Ciprofloxacin (5μg) and Trimetoprim Sulfamethoxazole (1.25/23.75μg). Results revealed resistance of all isolates to Ampicillin, Amoxicillin-clavulanic acid, Cefpodoxime, Ceftiaxone and Trimetoprim Sulfamethoxazole. All isolates were sensitive to Erythromycin, and Ciprofloxacin.Varying patterns were observed for Cephalothin, where only isolates from milk at pH 6.8 stored for 72 h and which exhibited suppressed toxigenic ability, were sensitive. Similarly, all isolates showed susceptibility to Tetracycline except for isolates at pH 6.3 which showed less sensitivity (intermediate response) to the antibacterial agent over a 48 h period. Findings suggest pH and storage conditions of foods could induce varying degrees of susceptibility to antibacterial agents in some enterotoxigenic B. cereus isolates.

Keywords: B. cereus, Haemolysin BL, low temperature storage, pH, antibacterial susceptibility