ผลของความเข้มข้นของตัวยับยั้งและปริมาณเซลล์เริ่มต้นต่อการเพิ่มจำนวนเชื้อบาซิลลัส (EFFECT OF INHIBITORS AND INITIAL CELL CONCENTRATIONS ON THE ENRICHMENT BACILLUS CEREUS )

Abstract

             Conventionally, the selectivity of Bacillus cereus in the enrichment step is only limited to the addition of 10 mg/L Polymyxin B sulfate. At this concentration, the growth of B. cereus is compromised and its selectiveness against other competing Gram-positive bacteria for industrial food samples is disputable. This research was aimed to study the effects of initial cell concentration and concentration of selective inhibitors on the recovery of B. cereus in selective enrichment step. The inhibitory effect of three conventional and alternative inhibitors (i.e., Polymyxin B sulfate, amoxicillin, and ceftriaxone) was evaluated at different initial cell densities (10¹, 10³, and 10⁵ CFU/mL) to determine the optimal B. cereus inhibitors and their suitable dosage. These selective inhibitors were added to Tryptic Soy Broth (TSB) and the recovery of B. cereus was evaluated using viable cell count on Tryptic Soy Agar (TSA). The concentrations of these selected inhibitors were formulated by doubling the suggested strength (recommended by most standard enrichment protocols) and reducing the standard concentration by half. Hence, the treatment of inhibitors was varied at 5, 10, and 20 mg/L. The cell recovery was performed at 30 ^oC for 24 h. In this research, ceftriaxone showed good preference towards the growth of B. cereus in TSB with ceftriaxone at 20 mg/L and significant toxicity against E. coli and S. aureus. At low initial cell concentrations, no viable B. cereus cell was detected in all amoxicillin treatments. Generally, higher concentrations of the selective inhibitors increased the toxicity of the selective broth and the initial cell concentration resulted in a subtle consequence and directly affected the recovery of B. cereus. The lower initial cell concentrations produced longer lag phase and sometimes produced false negative results due to the inhibitory effect of the strong inhibitor. The selectivity for B. cereus has to improve.