การหาสภาวะที่เหมาะสมในขั้นตอนการเพิ่มจำนวนเพื่อปรับปรุงการคัดเลือกเชื้อลิสทีเรีย (MEDIA OPTIMIZATION OF ENRICHMENT PROTOCOL TO IMPROVE LISTERIA SELECTIVITY)

Abstract

          Listeria innocua growth and selectivity during selective enrichment step were studied to formulate fundamental knowledge to enhance Listeria spp. detection protocol. L. innocua was used in this study as a non-pathogenic Listeria model because it shares similar physiological traits with Listeria monocytogenes. TSB, an effective non-selective enrichment medium, was selected to enhance the growth of L. innocua in the selective enrichment step. The effects of conventional inhibitors were studied by using common selective inhibitors suggested in most global icrobiological detection standards (i.e., NGFIS, IDF, USFDA, NMKL, ISO, AOAC, and USDA-FSIS). The TSB broth base was modified by adding some conventional selective agents to study the growth and selectivity of Listeria spp. The growth profiles showed that acriflavine significantly affected the growth of Gram-positive bacteria. Polymyxin B inhibited Gram-negative bacteria as well as Gram-positive bacteria but to a lesser extent. As low as 10 mg/L of Polymyxin enabled effective reduction of E. coli from 6 to less than 2 log CFU/mL within 2 h of incubation. Nalidixic acid only suppressed Gram-negative bacteria and was not as effective as Polymyxin B. Although lithium chloride did not significantly affect the inhibition of any target microorganisms. Response surface analysis indicated that the optimum selective agents included 5.7 mg/L of acriflavine, 10.0 mg/L of Polymyxin B and 20.7 g/L of lithium chloride. This new media formula was proved to isolate E. coli and S. aureus from L. innocua.