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ABSTRACT
Mannitol-yolk-polymyxin B agar (MYPA) was modified by supplementation with trimethoprim. The ability of the supplemented medium to select for and recover Bacillus cereus from pure cultures and food samples with high background microflora was compared with MYPA. For evaluation of the modified MYPA (mMYPA) in food samples with high background microflora, B. cereus was experimentally spiked into red pepper powder, fermented soybean paste, vegetable salad, and radish sprouts, and then it was recovered on MYPA and mMYPA for comparison. In all food samples, there was no difference in recoverability (P > 0.05) between mMYPA (red pepper powder, 3.34 ± 0.24 log CFU/g; fermented soybean paste, 3.52 ± 0.47 log CFU/g; vegetable salad, 3.51 ± 0.23 log CFU/g; radish sprouts, 3.32 ± 0.40 log CFU/g) and MYPA (red pepper powder, 3.18 ± 0.20 log CFU/g; fermented soybean paste, 3.33 ± 0.43 log CFU/g; vegetable salad, 3.36 ± 0.19 log CFU/g; radish sprouts, 3.33 ± 0.31 log CFU/g). However, mMYPA exhibited better selectivity than MYPA, because additional trimethoprim made the differentiation of suspected colonies easier by inhibiting competing flora. The addition of trimethoprim to conventional media could be a useful option to improve selectivity in foods with high background microflora.
Bacillus cereus has been increasingly recognized as a causative agent of food poisoning outbreaks (9, 15), and it has been found in various foodstuffs such as grains, vegetables, herbs, seasonings, dairy products, and meat (9, 19). Most food authorities establish quantitative microbiological standards for B. cereus in various types of foods, because B. cereus has to be present at high levels to produce sufficient toxins to cause foodborne illness (8, 11, 15, 19).
Direct plating on selective media is used as the standard quantitative evaluation method for B. cereus in food samples by many food authorities such as the U.S. Food and Drug Administration (FDA) and the International Organization for Standardization (ISO) (3, 16, 19). The key diagnostic schemes for the enumeration of B. cereus strains are their ability to hydrolyze egg yolk, but an inability to ferment mannitol (3, 19). Mannitol-yolk-polymyxin B agar (MYPA), which has been recommended by most food authorities including the FDA's Bacteriological Analytical Methods and ISO standard 7932, is based on these detection features (3, 10, 16, 17). However, the...