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Incidence and characterization of Bacillus cereus isolates contaminating dairy products
1Department of Microbiology, Soochow University, Taipei, Taiwan, Republic of China.
Abstract:
A total of 293 dairy products purchased from local markets were examined to determine the incidence of and characterize Bacillus cereus. Isolations were made on mannitol-egg yolk-polymyxin B agar medium and confirmed by several staining and biochemical tests. B. cereus occurred in 17% of fermented milks, 52% of ice creams, 35% of soft ice creams, 2% of pasteurized milks and pasteurized fruit- or nut-flavored reconstituted milks, and 29% of milk powders, mostly in fruit- or nut-flavored milk mixes. The average population of B. cereus in these dairy products was 15 to 280 CFU/ml or CFU/g (range, 5 to 800). The characteristics of these B. cereus isolates in terms of heat resistance, biochemical reactions, and antibiotic susceptibility were similar to previously reported data except for a higher utilization of sucrose. Some isolates were especially resistant to carbenicillin, nalidixic acid, streptomycin, and tetracycline. The MICs for the isolates were also determined. All of the tested isolates lysed rabbit erythrocytes; 98% showed verotoxicity, 68% showed cytotonic toxicity for CHO cells, and 3 of 11 selected isolates that showed strong hemolysin activity killed adult mice.
Insights
Bacillus cereus was detected in 17% of fermented milks and 52% of ice creams. This common bacterium showed varying levels of toxicity and antibiotic resistance in dairy products.
Area of Science:
- Food Microbiology
- Bacterial Pathogenesis
- Dairy Science
Background:
- Bacillus cereus is a foodborne pathogen that can cause illness through toxins.
- Dairy products are potential vehicles for B. cereus contamination.
- Understanding the prevalence and characteristics of B. cereus in dairy is crucial for food safety.
Purpose of the Study:
- To determine the incidence of Bacillus cereus in various dairy products.
- To characterize the biochemical and toxicological properties of isolated B. cereus strains.
- To assess the antibiotic susceptibility of B. cereus isolates from dairy sources.
Main Methods:
- Sampling of 293 dairy products from local markets.
- Isolation and identification of B. cereus using mannitol-egg yolk-polymyxin B agar.
- Confirmation through staining and biochemical tests.
- Evaluation of heat resistance, biochemical reactions, and antibiotic susceptibility.
- Determination of Minimum Inhibitory Concentrations (MICs).
- Assessment of hemolytic, verotoxic, and cytotoxic activities.
- Mouse lethality studies for highly hemolytic isolates.
Main Results:
- B. cereus was found in 17% of fermented milks, 52% of ice creams, 35% of soft ice creams, 2% of pasteurized milks, and 29% of milk powders.
- Average B. cereus populations ranged from 15 to 280 CFU/ml or CFU/g.
- Isolates exhibited characteristics similar to previous reports, with notable higher sucrose utilization.
- Some isolates showed resistance to specific antibiotics like carbenicillin, nalidixic acid, streptomycin, and tetracycline.
- All isolates lysed rabbit erythrocytes; 98% were verotoxic, and 68% showed cytotonic toxicity.
- Three of 11 highly hemolytic isolates were lethal to adult mice.
Conclusions:
- Bacillus cereus is prevalent in various dairy products, particularly ice creams and milk powders.
- Dairy-associated B. cereus strains possess pathogenic potential, including toxin production and antibiotic resistance.
- Further monitoring and control measures are necessary to ensure the safety of dairy products regarding B. cereus contamination.