Incidence and importance of Bacillus species in raw milk and in the dairy environment

Beattie, Sally Heather (1997) Incidence and importance of Bacillus species in raw milk and in the dairy environment. PhD thesis, University of Glasgow.

Abstract

The objective of this project has been to further existing knowledge of the incidence, seasonality and toxinogenicity of Bacillus cereus and other Bacillus species, particularly in milk and the dairy farm environment. The occurrence of psychrotrophic and mesophilic populations of Bacillus spp. in milk were seasonal. The study conducted here demonstrated that the same seasonality trends extend to raw milk, udder washes and faecal samples taken from dairy cattle. Mesophilic and psychrotrophic Bacillus spp. populations were not found to be exclusive of the other. Psychrotrophic sporeformers were detected out with the summer-autumn season when they were anticipated. This indicates that there is a potential problem arising from the presence of psychrotrophic Bacillus spp. in milk and dairy products throughout the year. Spores of psychrotrophic strains of Bacillus spp. occurred in raw milk in the survey reported here at low levels (<1 cfu/ml). The most commonly identified species of psychrotrophic bacilli were B. cereus and B.mycoides; in addition another 14 psychrotrophic Bacillus species were also recovered during the course of the survey. Contamination of raw milk appears to be derived from the udder teat surface. The teat surface may be contaminated by faecal material or soil, contamination may also occur from soiled winter bedding. The extent of udder contamination was affected by husbandry practices. Data for viable and sporeforming psycluotrophic, mesophilic and thermoduric populations occurring in raw milk and samples from the cow and the immediate farm environment over a 13 month period are presented. The work presented here describes an improved cell cytotoxicity assay which determines colorimetrically the inhibition of cell metabolism by Bacillus toxins. The cell cytotoxicity assay had good correlation with commercial B.cereus enterotoxin detection systems. Screening of B.cereus isolates for toxin production demonstrated their widespread ability to produce toxins. Although this was the case, there was also a wide spectrum of levels of toxin produced by the isolates. Species of Bacillus other than B.cereus were shown to be cytotoxic. These species included isolates of B.mycoides, B.thuringiensis, B.subtilis, B.lentus, B.brevis, B.circulans, B.licheniformis and B.polymyxa. The effectiveness of Fourier transform infrared spectroscopy (FTIR) for the discrimination between Bacillus spp. was investigated, and was found to be a useful tool for this purpose. It was possible to distinguish between the members of the Bacillus cereus group tested. Interactions between mesophilic and psychrotrophic Bacillus species were confirmed. The factor produced by B.subtilis which inhibits B.cereus is released into its growth environment. Strains of B.subtilis which caused inhibition also had a cytotoxic effect. It was unclear whether the inhibitor and the toxin were separate compounds, and therefore the potential for exploitation of the inhibitory factor to repress B. cereus in milk or other food products is removed.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Food science, microbiology.
• Subjects: Q Science > QR Microbiology; S Agriculture > S Agriculture (General)
• Colleges/Schools: College of Science and Engineering
• Supervisor's Name: Williams, Dr. Alan
• Date of Award: 1997
• Depositing User: Enlighten Team
• Unique ID: glathesis:1997-71365
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 10 May 2019 10:49
• Last Modified: 05 Sep 2022 10:36
• Thesis DOI: 10.5525/gla.thesis.71365
• URI: https://theses.gla.ac.uk/id/eprint/71365