Master of Science - Molecular and Cell Biology / The bacterial contamination profile of a typical commercial yeast factory was assessed by three replicate microbiological surveys. In order to detect low-level contamination in samples, this study made use of a preliminary incubation technique (24h at 37°C), which boosted bacterial counts for the identification of sources of contamination. Numbers of bacteria were quantified by standard pour- and spread-plate techniques and various selective media. Raw materials were negligible in contributing to the bacterial contamination of commercial yeast, with the exception of soda ash, used to control the pH of fermentations, which contained 2 log CFU/ ml Enterococcus and aerobic bacteria. It was found that the scale up of seed yeast biomass was the primary site for contamination with Enterococcus, which progressively increased in number as the product passed down the production line. Coliforms were present at low levels, with significant increases (P < 0.05) observed during the storage of yeast cream; extrusion of compressed yeast; and packaging of dry yeast. The environment surrounding the compressed yeast production line was identified as a potential source of airborne contamination. Although Salmonella spp. and S. aureus were not detected, L. monocytogenes was isolated from compressed and dry yeast products. In addition, Bacillus spp. commonly associated with the rope-spoilage of bread, were isolated from 67% of all dry yeast product samples. Shelf-life investigations, showed that cream and compressed yeast samples were spoiled with lengthened storage periods, and especially at higher temperatures (>10°C), whilst vacuum-packaged dry yeast remained bacteriologically stable. During shelf-life studies, isolates from spoiled cream and compressed yeast samples were predominantly Lactobacillus (up to 78%), while populations of Enterococcaceae predominated in vacuum-packaged dry yeast samples (up to 68%). The use of stainless steel surfaces, attached to processing equipment used in the manufacturing of Baker’s compressed yeast, in conjunction with SEM illustrated the accumulation of yeast and bacterial cells with early stages of biofilm formation, with time. Where populations of Gram-positive members of the lactic acid bacteria family, Lactobacillus and Enterococcaceae, were isolated in the highest proportion from processing equipment surfaces used in the manufacturing of Baker’s compressed yeast (81-100%).