Thermophilic sporeformers from dairy processing environments

Thermophilic sporeformers are a group of bacteria that proliferate at high temperatures (mostly above 50°C as optimum growth temperatures) and can form spores as a mean to survive in unfavourable environmental conditions. They are of primary concern for food processing plants that apply high temperatures. Even though they have been reported as not pathogenic, they could pose a threat to the quality of food products when they grow because their growth produces off flavour and changes the texture of the products. For this reason, they are sometimes used as a hygiene indicator in food processing environments. Customers of intermediate products also often set strict specifications for thermophilic sporeformers. Those specifications are difficult to achieve, thus present an important challenge to the dairy industry worldwide. To meet the strict specifications, a systematic approach to understanding the source of the contamination and the transmission routes of thermophilic bacilli in dairy-concentrate processing environments is needed.

In this thesis, the composition of the natural flora of thermophilic sporeformers present along an entire dairy-concentrate processing line is studied (Chapter 2), and the environmental and physiological factors contributing to their rapid growth in dairy-concentrate processing environments is explored (Chapter 2, Chapter 3).

Furthermore, it has been reported that biofilms contribute to the persistence of thermophilic sporeformers in dairy-concentrate processing environments. For this reason, specific genes that are involved in biofilm formation and development in thermophilic sporeformers, were also subjects of study in this thesis (Chapter 4). This has produced a better understanding of the mechanism of biofilm formation in thermophilic sporeformers. Lastly, we applied comparative analysis to explore the genomic and physiological characteristics of a group of dairy and non-dairy associated thermophilic bacilli in relation to the contamination in the dairy- concentrate processing environments (Chapter 5). Altogether, the research described here has produced new insights into the life cycle of thermophilic sporeformers in dairy-concentrate processing environments. First of all, the presence of thermophilic sporeformers was observed in dairy-concentrate processing lines, particularly at locations where high temperatures were applied. Secondly, for the sporeformers to readily grow in dairy environments, the ability to utilize lactose seems essential. If this ability is absent, monomeric sugar such as glucose and galactose is required for growth in such environments. Thirdly, genes involved in certain processes, such as methionine biosynthesis and phenylalanine utilization, seem to be relevant in the formation of thermophilic bacilli biofilms. Lastly, a niacin transporter, which is only present in thermophilic bacilli that can readily grow in skim milk, appears to be a relevant genetic marker for ability to grow in dairy-concentrate processing environments.