Zhen Li | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Zhen Li. This seminar is open to the general public to attend.

https://ualberta-ca.zoom.us/j/94131443113

Thesis Topic: Comparative analysis of the role of Bacillus species as food fermenting and food spoilage organisms

PhD with Dr. Michael Gaenzle.

Seminar Abstract:

Bacillus are fermenters in some food fermentation or spoilage organisms in some food products. For instance, Bacillus and fungi are the predominant organisms in daqu fermentation. However, the antifungal lipopeptides produced by Bacillus, including surfactins, fengycins and iturins, may impact fungi growth during daqu fermentation, thereby influence the final community composition. The interaction between Bacillus lipopeptides and fungi in daqu is unclear. Therefore, a simulated model of daqu was created to investigate the role of antifungal lipopeptides in situ. The results showed that iturin A displayed the strongest antifungal activities in vitro. B. velezensis FUA2155 exhibit higher antifungal activity than the two strains of B. amyloliquefaciens in the daqu fermentation model, due to the production of iturin A. Additionally, the hydrolytic enzymes produced by Bacillus spp. can cause ropy bread spoilage, which leads to economic losses in bakeries. Bacillus spp. are wheat grain endophytes and form heat resistant endospores, consequently, process hygiene and heating during baking are insufficient to prevent ropy spoilage. Therefore, to reduce the negative impact of Bacillus and increase the shelf life of bread, sourdough was investigated as a biopreservative. The results indicated more than 2 days delay of spoilage with an addition of 12 % sourdough fermented by lactic acid bacteria to bread. Bread with reutericyclin exhibited a bactericidal mode of action against strains of Bacillus.

In food industries, the high thermal loads required for inactivation of Bacillus spores can result in significant food quality losses. Pressure-assisted thermal sterilization is a promising approach for spore inactivation without the need for high thermal stress. Nevertheless, the pressure resistance of Bacillus spores varies greatly. The spoVA^2mob operon increases heat resistance of spores and redundant copies of the operons increase the heat resistance. However, the roles of spoVA^2mob operon in pressure resistance is not well explored. In this study, the pressure resistance of 17 strains of Bacillus were correlated with the copy numbers of the spoVA^2mob operon. Furthermore, to study the role of spoVA^2mob operon in pressure resistance and spore germination triggered by different germinants, the isogenic mutant strain of Bacillus subtilis 168 with insertion of spoVA^2mob operon was created. The spoVA^2mob operon increased pressure resistance and decreased pressure induced spore germination significantly.

Overall, this research provides insights into the interaction of Bacillus and other microorganisms, such as fungi and lactic acid bacteria in food fermentations, to better understand the role of Bacillus as fermenting or food spoilage organisms.