The signal-based microbial spoilage on New Zealand export lamb : thesis submitted to Massey University for the degree of Doctor of Philosophy

Abstract

Lamb is one of New Zealand’s primary exports; however, the spoilage of lamb causes considerable financial loss. Psychrotolerant, anaerobically growing spoilage bacteria are responsible for the spoilage of fresh chilled vacuum-packaged lamb exported from New Zealand. These spoilage bacteria interact with through a system called “quorum sensing”. Different types of quorum sensing signals are produced by spoilage bacteria in response to the bacterial cell population. Through this cell-to-cell interaction, the expression of certain genes is regulated, followed by changes in bacterial activity. The main objective of this research was to determine the influence of quorum sensing signals produced by those psychrotolerant, anaerobically growing spoilage bacteria on the spoilage of New Zealand lamb.

Quorum sensing of psychrotolerant, anaerobically growing spoilage bacteria was studied using 𝘪𝘯 𝘷𝘪𝘵𝘳𝘰 and 𝘦𝘹 𝘷𝘪𝘷𝘰 methods. Two types of in vitro quorum sensing signals (Type-I and Type-II) were identified for New Zealand reference type strains 𝘏𝘢𝘧𝘯𝘪𝘢 𝘢𝘭𝘷𝘦𝘪 and 𝘚𝘦𝘳𝘳𝘢𝘵𝘪𝘢 𝘭𝘪𝘲𝘶𝘦𝘧𝘢𝘤𝘪𝘦𝘯𝘴, and Type-II quorum sensing signal was found from a lamb isolate 𝘊𝘢𝘳𝘯𝘰𝘣𝘢𝘤𝘵𝘦𝘳𝘪𝘶𝘮 𝘥𝘪𝘷𝘦𝘳𝘨𝘦𝘯𝘴. These two types of signals were also discovered in spoiled chilled vacuum-packed lamb. After random EZ-Tn5 transposon mutagenesis to 𝘭𝘶𝘹𝘐/𝘙-𝘵𝘺𝘱𝘦 𝘨𝘦𝘯𝘦𝘴 and 𝘭𝘶𝘹𝘚 𝘨𝘦𝘯𝘦𝘴, which are responsible for Type-I and Type-II signals respectively in 𝘏. 𝘢𝘭𝘷𝘦𝘪, 𝘚. 𝘭𝘪𝘲𝘶𝘦𝘧𝘢𝘤𝘪𝘦𝘯𝘴, and 𝘊. 𝘥𝘪𝘷𝘦𝘳𝘨𝘦𝘯𝘴, wild type and mutant strains were compared. Type-I quorum sensing signaling molecules influenced the expression of 𝘭𝘪𝘱𝘉 that regulates the secretion of hydrolytic enzymes produced by 𝘏. 𝘢𝘭𝘷𝘦𝘪 and 𝘚. 𝘭𝘪𝘲𝘶𝘦𝘧𝘢𝘤𝘪𝘦𝘯𝘴. These enzymes are believed to contribute to lamb spoilage. Cinnamaldehyde added to fresh vacuum-packed New Zealand lamb as an inhibitor of quorum sensing enabled an extension of the shelf-life of lamb by 2-8 days, through deactivating the Type-I quorum sensing system. Inhibition of quorum sensing has the potential in lamb preservation.