Browsing by Author "Zhang, Yufeng"

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    The signal-based microbial spoilage on New Zealand export lamb : thesis submitted to Massey University for the degree of Doctor of Philosophy
    (Massey University, 2019) Zhang, Yufeng
    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.