Abstract
Food can be contaminated with disease-causing microorganism. With respect to rice and rice-containing products Bacillus cereus group spp. are of particular concern as are frequently found in rice and can cause diarrhoea and emetic episodes. In this study locally purchased samples of rice grain, rice products and paddy soil samples were tested for presence of B. cereus group spp. using selective media, molecular analysis, phenotyping, whole genome sequencing and RNA-seq.B. cereus was successfully isolated from 24% of the rice and rice-containing products and from 100% of the rice paddy soil samples. Ten out of 155 isolates obtained from rice grain or rice products were identified as B. thuringiensis. All isolates were shown to harbor genes associated with virulence, food poisoning outbreaks and environmental persistence. This included genes encoding diarrhoeal toxins but none of the isolates showed presence of the emetic toxin gene. All isolates also showed resistance to multiple antimicrobial compounds, tolerance to low pH and high osmotic stress. All strains investigated could therefore be classified as pathogens that harbor toxin genes as well as genes that facilitate resilience to a range of environmental stresses and could therefore, once established, potentially be difficult to eliminate from food production and clinical environments.
Eight strains were selected based on phenotypic characteristics to be genome sequenced and compared to publicly available genome data of B. cereus for identification of virulence and environmental stress resistance genes. Paired lllumina reads were mapped to publicly available B. cereus reference genomes as well as de novo assembled with velvet followed by genome annotation. All eight draft genomes generated showed presence of genes involved in osmotic stress response, antimicrobial resistance and toxin genes in their draft genome assemblies. Following this, RNA sequencing (RNA-Seq, lllumina Hiseq 4000) of one sensitive (F7B1) and one resistant (X22B) B. cereus strain was employed for comparison of gene expression responses to osmotic stress and/or tetracycline. Overall the sensitive F7BI showed a stronger response when exposed to stress, but there were also many overlapping responses and some genes that were only induced in the resistant X22B. The combined results implicate that genes involved in sigma B stress response regulation, I 111 operon, oppA (ABC transporter oligopeptide), ESAT-6 protein secretion system, sortase and capsular polysaccharides biosynthesis present good targets for the further study of the molecular mechanisms behind increased resistance to osmotic and tetracycline stress in environmental B. cereus strains.
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| Date of Award | Dec 2019 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Sponsors | Government of Thailand |
| Supervisor | Caroline Meharg (Supervisor), Christopher Elliott (Supervisor), Irene R. Grant (Supervisor) & Seamus Fanning (Supervisor) |