An investigation into the effects of a simulated human gastro-intestinal tract has on Bacillus cereus and Bacillus weihenstephanensis viability and pathogenicity

Food poisoning--Pathogenesis;Bacillus cereus;Bacillus weihenstephanensis;Gastrointestinal system--Physiology;Virulence (Microbiology)

Hillhouse, Elizabeth Ann ; Coote, Peter John ; Coote, Peter John

University:University of St Andrews

Department:Biology (School of)

关键词: Food poisoning--Pathogenesis;    Bacillus cereus;    Bacillus weihenstephanensis;    Gastrointestinal system--Physiology;    Virulence (Microbiology);   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/3203/ElizabethAnnHillhousePhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

Bacillus cereus is one of the known causes of diarrhoeal food poisoning. Intheir natural environment of soil surviving as spores facilitates theircolonisation of raw food ingredients enabling their access to the food chain.Recently psychrotrophic strains of B. cereus have been reclassified based ondivergent cold shock gene (cspA) sequences and renamed B.weihenstephanensis. It is the modified cspA gene that is thought to confer thepsychrotolerant phenotype witnessed by these strains. Aside from cspA, B.cereus and B. weihenstephanensis are closely related, leading to questionsabout its pathogenicity and ability to mediate diarrhoeal food poisoningoutbreaks.Food producers use a variety of processes to limit microbial contaminationwithin food products. Although effective against vegetative cells, spores areoften resistant and as such can persist within this environment. Chilledtemperatures (4°C) are often used to limit the growth of any contaminatingmicrobes. Under such conditions B. cereus spores would remain dormanthowever B. weihenstephanensis spores have been shown to germinate andoutgrow under refrigerated conditions. This could result in the consumption ofboth B. cereus and B. weihenstephanensis spores and vegetative cells. Theeffect that the human gastro-intestinal tract (GI) has on B. cereus and B.weihenstephanensis vegetative cells and spores is unclear. This studyshowed no difference in the viability of B. cereus or B. weihenstephanensisstrains to survive and grow within a simulated human GI tract. Vegetative cellswere revealed to die quickly in the stomach. Spore viability was shown toreduce in the stomach environment by approximately 10⁴-fold. With a largerinitial inoculum, 10⁷ spore/ml, viable spores were still recorded after 4 hours.These spores subsequently germinated within the small intestinal simulationand the resulting vegetative cells rapidly proliferated.Mass spectrometry illustrated the ability of vegetative cells from both B.cereus and B. weihenstephanensis to produce an array of secreted proteinswhose function were predominately related to virulence and pathogenesis. B.weihenstephanensis strain 10202 was shown to produce the potent cytotoxin,CytK-1, while other B. weihenstephanensis and B. cereus tested strainspossessed either or both Nhe and Hbl toxins. The primary diarrhoeal virulencefactor/haemolysin BL was shown to be present in the supernatant of eachstrain through western blotting. Significantly smaller concentrations of eachprotein were detected, however, under simulated human GI tract conditionswhen compared to optimal conditions. The effects of the simulated human GItract on virulence gene expression were monitored through real time PCR.No pattern between B. cereus and B. weihenstephanensis strains was foundconfirming that virulence gene expression is strain specific. Some genes wereshown to be significantly upregulated such as fur, (the ferric iron uptakeregulator and groEL, encoding a molecular chaperone. The expression ofothers however was reduced such as haemolysin BL components, hblA andhblC. Overall there were no significant differences detected between B.cereus and B. weihenstephanensis strains in their ability to survive the humanGI tract and express virulence factors associated with diarrhoeal foodpoisoning.