The Bacillus cereus sensu lato group is composed of six closely-related bacterial species, exhibiting a broad range of virulence activities. B. anthracis causes anthrax to human and mammals and B. thuringiensis is an entomopathogen against Lepidopteran, Coleopteran and Dipteran larvae. B. cereus s.s. and B. weihenstephanensis are opportunistic bacteria, causing gastro-enteritis and more acute diseases in immuno-compromised people. B. mycoides and B. pseudomycoides are generally considered as non-pathogenic. In addition to the specific virulence factors, several generic toxins have been described in the B. cereus s.l. members (phospholipases, enterotoxins, proteases and hemolysins). Among these virulence factors, cereolysin O was first characterised in 1967 by Cowell and Bernheimer, who suggested it to be a member of the cholesterol-dependent cytolysins. This toxin family cause the lysis of cholesterol-containing membranes and their activity, inhibited by oxidising agents, is restored by reducing agents. Moreover, small amount of free cholesterol irreversibly inactivate the toxins.
The first part of this thesis gives a description of the hemolytic activities of the different species of the B. cereus s.l. members, with regard to the international ISO norm for the characterisation of B. cereus s.s. in food. One hundred strains were analysed for their hemolytic and lecithinase activities on different blood-containing media, and the results showed that the most sensitive was the sheep erythrocytes-containing medium. The hemolytic activity of B. cereus s.l. is due to at least five hemolysins whose precise roles in pathogenesis are largely unknown. The continuation of this thesis was focused on one of these hemolysins, cereolysin O. PCR, RFLP and Southern blotting analysis pointed out that the clo gene was present and highly conserved in all the strains of the six tested species. The results also indicated that two copies of the clo gene are present in at least 20% of the strains. The last part of this work was to evaluate the hemolytic activity of Clo by knock-out and cloning experiments. The results revealed the importance of cereolysin O in the lysis of human blood as it is not the case on sheep blood.
| Identifer | oai:union.ndltd.org:BICfB/oai:ucl.ac.be:ETDUCL:BelnUcetd-09152005-143716 |
| Date | 06 July 2006 |
| Creators | Michelet, Nathalie C.M. |
| Publisher | Universite catholique de Louvain |
| Source Sets | Bibliothèque interuniversitaire de la Communauté française de Belgique |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://edoc.bib.ucl.ac.be:81/ETD-db/collection/available/BelnUcetd-09152005-143716/ |
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