Info

Pathogenicity of B. cereus

The B. cereus sensu lato group comprises five valid species: B. cereus, B. mycoides, B. thuringiensis, B. anthracis, and the newly described psychrotolerant species B. weihenstephanensis that is composed of B. cereus isolates capable of growing at below 7 °C [38-41]. There are several taxonomic problems with the species classification as B. cereus has a very high genomic variability [42, 43]. An overview of phy-logenetic coherences among strains mentioned in this review is given in Fig. 12.1. The chromosome size ranges from 2.4 Mbp up to 5.3 Mbp. It has been shown that despite this considerable range in genome size, the smallest, 2.4-Mbp genome corresponds to a conserved region of the larger genomes [44]. In addition, the genomes of several B. cereus isolates contain large, presumably linear plasmids with sizes ranging from 290 kbp to 730 kbp [45]. On the basis of these observations it has been proposed that the genome of B. cereus consists of a constant part and a less stable one, which is more easily exchanged by other genetic elements.

B. cereus B. th uringiensis ATCC 14579 tolworthii

B. cereus

B. thuringiei thuringiensi

B. thuring kurstaki

B. thuring israeliens

B. cereus B. th uringiensis ATCC 14579 tolworthii

B. cereus

B. thuringiei thuringiensi

B. thuring kurstaki

B. thuring israeliens

B. anthracis

B. weihenstephanensis

Fig. 12.1 Phylogenetic tree of members of the B. cereus sensu lato group. The unrooted tree derived from multiple-locus sequence typing. It was calculated using the neighbor-joining method. The scale bar represents the average number of nucleotide differences per site.

B. cereus ATCC 10987

B. anthracis

B. weihenstephanensis

Fig. 12.1 Phylogenetic tree of members of the B. cereus sensu lato group. The unrooted tree derived from multiple-locus sequence typing. It was calculated using the neighbor-joining method. The scale bar represents the average number of nucleotide differences per site.

B. cereus strains are the rare cause of food poisoning which is particularly associated with the consumption of rice-based dishes. The organism produces two forms of human food poisoning, characterized by either diarrhea and abdominal distress or an emetic syndrome with nausea and vomiting, induced by an entero-toxin or the emetic toxin, respectively. Other toxins are produced during growth, including phospholipases, proteases, and hemolysins, one of which, cereolysin, is a thiol-activated hemolysin [46-49]. These toxins may also contribute to opportunistic pathogenicity of B. cereus in nongastrointestinal diseases that may range from serious infections in immunosuppressed patients, neonates, and postsurgi-cal patients, to septicemia, meningitis, endocarditis, and osteomyelitis. Surgical and traumatic wound infections are other reported severe diseases [50-52]. In contrast to B. anthracis, virulent strains of B. cereus are often b-hemolytic, and contain b-lactamases rendering them resistant to b-lactam antibiotics.

As there is no clear genetic distinction between the members of the B. cereus group, it has long been debated whether the members of this group are varieties of the same species [53, 54] or separate species [38]. Insect-pathogenic members of the B. cereus group are currently classified as B. thuringiensis [39]. These bacteria produce toxins by virtue of plasmid-born genes in the form of parasporal crystal proteins that have been widely exploited in agriculture as an insecticide. Occasion ally, B. thuringiensis strains are responsible for human infections similar to those caused by strains of B. cereus [55, 56].

0 0

Post a comment