Abstract
The Bacillus cereus group of bacteria includes seven closely related species, three of which, B. anthracis, B. cereus and B. thuringiensis, are pathogens of humans, animals and/or insects. Preliminary investigations into the transport capabilities of different bacterial lineages suggested that genes encoding putative efflux systems were unusually abundant in the B. cereus group compared to other bacteria. To explore the drug efflux potential of the B. cereus group all putative efflux systems were identified in the genomes of prototypical strains of B. cereus, B. anthracis and B. thuringiensis using our Transporter Automated Annotation Pipeline. More than 90 putative drug efflux systems were found within each of these strains, accounting for up to 2.7% of their protein coding potential. Comparative analyses demonstrated that the efflux systems are highly conserved between these species; 70–80% of the putative efflux pumps were shared between all three strains studied. Furthermore, 82% of the putative efflux system proteins encoded by the prototypical B. cereus strain ATCC 14579 (type strain) were found to be conserved in at least 80% of 169 B. cereus group strains that have high quality genome sequences available. However, only a handful of these efflux pumps have been functionally characterized. Deletion of individual efflux pump genes from B. cereus typically had little impact to drug resistance phenotypes or the general fitness of the strains, possibly because of the large numbers of alternative efflux systems that may have overlapping substrate specificities. Therefore, to gain insight into the possible transport functions of efflux systems in B. cereus, we undertook large-scale qRT-PCR analyses of efflux pump gene expression following drug shocks and other stress treatments. Clustering of gene expression changes identified several groups of similarly regulated systems that may have overlapping drug resistance functions. In this article we review current knowledge of the small molecule efflux pumps encoded by the B. cereus group and suggest the likely functions of numerous uncharacterised pumps.
Highlights
The Bacillus cereus group is composed of seven species of low G+C Gram-positive spore-forming bacteria, which based on 16S rRNA sequence data form a separate cluster in the phylogenetic tree of Bacillaceae and Firmicutes [1]
To define the efflux potential of the B. cereus group, putative efflux systems were identified in the complete genome sequences of three reference strains, B. cereus ATCC 14579, B. anthracis Ames and B. thuringiensis konkukian 97–27, using the transporter automated annotation pipeline (TransAAP) [21]
Using the TransAAP we demonstrated that bacterial strains within the B. cereus group may devote more than 2.5% of their protein coding potential to the production of drug efflux pumps (Table 1)
Summary
The Bacillus cereus group is composed of seven species of low G+C Gram-positive spore-forming bacteria, which based on 16S rRNA sequence data form a separate cluster in the phylogenetic tree of Bacillaceae and Firmicutes [1]. The B. cereus group includes B. cereus (sensu stricto), B. anthracis, and B. thuringiensis, which are all well studied and are pathogens of animals, humans or insects, as well as B. weihenstephanensis, B. mycoides, B. pseudomycoides and B. cytotoxicus. The pathogenic species of the B. cereus group have different host preferences, mainly due to traits encoded on plasmids. B. thuringiensis strains produce proteinaceous crystal toxins (Cry or Cyt toxin) during sporulation which are the primary cause of their toxicity toward insects, and which are encoded by genes most often located on plasmids. B. thuringiensis strains do carry the chromosomal enterotoxin genes found in B. cereus, and the two species are genetically indistinguishable based on chromosomal characters [12, 13]. The plcR gene is present in B. anthracis strains, but carries a deleterious mutation making the protein non-functional and leaving the PlcR regulated genes non-transcribed [14]
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