Abstract
BackgroundClostridium botulinum strains can be divided into four physiological groups that are sufficiently diverged to be considered as separate species. Here we present the first complete genome of a C. botulinum strain from physiological group III, causing animal botulism. We also compare the sequence to three new draft genomes from the same physiological group.ResultsThe 2.77 Mb chromosome was highly conserved between the isolates and also closely related to that of C. novyi. However, the sequence was very different from the human C. botulinum group genomes. Replication-directed translocations were rare and conservation of synteny was high. The largest difference between C. botulinum group III isolates occurred within their surprisingly large plasmidomes and in the pattern of mobile elements insertions. Five plasmids, constituting 13.5% of the total genetic material, were present in the completed genome. Interestingly, the set of plasmids differed compared to other isolates. The largest plasmid, the botulinum-neurotoxin carrying prophage, was conserved at a level similar to that of the chromosome while the medium-sized plasmids seemed to be undergoing faster genetic drift. These plasmids also contained more mobile elements than other replicons. Several toxins and resistance genes were identified, many of which were located on the plasmids.ConclusionsThe completion of the genome of C. botulinum group III has revealed it to be a genome with dual identity. It belongs to the pathogenic species C. botulinum, but as a genotypic species it should also include C. novyi and C. haemolyticum. The genotypic species share a conserved chromosomal core that can be transformed into various pathogenic variants by modulation of the highly plastic plasmidome.
Highlights
Clostridium botulinum strains can be divided into four physiological groups that are sufficiently diverged to be considered as separate species
The toxins are proteins mainly produced by the species Clostridium botulinum but some Clostridium butyricum and Clostridium baratii strains are capable of producing botulinum neurotoxins (BoNTs)
In contrast to the previously completed plasmids originating from isolates before 1960, the BKT015925 sequences came from a recent isolate (2008)
Summary
Clostridium botulinum strains can be divided into four physiological groups that are sufficiently diverged to be considered as separate species. We present the first complete genome of a C. botulinum strain from physiological group III, causing animal botulism. Botulism is a paralytic disease caused by some of the most potent toxins known, the botulinum neurotoxins (BoNTs). The species C. botulinum is divided into four physiological groups (I-IV), which produce BoNTs of seven different serotypes (A-G) [4]. Physiological group I (proteolytic) includes C. botulinum strains producing toxins of serotype A, B or F and is mainly associated with human cases. Physiological group II (non-proteolytic) consists of strains producing toxins of serotype B, E or F; these strains cause human botulism. Group III produces toxins of serotype C or D and is associated with avian and nonhuman mammalian botulism
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