Summary auto-generated
Researchers characterized phage 493, a new filamentous bacteriophage isolated from Vibrio cholerae O139 strain AJ27-493. The phage has a 9.3 kb single-stranded circular DNA genome with an unusual 2 kb stem-loop structure. Phage 493 infects most O139 strains and early El Tor biotype strains isolated before 1994, but cannot infect classical O1 strains or recent El Tor isolates from 1995-1996. The phage receptor is not TcpA, distinguishing it from previously described filamentous choleraphages. Upon infection, phage DNA integrates both chromosomally and extrachromosomally in host cells, causing marked changes in colony morphology and producing an 18 kDa protein. Interestingly, phage 493 DNA sequences were found in most O139 strains and even in an encapsulated O31 strain isolated before O139 emerged. These findings suggest bacteriophages may have played a role in the rapid emergence, dominance, and subsequent decline of V. cholerae O139 in India and Bangladesh during the 1992-1997 period through horizontal gene transfer and selective pressure on susceptible populations.
Key findings
- Phage 493 is a lysogenic filamentous phage with a 9.3 kb genome containing an unusual 2 kb stem-loop structure, likely representing a transposable element
- The phage infects O139 and early El Tor strains but not classical O1 strains or recent El Tor isolates, suggesting it may have influenced the emergence and territoriality of V. cholerae strains
- Phage 493 uses a different receptor than TcpA (unlike the ctx4 phage) and integrates both chromosomally and extrachromosomally, providing a mechanism for horizontal gene transfer
- Phage 493 DNA sequences are widely distributed among O139 strains but absent from O1 strains, with some sequence variability detected
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Abstract
The emergence and rapid rise to dominance of Vibrio cholerae O139 in India and Bangladesh in 1992 led to the consideration that choleraphage might serve as both a selective mechanism and a means for horizontal transmission of genetic information. A filamentous phage '493' from O139 strain AJ27-493 has been purified and partially characterized. The phage was inactive on classical biotype V. cholerae O1 but it was active on El Tor biotype strains isolated prior to 1994 when El Tor re- emerged in Bangladesh. More recent El Tor isolates were all resistant to the phage. The phage was also active on O139 strains. Unlike the filamentous ctx phi, the receptor for 493 is not TcpA. The phage genome was a 9.3 kb closed circular single-stranded molecule containing a 0.4 kb double-stranded stem supporting a 2 kb single-stranded loop. A 283 bp fragment was cloned and used as a probe in Southern hybridization, in parallel with total phage 493 DNA. These probes hybridized both chromosomally and extrachromosomally with most O139 strains, but not with O1 strains. Infection of hybridization-negative El Tor or O139 strains resulted in the presence of hybridizing loci (both plasmid and chromosomal), in the appearance of an 18 kDa protein, and in marked alterations in colonial morphology. Phage 493 is clearly distinct from other O139 choleraphages which have been described. Phage 493 DNA hybridized with an encapsulated non-O1 (O31) strain (NRT36S) which was isolated before O139 was recognized. NRT36S also produces a phage which can infect El Tor strains with low efficiency. Further studies may reveal whether bacteriophage play a role in the emergence and the territoriality of new choleragenic vibrios.