CTX prophages in Vibrio cholerae O1 strains

Area of Science:

• Microbiology
• Genetics
• Epidemiology

Background:

• Vibrio cholerae O1 serogroup strains are classified into classical and El Tor biotypes, distinguished by their cholera toxin-encoding phages (CTX).
• Classical biotype strains (CTX(cla)) are extinct, while El Tor biotype strains contain CTX-1.
• Atypical El Tor strains have emerged, producing classical biotype cholera toxin, suggesting genetic exchange.

Purpose of the Study:

• To investigate the genetic makeup of CTX phages in atypical El Tor strains.
• To understand the evolutionary relationship between classical and El Tor biotype CTX phages.
• To characterize pre-seventh pandemic Vibrio cholerae strains from the US Gulf Coast.

Main Methods:

• Genome sequencing of CTX phages from atypical El Tor and pre-seventh pandemic strains.
• Comparative genomic analysis of CTX phage sequences.
• Phylogenetic analysis to determine evolutionary relationships.

Main Results:

• CTX phages in atypical El Tor strains are mosaics of CTX(cla) and CTX-1.
• Pre-seventh pandemic strains from the US Gulf Coast (1973-1986) harbor a distinct CTX phage, designated CTX(US Gulf).
• CTX(US Gulf) shares characteristics with both CTX(cla) and CTX-1 but is genetically distinct from CTX(cla).

Conclusions:

• Atypical El Tor strains represent a complex evolutionary lineage resulting from recombination between classical and El Tor CTX phages.
• The emergence of atypical El Tor strains highlights the dynamic nature of Vibrio cholerae evolution and O1 serogroup diversity.
• The identification of CTX(US Gulf) provides insights into the historical evolution of Vibrio cholerae O1 before the seventh cholera pandemic.