Design and construction of Vibrio cholerae O1 strains that contain various CTX phage array

Abstract

Cholera is a severe diarrheal disease caused by toxigenic strains of Vibrio cholerae serogroups O1 and O139. Toxigenic V. cholerae strains are generated by infection and lysogenization of a 6.9-kb single stranded DNA filamentous phage, called cholera toxin phage (CTX). V. cholerae O1 serogroup strains are further classified into the classical and El Tor biotypes, based on certain biochemical tests and genotypes of the CTX phages they harbor. Two major CTX phages within O1 serogroup of V. cholerae that are differentiated by rstR (repressor of CTX phage) have been described: CTX-cla in classical biotype strains and CTX-1 in El Tor biotype strains. The prototype El Tor strains that produced biotype-specific cholera toxin are now being replaced by atypical El Tor variant harboring classical cholera toxin. A model suggesting that type-specific CTX phages are acquired by each biotype strain in the evolution of toxigenic V. cholerae strains has been developed by demonstration of replication of CTX-1 under laboratory conditions. However, replication of CTX-cla has remained unverified in vitro. In this study, the replication of CTX-cla phage in vitro was confirmed by developing a plasmid-based CTX phage replication system and constructing El Tor strains that can be transduced by CTX phages under laboratory conditions. El Tor biotype strains don’t express TCP and cholera toxin under laboratory conditions. A point-mutation in the transcriptional regulator of Tcp and cholera toxin, ToxT, can up-regulate the expression of TCP and cholera toxin of El Tor strains under laboratory conditions. El Tor strains harboring different types of CTX phages and various array of CTX phages have been generated. The results in this study provide experimental evidence of the generation of toxigenic V. cholerae strains. The results in this study suggest various V. cholerae strains that contain diverse CTX arrays in nature. Strains developed in this study have potential to be used as vaccine strains considering they have characteristics of elevated expression of TCP and CT in addition to they contain different types of CTX phages.