A study of the distribution of regulatory genes controlling an expression of virulence genes among strains of Vibrio cholerae biovar eltor differing in their pandemic potential

Abstract

The evolution of the genome of Vibrio cholerae biovar eltor, a causative agent of the 7th global cholera pandemic, is conditioned by the acquisition of not only structural virulence genes, but also regulatory systems, which has been caused by a horizontal transfer of a genetic information. Using polymerase chain reaction, we have revealed the presence of the following regulatory genes, controlling the expression of virulence genes, in the chromosomes of prepandemic and pandemic strains of V. cholerae biovar eltor: toxR, toxT, tcpP, tcpH, luxS, luxO, hns, and pepA. We have revealed that the avirulent ATCC14033 strain, isolated in 1910 and proposed to be an ancestor of the biovar eltor, lost toxT, tcpP, and tcpH regulatory genes, located in a VPI-1 pathogenicity island and able to realize a positive control over the expression of genes from the ToxR regulon. Virulent strains, isolated during a local cholera episode in Indonesia (1937) from cholera patients, do not differ from the strains that caused the cholera eltor pandemic in 1961, in the composition of the regulatory genes investigated. Only one strain of the four studied isolates lost the tcpP gene. Two key regulatory genes, toxR and toxT, have been sequenced in all studied isolates. The toxR nucleotide sequence of three prepandemic strains has been shown to be indistinguishable from that of the pandemic isolates. At the same time, the MAK757 strain, isolated prior to the beginning of the epidemic, has demonstrated an altered toxR nucleotide sequence. Experiments with the intraintestinal infection of suckling rabbits have demonstrated a similar level of virulence for the prepandemic and pandemic clinical strains. These results are evidence of the in vivo activity of the toxT, tcpH, and tcpP genes, acquired by V. cholerae in the course of evolution and positively regulating the expression of virulence genes in prepandemic cholera strains.