Acquisition of classical CTX prophage from Vibrio cholerae O141 by El Tor strains aided by lytic phages and chitin-induced competence

Abstract

The El Tor biotype of Vibrio cholerae O1, causing the current seventh pandemic of cholera, has replaced the classical biotype, which caused the sixth pandemic. The CTX prophages encoding cholera toxin in the two biotypes have distinct repressor (rstR) genes. Recently, new variants of El Tor strains that carry the classical type (CTX class ) prophage have emerged. These ''hybrid'' strains apparently originate through lateral gene transfer and recombination events. To explore possible donors of the CTX class prophage and its mode of transfer, we tested environmental V. cholerae isolates for the presence of CTX class prophage and mobility of the phage genome. Of the 272 environmental V. cholerae isolates tested, 6 were found to carry the CTX class prophage; all of these belonged to the O141 serogroup. These O141 strains were unable to produce infectious CTX class phage or to transmit the prophage to recipient strains in the mouse model of infection; however, the CTX class prophage was acquired by El Tor strains when cultured with the O141 strains in microcosms composed of filtered environmental water, a chitin substrate, and a V. cholerae O141-specific bacteriophage. The CTX class prophage either coexisted with or replaced the resident CTX ET prophage, resulting in El Tor strains with CTX genotypes similar to those of the naturally occurring hybrid strains. Our results support a model involving phages and natural chitin substrate in the emergence of new variants of pathogenic V. cholerae. Furthermore, the O141 strains apparently represent an alternative reservoir of the CTX class phage genome, because the classical V. cholerae O1 strains are possibly extinct.hybrid Vibrio cholerae strain ͉ toxigenic Vibrio cholerae C holera caused by toxigenic Vibrio cholerae is a major public health problem confronting many developing countries, in which outbreaks occur frequently and are closely associated with poverty and poor sanitation (1, 2). Seven distinct pandemics of cholera have been recorded since the first pandemic in 1817. The current seventh pandemic, which originated in Indonesia in 1961, is the most extensive in terms of geographic spread and duration. The causative agent is V. cholerae O1 of the El Tor biotype. The sixth pandemic and presumably the earlier pandemics were caused by V. cholerae O1 of the classical biotype (2).Classification into biotypes is based on a set of phenotypic traits that include susceptibility to polymixin B, hemagglutination of chicken erythrocytes, hemolysis of sheep erythrocytes, the Voges-Proskauer test (which measures the production of acetylmethylcarbinol), and susceptibility to phages (1, 2). In toxigenic V. cholerae, the genes encoding cholera toxin (ctxAB) are located in the CTX prophage (3). Although most other parts of the CTX phage genomes are similar in the two biotypes of V. cholerae O1, the repressor genes (rstR genes) carried by CTX phages differ in CTX ET and CTX class (4,5). Two other varieties of the rstR gene carried by CTX Calc and CTX env also have been reported (6, 7...

Comparative genomics reveals mechanism for short-term and long-term clonal transitions in pandemic Vibrio cholerae

“…Given that V. cholerae O1 El Tor MAK 757, a clinical strain isolated in 1937, has this type of prophage, correlation of host biotype and prophage type is not considered significant. A recent study (19) showing the infection of CTX Class to V. cholerae non-O1 supports this finding.…”

Molecular keys of the tropism of integration of the cholera toxin phage

“…In this context, the integration of CTXϕ at dif2 in classical strains (3) and in recent El Tor isolates (4, 10-14) was puzzling. It was observed recently that El Tor strains can acquire the dif2-integrated classical copy of CTXϕ via chitin-induced competence and homologous recombination (18). However, such a mechanism left open the questions of how dif2 integration could be achieved initially and the frequency at which this integration could occur.…”

“…The mode of dissemination of this variant is of particular interest, because it allows the production of an elevated amount of cholera toxin (15,16) that seems to be implicated in a high proportion of severe infections associated with classical strains (17). It was proposed that this variant recently invaded the genome of El Tor strains through chitin-induced competence and homologous recombination (18). However, the transformation efficiency was very low, in the order of 10 −6 to 10 −4 .…”

Cholera Vibrios of nonO1/nonO139 Serogroups in Etiology of Acute Intestinal Infections: Current Situation in Russia and Around the World

“…В частности, в прибрежных водах Калифорнии обнаружены литические фаги, способные переносить CTX-элемент от штаммов Эль Тор НАГ-вибрионам [21]. Штаммы О141 серо-группы, неспособные образовывать инфекционные вирионы CTXφ (классического типа), успешно пе-редавали профаг штаммам Эль Тор в микрокосмах с хитиновым субстратом и О141-специфическим литическим бактериофагом за счет трансформации [56]. Описан конъюгативный перенос плазмиды, не-сущей несколько генов антибиотикорезистентности, от донора О1 серогруппы чувствительным штаммам НАГ-вибрионов [30].…”

“…Оба содержали 4 гептануклеотидных повтора в промотороной обла-сти ctxA и по последовательностям генов рРНК были близко родственными штаммам О1 серогруппы в от-личие от wbeN-негативных штаммов О26, попавши-ми в другой кластер [15]. Другие авторы, напротив, подчеркивают роль НАГ-вибрионов в возникнове-нии новых клонов эпидемически опасных штаммов О1 и O139 серогрупп [40,50,56]. На наш взгляд, эво-люция V. cholerae может происходить в обоих этих направлениях.…”