Assessment of Resistance to Lytic Effect of Diagnostic Cholera Bacteriophage El Tor in Non-Toxigenic Vibrio cholerae O1 Biovar El Tor Strains with Different Structure of Anti-Phage Systems

The aim of the work was to study the presence of anti-phage systems in the genome of non-toxigenic Vibrio cholerae O1 biovar El Tor strains and to determine the resistance of strains with different structures of these systems to the cholera diagnostic bacteriophage El Tor. Materials and methods. The work used 126 non-toxigenic (ctxA– tcpA+ and ctxA^– tcpA^– ) strains of V. cholerae O1 El Tor, isolated from the external environment and from patients with acute intestinal infections in the territory of the Russian Federation and neighboring countries between 1972 and 2018. Sequencing was performed on the MGI DNBSEQ-G50 platform. The following programs were used for bioinformatics analysis: fastp v0.23, unicycler v0.4.7, Blast 2.16.0 and MEGA X. The interaction of the cholera diagnostic bacteriophage El Tor with the surface of bacteria was studied using atomic force microscopy. Results and discussion. Phageinduced PLE islands and BREX systems were not detected in the genome of the studied strains. At the same time, 75 % of ctxA^– tcpA⁺ strains contain type I restriction-modification genes, while those genes were not detected in ctxA^– tcpA^– strains. The CBASS-system genes are present in single strains of both groups. The presence of type I CRISPR-Cas system was established in the genome of 36 (33 %) ctxA^– tcpA^– strains isolated in different regions of our country. Moreover, 78 % of strains containing this system are resistant to the cholera diagnostic bacteriophage El Tor. Thus, heterogeneity of the studied non-toxigenic strains of V. cholerae O1 El Tor in regard to the presence of anti-phage systems has been established, which expands the knowledge on their genetic organization. Non-toxigenic ctxAB^– tcpA^– strains of V. cholerae O1 El Tor are more resistant to the lytic action of the diagnostic cholera bacteriophage El Tor than ctxAB^– tcpA⁺ strains. One of the reasons for such resistance may be the presence of the type I CRISPR-Cas system.

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