Molecular biologic techniques have been applied to analyze V cholerae O1 and O139 strains in order to determine relationships and to deduce the origin or derivation of strains. The techniques include Southern blotting of restricted chromosomal DNA with specific probes, examination of restriction fragment length polymorphism (RFLP, ribotyping), pulsed-field gel electrophoresis (PFGE), multilocus enzyme electrophoresis (MEE, electropherotyping), comparison of DNA sequences, polymerase chain reaction methods, and full or partial genome sequencing.37-41 Application of these molecular subtyping methods has revealed that there are four major clonal groups of V cholerae O1 El Tor strains worldwide called: (1) seventh pandemic; (2) U.S. Gulf Coast; (3) Australia; (4) Latin America. The U.S. Gulf Coast and Australian strains, each representing an environmental reservoir, are closely related. The seventh pandemic and Latin American strains are also similar. It has also been shown that the O139 epidemic strains of 1992-1994 are closely related (indeed, virtually identical) to strains of the El Tor seventh pandemic clone.
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