RESEARCH ARTICLE Sequence polymorphisms

serotype shifts ai r s r an cholera toxin (CT) [1]. Cholera continues to be a major cholera. Seven pandemics caused by V. cholerae O1 have Liang et al. BMC Microbiology 2013, 13:173 http://www.biomedcentral.com/1471-2180/13/173display one of two serotypes, Ogawa or Inaba. A thirdDiseases, Hangzhou, People’s Republic of ChinaFull list of author information is available at the end of the articlepublic health concern in many developing countries [2,3]. Outbreaks of cholera have been increasing globally in the past decade, most recently in Haiti [4]. been reported since 1871. V. cholerae O139 emerged in late 1992 on the India subcontinent [7,8]. V. cholerae O1 exists as two biotypes, classical and El Tor, which are distinguished by a variety of phenotypic markers, and differ in the severity of their infections and ability to sur-vive outside the human intestine as well [3,9-11]. Two of the first six cholera pandemics are known to have been caused by the classical biotype, while the ongoing sev-enth pandemic, which began in 1961, is caused by the El Tor biotype. The vast majority of strains within the O1 serogrou