e to distinguish cholera strains, our study will definitely contribute to an improved understanding of possible application of the phages as biocontrol agents during cholera outbreak.
Most of the cholera cases during the period from 2010 to 2015 in India were reported from West Bengal, Karnataka, Assam, Madhya Pradesh, Maharastra, Gujrat, and Punjab. We incorporated V. cholerae strains in our present study which were received from these cholera endemic states of India as shown in [Table 1]. West Bengal reported 5,914 cholera cases in this time frame and other states were also considered to be cholera endemic due to the frequent cholera outbreaks in recent past.
We have characterized and confirmed all the strains used in this study as V. cholerae O1 biotype ElTor. The frequency of serogroup O139 has declined significantly over the past few years consistent with our findings as well. Analysis of serotype distribution in our present study indicates the dominance of Ogawa serotype among the circulating V. cholerae O1 strains in India irrespective of demography and period which matches well with the earlier findings in India., The capability of the strains belonging to Inaba serotype to convert into Ogawa serotype may explain the dominance of Ogawa serotype in our present study., Clinical symptoms of patients infected with Ogawa or Inaba serotype of V. cholerae showed different disease pattern as explained elsewhere. Infection with Ogawa strains are associated with shorter duration of diarrhea with more frequent abdominal pain, vomiting and need for more intravenous fluids when compared with infection with Inaba strain of V. cholerae. Recent Cholera outbreaks in Haiti indicates that toxigenic V. cholerae O1 serotype Ogawa can rapidly change into Inaba serotype with the potential to cause disease in individuals who have acquired immunity against Ogawa serotype which supports the hypothesis that periodic shift may occur due to a consequence of host immunity against different serotypes of V. cholerae O1. Findings of our present study may challenge the previous hypothesis of cyclic shift of V. cholerae O1 serotype due to induction of population-level immunity against dominant strain during an outbreak season. In India, so far, there is no report available regarding the shift in serotypes during different outbreak seasons irrespective of the geography or ethnicity of the infected host indicating equilibrium may have achieved between Ogawa and Inaba strains which require further investigation on responsible host factors associated with dominating serotypes to fine-tune the earlier proposed hypothesis.
Phage typing is a classical method to discriminate bacterial strains into different phage types based on the sensitivity pattern of a panel of typing phages with the bacterial host of the same species. It is a phenotypic characterization method of bacterial strains; much used in the microbiological community and has been around for several decades. The method itself requires that the different phages are available and therefore can generally only be performed at reference laboratories. Vibrio phage Reference Laboratory is involved in phage typing study over the decades using different phage typing schemes. Although Basu and Mukherjee scheme can classify V. cholerae O1 strains into six different types, over the years only two dominating types was found in circulation, reasons for abolition of other four types may demand a different study. In the current study, all of the 1882 strains clustered into two phage types only (either type 2 or type 4) when phage typing was performed using Basu and Mukerjee's scheme [Table 1]. Moreover 1% of the isolates remained untypeable. An earlier study also reported existence of significant number of untypable strains of V. cholerae O1 biotype ElTor when phage typing was performed using Basu and Mukherjee's scheme. Another drawback of this scheme is that the finer distinction is missing and our result indicates that this scheme lacks true identification of origin or clonality of the disease dissemination