 V. cholerae O1 biotype El Tor strains. Analysis revealed that V. cholerae O1 biotype ElTor strains were distributed into several different phage tapes of which (Types 27, 26, 23.24, 13, 21 and 25) were the major types that represented almost 81% of the total typed strains in this study [Table 2]. Majority of the 1882 strains belonged to phage type 27 (60.41 %). Phage type 26 (8.66%) was the next major phage type followed by type 23 (4.94%), type 24 (4.41%), type 13 (3.82%), type 21 (3.08%) and type 25 (2.92%) [Table 1]. It was found that the V. cholerae O1 biotype ElTor strains belonging to phage type 27 were widely and almost equally distributed in all the nine states from where strains were selected for the present study.
|Table 2: Major Phage type classification based upon sensitivity to phages N4, S5, S20, M4 and D10. (+) indicates sensitive to specific phage, (-) indicates resistance to specific phage|
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Identification of candidate phage for an alternative therapy
Sensitivity of the circulating V. cholerae strains against the five phages of the new phage typing scheme was calculated and it was found that all the phages have the ability to lyse at least 80% of the circulating strains in India. As shown in [Table 3], phage N4 can lyse 91.17% strains, phage S20 can lyse 83.95%, phage M4 can lyse 86.29% of strains and phage D10 can lyse 80.49%. However, phage S5 has shown highest lethality which can kill or lyse 94% of the circulating V. cholerae O1 biotype El Tor strains.
|Table 3: Comparative analysis of vibriophage susceptibility. All the 1882 strains used in the present study were analyzed for sensitivity against the individual phages. Maximum number of strains were found sensitive to phage S5 followed by N4, M4, S20 and D10|
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| Discussion|| |
We are using phage typing as one of the primary methods in monitoring the V. cholerae strains responsible for cholera infection in India at Vibrio Phage Typing Reference Laboratory. Besides potential application of phage typing scheme 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 