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. Therefore, a different scheme with better discrimination power and better typeability was essential. New phage typing scheme which was developed based upon phages isolated from NICED, Kolkata can distinguish V. cholerae O1 strains into 27 different types and is useful to achieve more type distinction and improved discrimination within the biotype ElTor strains to get a better idea on the origin and clonality route of the outbreak. The present study has shown that type 27 is the dominant phage type in every state with maximum percentage [80%] from West Bengal [Figure 1], indicating the dissemination of cholera outbreak may have occurred from common precursor strain. These observations reveal a common origin and probably the pathogenic strains of cholera belong to the same clone. O1 strains from Maharashtra, Andhra Pradesh and Gujarat exhibited the maximum number of phage types where chorea is endemic over last 5 years. Co-occurrence type 27 and incidence of cholera indicates that V. cholerea O1 strains belonging to phage type 27 might be primarily responsible for outbreaks in recent past in India.
|Figure 1: Distribution of strains belong to phage Type 27 [T-27] in different states of India isolated from 2012 to 2017|
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Apart from the phages characterized in our laboratory to distinguish V. cholerae O1, naturally occurring similar kind of lytic phages may play an essential role in controlling the spread of cholera infection. In the recent past, a model was proposed to understand how equilibrium exists between vibriophages and its host contributing to an outbreak. In an outbreak, increase load of V. cholerae leads to an increase in vibriophage population, which subsequently results in declining of vibrio population and helps in controlling outbreak. A recent study suggests that a cholera patient having vibriophages in their stool less likely to transmit cholera infection through household contact when compared with patients without any vibrophage in their stool. Later it was found that phage posi