More than 4 million people are affected by cholera and more than 1 lakh deaths occur yearly.² India reported about 37,000 cases from 68 outbreaks between 1997 to 2006.¹¹ The number of cases in reality are predicted to be six times higher than reported.^{11, 12} Large, diverse population, suboptimal water sanitation and warm climate are conducive for epidemics in India.¹¹

Cholera is a self-limiting disease but can cause severe dehydration and death in a matter of hours.¹³ Rehydration is the mainstay of treatment. Antibiotics are indicated in cases of severe dehydration. Antibiotics reduce the total volume of stool passed, stopping diarrhea in 48 hours.^{1, 2} Hence antibiotics play a very important role in the treatment of cholera. However, emergence of drug resistance among V. cholerae has narrowed the choices of antibiotics. There has been sudden surge of multidrug resistance (MDR) in Vibrio in last 20 years. ^{3-5, 14-16} Besides, reports of biotype variation among V. cholerae from Inaba to Ogawa and vice versa are also known.^13-15 Such variation is due to immune selection pressure in the local population. There have been reports of emergence of atypical variants of V. cholerae presenting with severe illness and MDR.^4,5,16 Hence it is important to monitor biotypes and resistance pattern of the isolates.

In our study 106 V. cholerae O1 strains were isolated and all of them belonged to biotype El Tor, the predominant serotype was Ogawa. Phage typing revealed types T2 (Basu & Mukerjee) and T27 (new scheme) were most common. Workers from different parts of India during the same period also reported similar findings.^7,18,19 Classical and O139 serotypes were not isolated at this centre indicating their absence in this geographical area during the study period. Over the years, frequency of Classical and O139 serotypes of V. cholerae has declined considerably throughout India.¹¹ However a study conducted during 2000-2004 reported presence of non O1 and non O-139 strains.²⁰

Various studies have tested a variety of antibiotics for V. cholerae including cephalosporins, fluoroquinolones and aminoglycosides. But CLSI recommends testing of only four drugs, i.e., Ampicillin, Co-Trimoxazole, Tetracycline and Chloramphenicol.¹⁰ The present study showed a high resistance to Ampicillin and Co-Trimoxazole throughout the study period. This may be linked to plasmid transfer from resistant colonic flora to V. cholerae in the gut environment.^14,21 Variable resistance patterns was seen towards Tetracycline. A similar picture was observed, also among Enterobacteriaceae. Plasmid-transfer from gut flora to V. cholerae may be the reason for variable pattern of antibiotic resistance. We found that the variable resistance of our isolates to Tetracycline was not associated with the serotype and biotype of the isolate. Our isolates showed least resistance towards Chloramphenicol. However, Chloramphenicol has side effects when used in children, abridging its use as drug of choice.

Apart from plasmid transfer of antibiotic resistance, there are other reasons for emergence of resistance among V. cholerae. Extensive antibiotic usage in veterinary and poultry industry, in order to increase the diary and meat production, is one of the important reasons.²² Pressure exerted by the antibiotics lead to selection of resistant mutants. Studies have also shown that during an outbreak, if antibiotics are used for treatment, there is rapid development of resistance.²³ Biotype switching is another cause of shift in drug resistance.^7,24 Presence of a single biotype of V. cholerae in a geographical area leads to development of increased immunity levels in the community and in such situations, V. cholerae are known to exhibit a revolutionary strategy by changing the biotype from Ogawa to Inaba and vice versa. Such biotype variation is linked to variation in antimicrobial resistance in some studies.^14,17

Since antibiotics play important role in limiting the disease severity, they should be wisely used. Clinical microbiology laboratories should adhere to the CLSI guidelines of testing only four antibiotics for V. cholerae.¹⁰ Testing and reporting sensitivity to antibiotics, not recommended by CLSI, would lead to their injudicious use adding to the menace of drug resistance. Many studies have also documented reversal of resistance with judicious use of antibiotics.^{7, 24}

Clinical laboratories do not test antibiotic susceptibility of V. cholerae routinely. Therefore, unlike the other gram negative bacteria, only a little data on sensitivity pattern of this epidemiologically important pathogen exists.

The risk of cholera to community can be prevented by availability of good quality potable water and sanitation measures. Use of vaccines also is highly effective in halting the spread of cholera. Vaccination is an effective measure to prevent drug resistance in pathogens.²⁵

Our study had a few limitations. Only those cases presenting to our hospital were included in this study and the data does not include isolates from cases treated at other government or private health care centers in this area. And also we could not perform the minimum inhibitory concentration (MIC) of Tetracycline for resistant isolates. Inclusion of most isolates from a region would provide a comprehensive perspective of the prevalence of a pathogen.