Emergence of Fluoroquinolone-Resistant Strains of Vibrio cholerae O1 Biotype El Tor among Hospitalized Patients with Cholera in Calcutta, India

Emergence of Fluoroquinolone-Resistant Strains of Vibrio cholerae O1 Biotype ElTor among Hospitalized Patients with Cholera in Calcutta, IndiaCiprofloxacin and norfloxacin are broad-spectrum fluoroquinolones and possess excellent activity against Vibrio cholerae O1 and O139 serogroups (6).Clinical studies have shown that these drugs are effective in treatment of cholera in adults and children (2-4).For the past 11 years, we have been monitoring the incidence of antibiotic susceptibility and genotypic changes in V. cholerae isolates from cholera patients admitted at the Infectious Diseases Hospital, Calcutta, India.Here, we report the emergence of fluoroquinolone-resistant strains of V. cholerae O1 Biotype El Tor among hospitalized patients with cholera.Antimicrobial susceptibility analysis of V. cholerae strains was performed by disk diffusion (1) on Mueller-Hinton agar (Difco, Detroit, Mich.) with commercial disks (HiMedia, Mumbai, India).The following antibiotic disks were used: ciprofloxacin, 5 g; nalidixic acid, 30 g; norfloxacin, 10 g; and tetracycline, 30 g. Characterization of strains as susceptible, intermediately resistant, or resistant was based on the size of the inhibition zone according to the manufacturer's instructions.These zone size interpretive criteria for susceptibility corresponded to MICs of 0.25, 0.06, and 0.06 g/ml for nalidixic acid, norfloxacin, and ciprofloxacin, respectively.Strains showing intermediate zones of growth inhibition were classified as resistant on the basis of previous MIC studies with V. cholerae (6).We have reported considerable increases in fluoroquinolone resistance among V. cholerae strains belonging to non-O1, non-O139 serogroups during 1996 (5).All the V. cholerae strains of serogroup O1 isolated in or before 1994 are susceptible to ciprofloxacin.From 1995, we have recorded progressive increases in ciprofloxacin and norfloxacin resistance among V. cholerae O1 strains, with the highest occurrences of 38.8% in 1999 and 25% in 2000, respectively (Table 1).To our knowledge, this is the first report on such high incidence of fluoroquinolone resistance among toxigenic V. cholerae O1 strains.The MICs of ciprofloxacin and norfloxacin for ciprofloxacinresistant V. cholerae strains ranged between 9 and Ͼ32 g/ml and between 192 and Ͼ256 g/ml, respectively, when tested