Indian Journal of Marine Sciences Vol. 32(4), December 2003, pp. 334-336 Short Communication Antibiotic susceptibility of luminous bacteria from shrimp farm environs of West Bengal Tapti Sengupta, Debasis Sasmal & T. Jawahar Abraham* # Department of Fishery Pathology and Microbiology, Faculty of Fishery Sciences, West Bengal University of Animal and Fishery Sciences, Mohanpur, Nadia-741 252, West Bengal, India # [ E-mail: jawaharabraham@rediffmail.com ] Received 9 December; revised 22 September 2003 Sensitivity of 175 isolates of luminous bacteria from various shrimp farming systems of West Bengal was tested against six antibiotics, viz., chloramphenicol (30μg), ciprofloxacin (5μg), co-trimoxazole (25μg), gentamycin (10μg), nitrofurantoin (300μg) and oxytetracycline (30μg). Besides, the minimum inhibitory concentration (MIC) of the antibiotics was determined against 60 isolates of Vibrio harveyi. The isolates were least susceptible to oxytetracycline followed by ciprofloxacin and nirofurantoin, and highly susceptible to chloramphenicol. Vibrio harveyi was the most resistant species. About 56% of the luminous isolates exhibited multiple antibiotic resistance. The MIC of oxytetracycline was observed to be in the range of 1.56->200 μg/ml. Majority of the isolates were inhibited at a concentration of 0.10-0.78 μg/ml of ciprofloxacin and 0.20-6.25 μg/ml of chloramphenicol. The observations on the antibiotic resistance did not indicate that the luminous bacteria of certain shrimp culture systems of West Bengal is developing resistance to broad-spectrum antibiotics. [ Key words : Antibiotic resistance, luminous bacteria, resistance profile, shrimp farming, Vibrio harveyi ] With rapid intensification in shrimp aquaculture globally, various kinds of diseases have surfaced resulting in serious losses in the production and increased use of aquaculture drugs 1 . Luminous vibriosis due to Vibrio harveyi is an important disease, causing mass mortalities in hatchery reared penaeid shrimp larvae 2 , but its occurrence has not been rampant in cultured juveniles in grow-out ponds until the last quarter of 1993. Since then, larval and juvenile shrimp mortalities associated with luminous Vibrio spp have been reported in epizootic proportions in grow-out ponds 3,4 . Antibiotics have been used to control luminous vibriosis in grow-ponds with little success. The principal drawback of antibiotic application is the development and spread of antibiotic resistant organisms 2,5 . West Bengal, one of the important maritime States in the eastern India, has vast brackishwater resources where shrimp farming is carried out by traditional (in bheries), improved traditional, modified extensive and semi- intensive methods. This communication reports the study carried out between May 2000 and September 2001 on the sensitivity, resistance profile and resistance pattern of the luminous bacteria from various shrimp farming systems of West Bengal to six broad-spectrum antibiotics. A total of 175 luminous bacterial isolates from diseased shrimp, pond water, pond sediment and source water of improved traditional, modified extensive, semi-intensive and stagnant pond culture systems of Midnapore and 24 Parganas (South) districts, West Bengal, India, identified to be Vibrio fischeri (1), V. harveyi (166) and V. splendidus biotype 1 (8) as described in Abraham et al . 6 , were screened for their antibiotic sensitivity to six potential antibiotics by agar disc diffusion method 7 on tryptic soy agar (TSA) supplemented with 1.0 % (w/v) sodium chloride. The antibiotic impregnated discs (HiMedia, Mumbai) used include chloramphenicol (30 μg), ciprofloxacin (5 μg), co-trimoxazole (25 μg), gentamycin (10 μg), nitrofurantoin (300 μg) and oxytetracycline (30 μg). The plates were incubated for 20-24 h at 30 ± 2 °C and the diameter of zone of inhibition (in mm) was measured. The resistance profiles and resistance pattern were determined from the antibiogram data. The minimum inhibitory concentration (MIC) of the antibiotics was determined against a total of 60 V. harveyi isolates following the agar dilution method 8 . The significance of difference in the multiple antibiotic resistance patterns among _____________________ *Corresponding author