Antibiotic Resistance and Chromium Reduction Pattern Among Actinomycetes

Abstract

Actinomycetes, one of the most important groups of microbes, exhibit many interesting activities such as degradation and transformation of organic and metal substrates together with production of antibiotics. With these bioactivities, actinomycetes would play an important role in the webs of the marine environment. The present study was designed to evaluate the antibiotic resistance pattern, antibiotic producing potential and chromium resistance as well as chromium reduction potential of a range of actinomycetes isolated from marine environments. Actinomycetes were isolated from marine sediment samples obtained from St. Martin's Island in Bangladesh. Antibiotic resistance among the selected isolates was studied against 10 different antibiotics by disc diffusion method and antibiotic producing potential was assessed by the perpendicular streak method. The isolates were screened for resistance towards heavy metal Cr(VI) on culture plates supplemented with Cr(VI) at concentrations ranging from 1-5 mM of Cr(VI). Highly resistant isolates were subjected to screening for Cr(VI) reduction activity, which was estimated using the Cr(VI) specific colorimetric reagent 1, 5-diphenylcarbazide. Out of the total 30 different selected isolates, 25 (83.33%) showed resistance against more than three antibiotics and 6 (20%) showed resistance to more than six antibiotics. Ninety three percent of the isolates showed MAR index greater than 0.2 and tolerance to Cr(VI) at 1mM of initial Cr(VI). None of the isolates displayed antimicrobial activity against the organisms tested. Among the isolates tested for chromate reduction, two were most efficient showing complete reduction of 1mM Cr(VI) within 24 h. These two isolates were capable of reducing chromate even at high initial Cr(VI) concentrations. Remarkably, the isolate SM-11 was found to reduce 82.67%, 44.34% of Cr(VI) at 2.5mM, 5mM of initial Cr(VI) concentrations respectively, within 72h of incubation. The majority of the actinomycetes isolates displayed resistance to both antibiotics and heavy metal chromium which indicates the possible acquisition of resistance factors due to environmental or human activities. The study also demonstrates possible correlation between antibiotic resistance and metal tolerance. Two of the isolates which showed considerable chromium reduction activity even at high chromium concentrations, may find potential application in bioremediation approaches.