RESUMEN
Aims: This study was conducted to determine the effect of different doses of gamma radiation on Multi-drug resistant Pseudomonas aeruginosa isolated from River Nile at Rosetta branch and associated drains in Egypt. Place and Duration of Study: The study was started with samples collection in August 2010 through April 2011 in the Microbiology Dep., Central Laboratory for Environmental Quality Monitoring (CLEQM), National Water Research Center (NWRC), Cairo, Egypt and the National Centre for Radiation Research and Technology (NCRRT), Egyptian Atomic Energy Authority (EAEA), Cairo, Egypt. Methodology: Water samples were processed using membrane filtration, 144 strains of P. aeruginosa were isolated and identified and their antibiotic susceptibility was determined against 20 different antibiotics using agar disc diffusion method. Irradiation of bacterial isolates was processed using gamma irradiation unit of cobalt (Co60) and the D10-value was calculated from the survival curve. Results: Isolates were categorized as multi-drug resistant Pseudomonas aeruginosa (MDRPA). 125 (86.8%) were found to be extensively drug resistant (XDR) and 19 (13.2 %) were characterized as possible pan drug resistant (PDR). The highest resistance (100%) was mostly directed to amoxycillin/clavulanic acid, ampicillin, carbenicillin, methicillin, cephalothin, kanamycin, vancomycin, tetracycline, erythromycin, clindamycin, trimethoprim/sulfamethoxazole, nitrofurantoin and chloramphenicol. More than 75% of isolates were sensitive to norfloxacin (82.6%), piperacillin (81.2%), amikacin (79.2%) and tobramycin (77.8%). 63.2%, 26.4% and 14.6% of isolates were sensitive to ofloxacin, cefotaxime and ceftriaxone, respectively. The viable counts of MDRPA decreased with increasing radiation doses of gamma rays up to the lethal dose (3 kGy). The counts of 0.5, 1.0, 1.5, 2.0, 2.5 kGy irradiated samples were respectively 7.8, 6.5, 4.7, 2.3& 1 log10 and the D10-value calculated from the survival curve was 0.27 kGy. Conclusion: Contaminated fresh water may act as reservoirs for antibiotic resistant pathogens. Regular monitoring of Multi-drug resistant pathogens in aquatic environments should be adopted constantly. Gamma radiation demonstrates a potential value for wastewater treatment and pollution control.