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Objective: Infectious endophthalmitis is among the most serious complications of cataract surgery. Gram-negative bacteria, including Pseudomonas aeruginosa, are responsible for less than 30% of cases; however, their rapidity of infection and virulence often results in poor visual outcome despite prompt antibiotic treatment. The purpose of this study was to investigate an outbreak of post-cataract surgery P. aeruginosa endophthalmitis in India. Design: Hospital-based case series. Participants: Twenty patients with acute postoperative endophthalmitis who underwent cataract surgery at one of the peripheral centers of Joseph Eye Hospital, Tiruchirapalli, Tamil Nadu, India, from February 23 to April 2, 2008. Interventions: Vitreous aspirates and environmental surveillance specimens were inoculated for culture. Antibiotic susceptibility testing was performed by agar diffusion method. Polymerase chain reaction (PCR) with enterobacterial repetitive intergenic consensus (ERIC) primers (ERIC-PCR) was used to establish the clonal relationship between clinical and environmental isolates. Main Outcome Measures: Post-cataract surgery P. aeruginosa endophthalmitis. Results: Pseudomonas aeruginosa was isolated from 20 eyes with postoperative endophthalmitis, the phacoemulsifier''s internal tubes, the povidone-iodine solution, and the operating theater air-conditioning system. All strains were multidrug-resistant to cefazolin, chloramphenicol, tetracycline, aminoglycosides, and fluoroquinolones; conversely, most of them were susceptible to polymyxin B. Polymerase chain reaction with enterobacterial repetitive intergenic consensus primers disclosed 2 major clusters: six genetically identical clinical isolates shared 94% of similarity with the air-conditioning isolate; 11 other clinical isolates had 88% of similarity with the former strain. Despite the prompt use of intravitreal antibiotics, 10 patients had evisceration or phthisis of the affected eye. Conclusions: The outcome of cataract surgery-related P. aeruginosa endophthalmitis is poor. The detection of multidrug-resistant isolates is a serious problem, jeopardizing an appropriate choice of treatment. Polymerase chain reaction with enterobacterial repetitive intergenic consensus results strongly suggest that the main source of infection in this outbreak was the contaminated air-conditioning system. Polymerase chain reaction with enterobacterial repetitive intergenic consensus is an inexpensive, fast, reproducible, and discriminatory DNA typing tool for effective epidemiologic surveillance of clinical and environmental isolates of P. aeruginosa. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article. [Copyright &y& Elsevier]