Isolation of Methicillin Resistant Staphylococcus aureus Strains from Hospital Sewage in Tehran in 2015

Staphylococcus aureus is known as a bacterial pathogen responsible for a wide range of human infections in hospitals and communities that are difficult to treat. The aim of this study was isolation and determination of antibiotic resistance patterns of methicillin-resistant S. aureus strains isolated from treated hospital sewage in Tehran during 2015.
The filtration system and the oxacillin containing chromogenic Hicrome aureus agar medium was employed to isolate methicillin resistant bacteria. All isolates were identified at the species level using standard biochemical tests and also PCR assay using specific primers for nucA gene, and the presence of mecA gene was also tested. Susceptibility of all MRSA strains to 17 antibiotics was determined using disc diffusion method according to guidelines of Clinical Laboratory and Standard Institute (CLSI). Minimum inhibitory concentrations (MIC) of oxacillin and vancomycin were also detected using broth microdilution assay
All 79 isolated colonies from the oxacillin containing medium showed resistance to cefoxitin and were also positive for nucA and mecA genes and were identified as MRSA. All MRSA strains were susceptible to vancomycin, linezolid, quinopristin-dalfopristin and chloramphenicol. Their resistance to penicillin, ciprofloxacin, kanamycin, tobramycin, erythromycin, amikacin, tetracycline, clindamycin, rifampin, trimethoprim-sulfamethoxazole, minocycline, gentamicin and nitrofurantoin were 100, 92, 89, 89, 87, 79, 79, 79, 62, 58, 48, 45 and 3% respectively. In this study, 25 antibiotic resistance patterns were identified in which 8% of strains showed susceptibility to all classes of antibiotics expect for penicillin and 92% of strains were resistant to 3-12 antibiotics.
The results of the present study illustrated the presence and persistence of highly resistant different clonal group of MRSA strains in treated hospital sewage. Emergence of these highly resistant potentially pathogenic bacteria in the treated sewage in Tehran could be a serious threat for the public health, which indicating the needs of urgent review to improve the efficiency of hospital sewage treatment systems.