E.coli is one of the bacteria isolated from hospital sewage and can be a source for dissemination of antibiotic resistance genes (17). Since antibiotic resistance genes are often located on plasmids or transposons that capable to transfer to other microorganisms, the presence of these genes in hospital sewage can be a serious threat to public health (5). Therefore hospital wastewater should be properly collected, treated and returned to the natural environment (1).
In our study, all pre and post sewage treatment isolates were totally resistant to penicillin and highly resistant to other classes of antibiotics indicating the presence of high resistant strains of E. coli in hospital sewage. The both pre and post sewage treatment isolates was mostly susceptible to aminoglycosides (gentamicin and amikacin). The resistance rate of ESBL-producing isolates to most antibiotics tested was slightly lower in post sewage treatment isolates; however, the resistance rate in non-ESBL isolates for most antibiotics was higher in post treated isolates. It has been shown that in many cases, sewage treatment process cannot significantly eliminate resistant strains or resistance genes (7). Furthermore, there are some reports that showed the increase in both antibiotic resistance rates and antibiotic resistance genes for post treatment sewage isolates (18, 19). For instance, research has shown that sewage treatment process caused an increase in antibiotic resistance of E. coli to nalidixic acid, sulfamethoxazole, cephalexin and ceftriaxone (20). These results suggest that the effect of sewage treatment process on various classes of antibiotics may be different.
The rate of ESBL-producing E.coli strains have been reported higher than our results which may reflect difference in the regional frequencies of resistance genes (7, 21). In the present study, the rate of ESBL-producing E. coli strains was slightly higher in the post treatment sewage isolates, which may reflect the transferring of EBSL among bacteria within sewage. Since the ESBL genes mostly located on plasmid in E.coli the horizontal transferring of ESBL genes between bacteria is reasonable. The bla SHV, including bla SHV-5 and bla SHV-12 have been found in hospital sewage isolates (8). A high incidence of SHV strains in untreated wastewaters in Australia has been reported, possibly due to the transmission of these genes in liquid environments (6). In our study, the frequency of bla SHV-5 and bla SHV-12 genes showed no significant changes in isolates of pre and post treatment sewage. The rate of CTX-M ESBL subgroups are raising among enterobacteriaceae isolates (22).
The low effect of sewage treatment processes on bla CTX-Ms can be attributed to the presence of this group in a wider range of gram-negative and even gram-positive microorganisms present in sewage. The majority of the microorganisms entering the sewage are gram negative with intestinal origin (23). The ESBLs genes in the presence of antibiotics can give bacteria a selective and competitive survival advantage inside hospital sewage (24). A research on the sewage treatment effect on resistance to third-generation cephalosporins in the enterobacteriaceae showed that the frequency of the bla CTX-M gene in the post treatment sewage isolates increased because of easily transmission of these genes among gram-negative bacteria (25). It has been shown that the prevalence of bla CTX-M-1 and bla CTX-M-2 genes increased by 15% among isolates of post treatment sewage process (23). These findings are consistent with our results for the increased rate of bla CTX-M-1 and bla CTX-M-2 genes after swages treatment. Studies on hospital sewage isolates have also indicated a low frequency of qnr genes. The qnr genes can be easily transferred through plasmids to other bacteria and, together with chromosomal mutations, can result in the high levels of resistance to fluoroquinolones (6, 15, 26). In a large study in China (2002–2005), the frequency of qnrA, qnrB and qnrS genes in 514 E. coli isolated from hospital sewage was 0.4, 1.2 and 2.7%, respectively (6) which support our results for the low frequency of qnr genes among isolates.