This study was designed to determine the extent of contamination of water storage tanks by non-lactose fermenter Enterobacter spp, and to characterize the chlorine and antibiotic resistance status. Moreover, to find the correlation between biofilm formation and resistance to chlorine. For this purpose, a total of 60 water samples were collected from residential and restaurant water storage tanks. Bacterial analysis and antibiotic susceptibility profiles of the samples were assessed by the Most Probable Number (MPN) and Vitek 2 compact tests, respectively. The biofilm formation was quantified by crystal violet staining method and chlorine resistance test by microdilution technique.

Obtained results indicated that water samples were contaminated by Escherichia coli (35%). Additionally, water samples that were assessed to be potable by the MPN test showed that (44%) positive for Enterobacter cloacae. Results of chlorine resistance test revealed variation in resistance of E. cloacae to different concentrations of chlorine, and relatively similar antibiotic susceptibility profiles. Moreover, biofilm analysis showed the isolates that were resistant to concentration of chlorine 400 mg L-1, have formed significantly more biofilm than those that were resistant to other concentrations. A positive non-linear correlation (r = 0.72) was found between the degree of biofilm formation and the ability of isolates to resist different chlorine concentrations, and no correlation has been detected between antibiotic and chlorine resistance.

It can be concluded that the presence of chlorine resistant E. cloacae in drinking water can pose a real public health threat. The routine microbial water analysis should be modified to include detection of non-lactosefermenter Enterobacter.


MPN, Chlorine treatment and chlorine resistant, Antibiotic resistant, Water storage tanks, Enterobacter cloacae, Biofilm formation.





Document Type

Original article



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.