Occurrence And Distribution of Gram Negative Bacteria in Chlorinated Drinking Tap Water and Their Susceptibility To Chlorine and Antibiotics

Abstract

Viable microorganisms are often recovered from potable chlorinated water distribution systems; suggesting the presence of chlorine-resistant microorganisms. Chlorine and antibiotic resistant microorganisms pose a global concern in sanitation and public health. The objective of the study was to explore the occurrence and distribution of gram negative bacteria in chlorinated drinking tap water and their susceptibility to chlorine and antibiotics.

This study was conducted at Environment and Climate Change Laboratory of Nepal Academy of Science and Technology from April 2009 to August 2010. A total of 107 water samples were collected randomly from Bansbari (52) and Mahankalchaur (55) treatment and distribution plant. Physico-chemical parameters of drinking water like pH, turbidity, conductivity, temperature, free residual chlorine, combined residual chlorine and total residual chlorine and microbiological parameters like heterotrophic plate count and total coliform count were conducted by standard methods. Chlorine resistance and antibiotic susceptibility test of selected isolates were performed by liquid assay and modified Kirby Bauer disc diffusion method respectively.

Sixty (56.1%) water samples crossed the permissible limit of WHO guideline value in heterotrophic plate count and total coliform count each. Ten different genera of gram negative bacteria were recovered in which E. coli was predominant followed by Citrobacter spp., Shigella spp., Enterobacter spp., Providencia spp., Klebsiella spp., Salmonella spp., Pseudomonas spp., Proteus spp. and Edwardsiella spp. Higher the temperature of water sample, higher the bacterial growth was obtained (p= 0.002) and similarly higher level of free residual chlorine in water reduced the bacterial growth (p= 0.037) whereas increase or decrease of pH (p= 0.454), turbidity (p= 0.164) and conductivity (p= 0.969) didn't affect the microbial growth. A negative correlation (r= -0.162) between heterotrophic plate count and free residual chlorine was observed however without statistical significance (p= 0.096). Similarly, a negative correlation r= -0.383) between total coliform count and free residual chlorine was observed with statistical significance (p= 0.001). In chlorine assay, all tested eight different genera of gram negative bacteria were found to be chlorine resistant at 0.2 mg/lt for a contact time of 30 minutes. All of the tested gram negative chlorine resistant bacteria were found to be antibiotic resistant to at least one antibiotic. Average time required for T 99.9 (3-log) and T (4-log) reduction of viable isolates from initial population of 2×10 6 99.99 cells/ml were found to be less than 30 minutes and greater than 60 minutes respectively. Log inactivation of various bacterial isolates with chlorine concentration of Nepal Standard (0.2 mg/lt for a contact time of 30 minutes) were found to be ranged from 3 to 3.5-log. Among the isolates, Ampicillin was found to be 100% resistant followed by Amoxycillin, Cotrimoxazole, Tetracycline, Ceftazidime, Ceftriaxone, Chloramphenicol, Gentamicin and Ofloxacin. The isolates of water samples were found to 78.7% Antibiotic resistant (AR) and 27.7% Multiple antibiotic resistant (MAR).

Emergence of chlorine and antibiotic resistant organisms in drinking water probably demands alternate disinfection or mitigation strategy.

Keywords: Antibiogram, Chlorine Resistance, Drinking Water, Disinfection, Gram Negative Bacteria