Antibiotic Resistance of Heterotrophic Bacteria Isolated from Drinking Water – from the Water Source to the Consumers’ Taps

The aim of this study was to determine the antibiotic resistance (ABR) phenotype of heterotrophic bacteria isolated from a Bulgarian drinking water supply system. The disc diffusion method of Kirby-Bauer was used in the bacterial isolates susceptibility testing toward 13 antibiotics from 7 classes. The ABR phenotype of 233 bacterial strains was determined and the influence of drinking water treatment and its transportation in the water supply network on the ABR prevalence was assessed. Among the isolates, the ABR incidence was low or medium, only tetracycline resistance was very rare, and no gentamicin resistance was detected; 42% of the isolates were resistant to one class of antibiotics, and 11% exhibited multiple resistance. Water chlorination has led to restructuring of the aquatic community and changes of the predominant ABR phenotypes. The ABR phenotype of the isolates differed depending on the drinking water sampling point demonstrating the local influence of the water supply network. Although there is no regulation of the ABR of drinking water, the data demonstrated prevalence of bacterial resistance to different antibiotics and need of a greater concern the microbiological water quality.