Seasonal incidence of and antibiotic resistance among Aeromonas species isolated from domestic wastewater before and after treatment in stabilization ponds.

The efficiency of stabilization pond treatment of domestic wastewater in removing culturable cells of motile Aeromonas and its influence on the incidence of resistance to seven antibiotics were investigated in this study. Removal efficiency was higher (P < 0.001) in the warm months (98.8%) than in the cold months (97%). Among the 264 isolates, 163 were Aeromonas caviae, 24 were A. hydrophila, and 54 were A. sobria. Twenty-three isolates could not be identified to the species level. In the influent, A. caviae dominated in both cold and warm months. In the water samples originating from the influent, A. sobria was present at higher percentages in the warm period. All the isolates were resistant to amoxicillin and most of them (73%) exhibited resistance to cephalothin. Of the three species tested, A. sobria was more susceptible to antibiotics than either A. caviae or A. hydrophila. The most striking difference among the species was seen in resistance to cephalothin. There were 91 % of A. caviae strains and 96% of A. hydrophila isolates that were resistant to cephalothin. However, only 9% of A. sobria strains exhibited resistance to this drug. The high incidence of resistance in raw sewage was connected with a high proportion of A. caviae, whereas in the water samples collected from the effluent during the warm months, a high proportion of A. sobria decreased the total amount of multiple-resistant bacteria. Results demonstrated the need for identification to the species level.