Effect of wastewater stabilization ponds on antimicrobial susceptibility and haemolysin occurrence among motile Aeromonas strains

Aeromonas strains (total = 953) isolated from raw wastewater, stabilization pond effluent and sediments were evaluated for their susceptibilities to 17 antibiotics and for their ability to produce haemolysins. Stabilization ponds did not seem to select highly resistant strains of aeromonads. There were no differences in the resistance patterns of isolates from raw sewage, stabilization pond effluent and sediments. All strains were found to possess multiple resistance, most commonly to ampicillin, amoxicillin and novobiocin. Almost 90% of the strains of A. hydrophila and A. ravine were resistant to cephalothin, whereas more than 80% of A, sobria isolates were found to be susceptible to this antibiotic. Resistance to trimethoprim, oxytetracycline, nalidixic acid, chloramphenicol, tetracycline, trimethoprim-sulphamethoxazol, polymyxin B, kanamycin or erythromycin among all isolates did not exceed 10%. Moreover, no strain was found to be resistant to gentamycin and only 9 of the 953 isolates exhibited resistance to cefotaxim. The percentage of haemolytic strains was significantly higher in the stabilization pond effluent than in raw sewage. This high incidence of haemolytic activity was connected with a high proportion of A, sobria whereas, in samples from the raw sewage or stabilization pond sediments a high proportion of A. ravine decreased the total amount of haemolytic aeromonads. The high incidence of haemolytic activity (alpha + beta) was associated particularly with A. sobria (93.3%) and A. hydrophila (88.7%) whereas A. caviae was found to be the lowest haemolytic species (16.3%).