[Development of antibiotic resistance in purified sewage effluents subjected to chlorination]

Antibiotic-resistance is widely spread phenomenon in the environment because of uncontrolled discharge of urban and animal wastewaters. Sewage treatment can significantly reduce the number of both sensitive and resistant bacteria. A reduction of about 1.5 logarithmic units in faecal coliforms was observed during biological treatment (3, 7), but a simultaneous increase in the percentage of resistant strains occurred because of not well understood selection phenomena. The above reported bacterial reduction is not always sufficient to meet the quality standards of Italian legislation required to discharge the treated effluents into surface waters, and so, chlorination become a compulsory additional treatment whose impact on both sensitive and resistant microflora must be evaluated. The results obtained in the present research have demonstrated that chlorine concentrations in the range of 0.5-2 ppm are able to reduce significantly the faecal coliforms concentrations and, in particular, treatment with 1 ppm of chlorine for 1 hour reduces the concentration of the above reported bacteria to the extent of 2 logarithmic units, so that their final concentration are of the about 10(2)/100 ml. The surviving chlorine tolerant bacteria seem to be antibiotic resistant in higher percentage than the chlorine sensitive ones and so, as a consequence, a significant increase in the antibiotic resistance and multiresistance was observed in the chlorinated effluents. In this context it is interesting to underline the larger variety of resistance patterns observed in the chlorine-resistant bacteria in comparison with the uniformity in the resistance patterns observed in isolated from unchlorinated effluents. The selected chlorine-tolerant strains seem to be less able to transfer their resistances under laboratory conditions, not because of curing effect of chlorine on the plasmids but, probably, because of the damage to cellular cell envelopes.