Use of a non-radioactive DNA hybridization technique to study the effect of quinolone antibiotics on plasmid replication and curing.

A biotin-labelled DNA probe was used to detect the presence of bacteria carrying plasmid DNA and to study the effect of ciprofloxacin and nalidixic acid on plasmid carriage. The colour intensity generated in dot-blot hybridization tests was measured with a computer-controlled image analysis system and was shown to be related to the number of plasmid copies present. Ciprofloxacin, at about 0.5 x MIC, reduced the plasmid copy number in a bacterial population, while nalidixic acid, used at a similar ratio to MIC, had a much smaller effect. The method could potentially be used to study the effect of antibiotics on plasmid replication and curing in any bacterial system for which a suitable DNA probe is available.