Sporicidal activity of local anaesthetics and their binary combinations with preservatives

The sporicidal activity of 1% solutions of five local anaesthetics and five preservatives (cetrimide, chlorocresol, chlorhexidine, phenoxyethanol and phenylmercuric nitrate) at their pharmacopeial concentrations, alone and in binary combinations was determined against Bacillus subtilis and Aspergillus niger spores at different temperature levels by the surface viable count technique. The sporicidal activity of all tested systems were temperature dependent. A. niger spores were much more sensitive to the tested systems than B. subtilis spores. The temperature at which 99% of all spores were killed after 30 min exposure time of the anaesthetics were calculated. In case of the local anaesthetics alone against spores of A. niger the recorded temperatures were 30 degrees C for amethocaine, 45 degrees C for both amylocaine and cincochaine, 48 degrees C and 50 degrees C for lignocaine and procaine, respectively, in contrast to 58 degrees C in a control with saline solution. Much higher temperatures were calculated against B. subtilis spores. Cincochaine was the most effective local anaesthetic with a recorded temperature of 60 degrees C, where 99% killing occurred. Amylocaine and amethocaine showed temperatures of 84 degrees C and 90 degrees C, respectively. Procaine, lignocaine as well saline solution as a control caused a 99% killing effect at temperatures higher than 100 degrees C. Among the 25 tested binary combinations of local anaesthetics and preservatives, the highest incidence of potentiation of the sporicidal activity was recorded with chlorocresol combinations, while other combinations of the four remaining preservatives showed different types of interactions at different percentages.