Sporicidal activity of local anaesthetics and their binary combinations with preservatives

The sporicidal activity of a 1% solution of five local anaesthetics and five preservatives (cetrimide, chlorocresol, chlorhexidine, phenoxyethanol and phenylmercuric nitrate) at their commonly used concentrations, alone and in binary combinations, was determined against Bacillus subtilis and Aspergillus niger spores at different temperature levels by surface viable count technique. The sporicidal activity of all tested systems was temperature dependent and A. niger spores were much more sensitive to the effect of the test systems than B. subtilis spores. The temperatures at which 99% kill is achieved after 30 min exposure were calculated. For local anaesthetics used singly against A. niger the recorded temperatures were 30 degrees C for amethocaine, 45 degrees C for amylocaine, 43 degrees C for cincochaine, 48 degrees C and 50 degrees C for lignocaine and procaine, respectively. A control temperature of 58 degrees C for saline solution was observed. Much higher temperatures were recorded for B. subtilis spores. Cincochaine was the most effective local anaesthetic with a recorded temperature of 60 degrees C for a 99% kill while amylocaine and amethocaine showed temperatures of 84 and 90 degrees C respectively. Procaine, lignocaine as well as the control saline solution recorded temperatures higher than 100 degrees C. Among the 25 binary combinations of local anaesthetics and preservatives tested, the most pronounced potentiation of the sporicidal activity against fungal spores was recorded with chlorocresol combinations, while other combinations of the four remaining preservatives showed different types of interactions at various temperature levels.