Inhibition of bacteria on agar surfaces by vapour phase triclosan

Sir, Triclosan is the most widely used member of the bisphenol family of disinfectants, which are characterized by their high intrinsic activities and proportional low solubility in water. Unfortunately, they also share ineffectiveness against pseudomonads and an ability to select for resistance in Staphylococcus aureus. In recent years, an increasing number of domestic plastic and cleaning products incorporating triclosan have been marketed under the Microban trade mark. In their paper in this journal 1 on triclosan-impregnated food-storage boxes, Braid & Wale described, but were “unable to account for the complete suppression of growth of S. aureus on one side of open agar plates that were directly adjacent to the triclosan-impregnated wall of the 9 L boxes used”. This effect is reproducible and we have also demonstrated it, under experimental conditions, by exposure of strains of both S. aureus and Escherichia coli to a proprietary Microban household cleaning fluid and a 5% solution of triclosan. The degree of inhibition appears to be proportional to both the concentration of the agent and the respective susceptibility of the test strains. The authors 1 also found it difficult to equate the vapour phase of triclosan with the pattern of inhibition found, asking why all of their plates had not been affected equally and evenly; however, we believe that the pattern of suppression seen can be entirely explained in terms of classical zone formation theory.2 In our experiments, we inoculated nutrient agar plates with our test organisms, as if for a conventional susceptibility test. We then aseptically removed one-third of the agar and, into the space, we placed an inverted plastic cap from a 5 mL bottle, filled with either the Microban cleaning fluid or 5% triclosan. The plates, with lids, were incubated overnight at 37°C and examined for inhibition. Large, rather flattened, zones of inhibition were seen immediately opposite the reservoir of cleaning fluid. We also used modified Petri dishes, in which spacing shims held the inoculated agar surfaces at varying heights above reservoirs of the test solutions. Large zones of inhibition were formed at heights of up to 6 cm. It was noted that the label on the cleaning fluid bottle stated that the product also contained formaldehyde. However, tests indicated that the inhibition zones produced were consistent with those produced by triclosan and the differences in zone sizes seen between the test strains were consistent with the differing MICs of triclosan for those organisms.