Selection of optimum laboratory tests for the identification of Moraxella catarrhalis

We evaluated a variety of conventional and rapid tests and examined the erythromycin susceptibility of a collection of Moraxella catarrhalis and commensal neisseria strains in order to determine the optimum method for routine identification. One hundred and fifty three strains were tested by Gram stain, catalase, oxidase, carbohydrate degradation by two methods and the presence of esterases using indoxyl acetate, 4-methylumbelliferyl butyrate (MUB). Tween 80 and tributyrin as substrates. Erythromycin MICs and zone diameters around 1, 5 and 15 micrograms discs were determined by the NCCLS method for 151 of the strains. A combination of Gram stain, oxidase and either indoxyl acetate, spot MUB or tributyrin hydrolysis test proved to be reliable and potentially the most convenient for routine testing. MICs and zone diameters easily distinguished between the erythromycin-sensitive M. catarrhalis and the erythromycin-resistant commensal neisserias and would provide confirmation of identification if used for susceptibility testing.