EVALUATION OF METHODS FOR DISTINGUISHING GRAM-POSITIVE FROM GRAM-NEGATIVE ANAEROBIC-BACTERIA

Several species of anaerobic bacteria display variable Gram stain reac tions which often make identification difficult. We have evaluated the Gram stain, the potassium hydroxide test (3% KOH), an antibiotic susc eptibility test (10 mg/L vancomycin), and the L-alanine aminopeptidase test (utilising fluorogenic L-alanine-4-methoxy-beta-naphthylamide [L AMNA] and chromogenic L-alanine-4-nitroanilide [LANA] substrates) for the preliminary grouping of Gram-positive and Gram-negative anaerobes. By testing 88 clinical isolates, 35 Gram-positive and 53 Gram-negativ e, we obtained the following results: sensitivity, 49, 94, 21 and 23%; specificity, 100, 98, 91 and 91%; positive predictive value, 100, 97, 79 and 80%; negative predictive value, 56, 96, 43 and 44%, for the KO H test, the vancomycin susceptibility test, L-alanine-4-methoxy-beta-n aphthylamide test and L-alanine-4-nitroanilide test, respectively. The KOH test correctly grouped all the Gram-positive strains but incorrec tly grouped 27 of 53 Gram-negative strains (e.g. Prevotella spp., Fuso bacterium spp., Veillonella spp, and Leptotrichia spp.). The vancomyci n test correctly identified 33 of 35 Gram-positive strains (resistance with Clostridium innocuum and Actinomyces odontolyticus) and 52 of 53 Gram-negative strains (susceptible with Prevotella melaninogenica). B oth LAMNA and LANA tests correctly grouped 32 of 35 Gram-positive stra ins but the LAMNA test incorrectly grouped 42 of 53 Gram-negative stra ins whilst the LANA test incorrectly grouped 41. From the methods exam ined we conclude that the vancomycin susceptibility test is the most u seful for the preliminary classification of anaerobes in the clinical laboratory. (C) 1996 Academic Press