Several species of anaerobic bacteria display variable Gram stain reactions which often make identification difficult. A simple, rapid method utilizing a 3% solution of potassium hydroxide to distinguish between gram-positive and gram-negative bacterial was tested on 213 strains of anaerobic bacteria representing 19 genera. The Gram stain reaction and KOH test results were compared with the antibiotic disk susceptibilities (vancomycin and colistin) the preliminary grouping of anaerobic bacteria. All three procedures were in agreement for the majority of strains examined. Some strains of clostridia, eubacteria, and bifidobacteria stained gram negative or gram variable; the KOH and antibiotic disk susceptibility tests correctly classified these strains as gram-positive. The KOH test incorrectly grouped some strains of Bacteroides sp., Fusobacterium sp., Leptotrichia buccalis, and Veillonella parvula, but all Gram stain results for these strains were consistent for gram-negative bacteria. The KOH test is a useful supplement to the Gram stain and antibiotic disk susceptibility testing for the initial classification of anaerobic bacteria.

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The hundred twenty-four isolates of anaerobic and microaerophilis organisms were examined with a polyvalent direct immunofluorescent antibody (DFA) reagent directed against Legionella pneumophila, serogroups (SG) I-IV. Three of 53 isolates of Bacteroides fragilis cross-reated with the SG I component of the reagent. Rabbit antisera to three SG I strains of L. pneumophila cross-reacted with these three B. fragilis isolates, and cross-reactivity was not due to preexisting rabbit antibodies; antisera to a fourth SG I strain did not cross-react with these isolates. Indirect fluorescent antibody (IFA) examinations of convalescent-phase sera from eight of 41 patients with B. fragilis group infections showed titers of greater than or equal to 1:128 or a fourfold or greater seroconversion against L. pneumophila, SG I. All eight patients had bacteremia due to B. fragilis, but five B. fragilis isolates available from these patients were DFA-negative. Thus, false-positive reactions may result from DFA as well as IFA examinations of sera from patients with B. fragilis infections.