The identification of the four genera described in this chapter must be considered along with that of Actinomyces, Bifidobacterium, and Propionibacterium spp. Although the latter genera are usually considered with the anaerobic bacteria, they grow on routine laboratory media in 5% to 10% CO2. Some are catalase negative. Therefore, as shown in Table 1, these organisms must be considered together when a laboratory encounters catalase-negative, gram-positive, non-spore-forming rods.
Table1. Biochemical and Physiologic Characteristics of Catalase-Negative, Gram-Positive, Aerotolerant, Non–Spore-Forming Rods
Several commercial systems for fastidious gram negative bacterial identifications will adequately identify Gardnerella. The HNID panel (Haemophilus-Neisseria identification panel, Dade MicroScan, West Sacramento, California) works particularly well. However, rapid identification panels usually are used for isolates from extra genital sources (e.g., blood).
Comments Regarding Specific Organisms
A presumptive identification of G. vaginalis is sufficient for genital isolates, based on typical appearance on Gram stain, beta-hemolysis on HBT agar, and negative tests for oxidase and catalase. Corynebacterium lipophiloflavum, a bacteria isolated from females with bacterial vaginosis, is catalase positive.
The beta-hemolytic Arcanobacterium spp. resemble the beta-hemolytic streptococci but can be differentiated from them by Gram stain morphology. A. haemolyticum and A. pyogenes can be differentiated based on liquefaction of gelatin; A. pyogenes is positive and A. haemolyticum is negative. A. bernardiae is nonhemolytic.
Erysipelothrix sp. is the only catalase-negative, gram positive non-spore-forming rod that produces hydrogen sulfide (H2S) when inoculated into triple sugar iron (TSI) agar (Figure 1). Some Bacillus spp. also blacken the butt of TSI, but they are catalase positive and produce spores. Automated identification with the Vitek2 and Phoenix systems and the API ID system is reliable for identification.
Fig1. H2S production by Erysipelothrix rhusiopathiae in TSI (B). A negative TSI (A) is included for comparison.
Lactobacillus spp. are usually identified based on colony and Gram stain morphologies and catalase reaction (negative). Differentiation from viridans streptococci may be difficult, but the formation of rods in chains rather than cocci in thioglycollate broth is helpful. Alter natively, a Gram stain of growth just outside the zone of inhibition surrounding the 10-U penicillin disk placed on a blood agar plate inoculated with a lawn of the organ ism should show long bacilli rather than coccoid forms if the organism is Lactobacillus spp.
