The gram-positive anaerobic cocci typically are found as part of the normal flora of the oral cavity, upper respiratory tract, gastrointestinal tract, female genitourinary tract, and the skin. The anaerobic cocci are non-spore forming and may appear slightly elongated. The cells vary in size and may be arranged in tetrads, chains, clusters, pairs, or clumps. Carbohydrate utilization varies among the genera. In addition, organisms typically classified as aerobic, such as Staphylococcus epidermidis, include strictly anaerobic strains. Staphylococcus saccharolyticus and Staphylococcus aureus subsp. anaerobius grow under anaerobic conditions, although after subculture they may develop aerotolerance. If a gram-positive coccus demonstrates resistance to metronidazole (5 µg) after 48 hours of incubation, it is likely a Streptococcus species.

The gram-negative anaerobic cocci are part of the normal flora of the oral cavity and the gastrointestinal, genitourinary, and respiratory tracts of humans. Among anaerobic gram-positive cocci, the genera of clinical importance are Peptostreptococcus, Finegoldia, Gallicola, Parvimonas (previously Micromonas), Peptoniphilus, Murdochiella, Staphylococcus, and Anaerococcus.

The category of anaerobic gram-negative cocci is based on Gram stain morphology. This category includes the genera Veillonella, Megasphaera, Anaeroglobus, Negativicoccus, and Acidaminococcus. The genus Veillonella is ubiquitous as part of the normal flora of the human oral cavity and the genitourinary, respiratory, and gastrointestinal tracts.

Laboratory Diagnosis

 Direct examination of clinical specimens reveals gram positive or gram-negative cocci in chains, pairs, or singly. Follow-up tests to identify the organism are determined by interpretation of the initial Gram stain results. Organ isms typically are isolated on anaerobic blood agar, and they can be differentiated using the special-potency anti biotic disks previously described in this chapter. Gram positive cocci are sensitive to vancomycin and resistant to colistin. Gram-negative cocci typically are resistant to vancomycin. Peptostreptococcus anaerobius and Parvimonas micra demonstrate sensitivity to sodium polyanetholsul fonate (SPS). P. micra also produces a milky halo around the colonies on blood agar. Interpretation and identification of either gram-positive or gram-negative cocci from a clinical specimen should be reported with caution and should correlate with the patient’s signs and symptoms. Table 1 provides an identification scheme for representative anaerobic organisms.

Table1. Differentiation of Representative Gram-Negative Bacilli and Gram-Positive Cocci

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