Depending on the culture medium (liquid or solid) used to isolate the organism, certain cellular arrangements may not be apparent. This is because the Gram stain morphology is often less characteristic when smears are prepared from colonies growing on the agar surface, compared with smears prepared directly from the clinical specimen. The information contained in the table can still prove helpful, especially for isolates recovered from broth.
| Oxygen Requirement/Gram Stain Characteristics | Common Genus | Characteristic Gram Stain Morphology/Comments |
| Aerobic, gram-positive cocci (GPC) | Staphylococcus Streptococcus Enterococcus | - Staphylococcus species: GPC in clusters or tetrads.
- Streptococcus species: GPC in singles, pairs, or short to long chains.
- S. pneumoniae: GPC in pairs that are lancet-shaped, occasionally with a capsule that appears as a "halo" surrounding the organisms.
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| Aerobic, gram-positive rods (GPRs) | Bacillus Lactobacillus Nocardia | - Bacillus, Lactobacillus, and Mycobacterium species RARELY exhibit a diphtheroid or "picket-fence" morphology.
- Bacillus: Large GPRs, often in pairs or chains, with rounded or square ends and usually a single endospore. The location of the spore is different in various species. Characteristically, Bacillus species tend to stain gram-negative as they age.
- Lactobacillus: Long, slender GPRs, often in long chains like spaghetti.
- Nocardia: Branching, long, thin filaments. May demonstrate the diphtheroid appearance; tend to stain with a beaded, gram-variable pattern.
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| Anaerobic, GPRs | Clostridium Actinomyces | - Clostridium species RARELY exhibit a diphtheroidal morphology. They are large, "box-car" shaped bacilli that may contain spores. May decolorize easily, appearing to be gram-negative.
- Actinomyces is a thin, filamentous (branching) GPR that can demonstrate a diphtheroidal appearance.
|
| Facultative anaerobic, GPRs | Listeria Erysipelothrix Corynebacterium | - These organisms can demonstrate a diphtheroid or "picket-fence" arrangement.
- Listeria: Short, coccobacillus
- Erysipelothrix: Short, slender, slightly curved, and sometimes forming long filaments.
- Corynebacterium: Small, palisading GPRs
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