The Gram stain provides direction for the workup and identification of the anaerobe recovered from the culture. Refer to the attached Centers for Disease Control and Prevention Job Aid for a review of the Gram Stain procedure.13
Some anaerobic bacteria may appear gram negative when they are actually gram positive. Be mindful of these challenges by:
- Checking for the presence of cellular material (white and red blood cells) on a direct smear to compare color (rule out true overdecolorization).
- Looking for spores, as these may provide a hint that rods are actually gram positive.
- Considering the smear may have been made from an old culture or plate that has been left out in the ambient air for a time. The bacteria may be dead or dying, causing them to lose their gram-positive appearance.
- Comparing the direct Gram stain and any stains made from colony growth on agar plates and any broth cultures. Be aware that bacteria may appear more pleomorphic on Gram stains made from broth cultures.
Anaerobes may present as:
- Gram-positive bacilli
- Gram-negative bacilli
- Gram-negative cocci
- Gram-positive cocci
Appearance of the bacterial organism is important
Ask: Large or small? Short or elongated? Document findings, as this information is useful when working toward an identification.
Rods:
- Fusiform
- Spore-former
- Pleomorphic
- Coccobacillary
- Boxcar ends
- Branching
Cocci:
