Specimens may contain a lot of (inflammatory) polymorphonuclear white blood cells, as well as granular tissue and fibrosis. Expected microbiology smear findings for the aerobic actinomycetes are as follows:
Gram stain
Gram-positive organisms appear purple/blue. Gram-negative organisms appear red.
- Thin, delicate filamentous, beaded, branching organisms (although Rhodococcus appears coccoid and chaining) are present. Additional characteristics of the Gram stain will be reviewed in the section on genera and species.
- Aerobic actinomycetes stain weakly to strongly gram positive.
- On a direct smear, bacteria are usually surrounded by polymorphonuclear white blood cells.
Modified acid-fast stain (MAS) - using a weak acid decolorizer
Using the modified Kinyoun method, modified acid-fast organisms appear pink-red (positive). A negative staining reaction is blue-green. Sometimes, organisms stain weakly positive, or variably (some appear positive, some appear negative).
Limitation: Some of the organisms containing mycolic acid in cell walls do not stain acid-fast if grown on blood agar.7
- Nocardia species stain weakly acid-fast; Rhodococcus, Gordonia, and Tsukamurella vary upon staining with a modified technique.
- Streptomyces species stain negative. However, some spores from Streptomyces hyphae may stain acid-fast.
- Note: Mycobacterium stain strongly acid-fast.
7. Leber A. (Ed.). (2016). Clinical microbiology procedures handbook (4th ed.). American Society for Microbiology Press.Figure 8. CDC/Hollinger. (1968). Image #21042. Under a magnification of 1200X, this photomicrograph of a lung tissue specimen, which had been harvested from a patient, who had been diagnosed with a case of nocardiosis, revealed some of the histopathologic changes caused by the Gram-positive bacterium, Nocardia asteroides. [Now no longer referred to as asteroides, see recent changes in taxonomy.] PHIL public domain. https://phil.cdc.gov/Details.aspx?pid=21042
Figure 9. CDC/Ajello. (1968). Image #21346. Under a magnification of 1125X, this photomicrograph depicted numerous, Nocardia asteroides [now no longer referred to as asteroides; see recent changes in taxonomy] bacteria, in this acid-fast-stained specimen. PHIL public domain. https://phil.cdc.gov/Details.aspx?pid=21346
