Actinomycetes is a diverse and a large group
of gram positive filamentous and/ or branching bacilli. Actinomycete organisms are bacteria
and are usually handled in mycology since they are filamentous and may take few days to
several weeks to grow. The most common
organism in this group is Nocardia that is responsible to
cause a variety of infections including Mycetoma. Sulfur granules
may be seen by the naked eye in the specimen and under the microscope
during direct microscopic examination of the specimen stained by H
& E stain.
Actinomycetes are ubiquitous in nature, correct
identification of Nocardia species is essential to rule out
Streptomyces species that is usually considered nonpathogenic.
Growth rate few days to 4 weeks. Nocardia species must also
be compared with rapid grower mycobacteria that may mimic Nocardia
both by macroscopically (culture) and microscopically (direct specimen
Simple tests that are necessary to identify
Actinomycetes are growth rate, susceptibility to antibacterial agents,
morphology information, distinct musty order Gram stain, Modified
Kinyoun Stain (MK), Ziehl-Neelsen Stain (ZN), Casein Hydrolysis and
growth on selective and differential medium such as sodium pyruvate
(PYR) for branching and fragmentation by Nocardia species.