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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 or culture).

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.


Actinomycetes Diagnostic Testing Flow Charts
Click circular thumbnails for larger images and descriptions.


H&E Stain
Wet Prep
Gram Stain
Gram Stain
Modified Kinyoun Stain
Blood Agar
PYR Media
Modified Kinyoun Stain
Ziehl-Neelsen Stain
Modified Kinyoun Stain
PYR Media
Casein Agar
Casein Agar
Casein Agar
Casein Agar
Sabouraud and PYR Agar
Sabouraud Agar
Sabouraud Agar
Sabouraud Agar
Sabouraud Agar
Sabouraud Agar
Modified Kinyoun Stain
Sabouraud Agar
Sabouraud Agar


Copyright 1999-2007 Department of Microbiology, Mount Sinai Hospital, Toronto, Canada. All rights reserved.