Introduction: This course is intended as a basic-to-intermediate level introduction to aerobic actinomycetes. Participants should already have a basic knowledge of the tests performed in the medical (clinical) microbiology laboratory. Updates in taxonomy and nomenclature are presented in an effort to provide the most recent information in a rapidly evolving field of microbiology.
A note regarding image footnotes: Several public domain images provided in this module contain original captions which utilize outdated nomenclature. To avoid confusion, updates will be inserted using brackets ([ ]) with a note referencing the most recent nomenclature.
Background:
The actinomycetes are filamentous bacteria that stain gram-positive. Some genera are anaerobic (eg, Actinomyces), while others are aerobic (eg, Nocardia, Streptomyces). This course will focus on the more commonly isolated, medically important aerobic actinomycetes (Nocardia, Streptomyces, Rhodococcus, Gordonia, Acinomadura, Tsukamurella, Williamsia, and other similar organisms), but will not include Mycobacterium or Corynebacterium (which belong to the same order, - refer to 'Classification' below).
These bacteria were once thought to be fungi because of their branching appearance upon staining, growth characteristics (some form aerial mycelia), and clinical manifestations. Actinomycetes are not fungi but do appear closely related to the mycobacteria.
The aerobic actinomycetes cause a wide range of human diseases as well as high morbidity and mortality among the immunocompromised. They can be difficult to identify, and challenging to treat. Treatment may require a long course of antibiotics, as well as wound care or surgery.
Classification1:
Kingdom - Bacteria
Phylum - Actinobacteria
Class - Actinobacteria
Subclass - Actinobacteridae
Order - Actinomycetales
- The suborder, Corynebactereae contains the following genera: Gordonia, Nocardia, Rhodococcus, Tsukamurella, Williamsia, Corynebacterium, Mycobacterium, and others.
- The suborder, Streptosporangineae contains the genus: Actinomadura
- The suborder, Streptomycineae contains the genus: Streptomyces
Safety:
Use Biosafety Level 2 (BSL2) precautions when working with any of the actinomycetes.
Primary laboratory hazards include:
- mucocutaneous contact with the infective agent,
- accidental parenteral inoculation, and
- exposure via aerosol.
Note: Whenever aerial mycelia (eg, powdery growth) is present, handle the isolate as a fungal isolate. Work should be performed in a biological safety cabinet (BSC). Sealing agar plates closed using approved tape or film is recommended.
1. Conville PS, Brown-Elliott BA, Witebsky FG. Nocardia, Rhodococcus, Gordonia, Actinomadura, Streptomyces, and other aerobic actinomycetes. In: Carroll KC, Pfaller MA, Landry ML, McAdam AJ, Patel R, Richter SS, Warnock DW, eds. Manual of Clinical Microbiology, 12th ed. ASM Press; 2019:525-557.
2. CDC/Kaplan. Image #18868. This slant culture containing a Sabouraud dextrose agar (SDA) growth medium, demonstrated the colonial morphology produced by Nocardia asteroides [no longer known as asteroides, but as specific species] bacteria. Nocardia spp. colonies can be smooth, but in this case, these organisms produced colonies that may be described as mold-like, or verrucous, and exhibited a pinkish coloration. PHIL public domain. Created 1970. Accessed October 27, 2022. https://phil.cdc.gov/Details.aspx?pid=18868
