Actinomycetes

The term Actinomycetes is used to indicate organisms belonging to the Actinomycetales, a major subdivision of the Prokaryotae, the kingdom that comprises all organisms with a prokaryotic cell. Sometimes the name "Actinomycetes" is used restrictively for the members of the genus Actinomyces only. Actinomycetes were long regarded as fungi, as is reflected in their Greek name ("aktino" = ray, "mykes" = mushroom or fungus). Actinomycetes are transitional forms between bacteria and fungi, sometimes called the "higher bacteria," or filamentous bacteria. Like bacteria they possess cell walls containing muramic acid. They also have prokaryotic nuclei and are susceptible to antibiotics. But like fungi they form filaments called hyphae similar to the hyphal forms of fungi. Actinomycetes differ from fungi in the composition of cell wall; they do not have chitin and cellulose that are commonly found in the cell wall of fungi. The Actinomycetes include a wide variety of bacteria found in soil, decaying vegetation, humans and animals.^1,2 Some Actinomycetes establish symbiotic nitrogen fixing association with about 200 species of plants including soapberry, alders, bitterbrush, coffee berry, Australian pine, mountain mahoganies, etc.³

A number of actinomycetes form close association with plants and act as growth-promiting and biocontrol agents. Some actinomycetes form lichen-like associations with green algae called actinolichens. Though most actinomycetes are saprophytes (feeding on decaying organic material), some cause diseases in plants (e.g. potato scab, gall, and wilt) and animals. Most members of actinomycetes are aerobic, but a few can grow under anaerobic conditions.⁷

Actinomycetes are divided into several families:

• Actinomycetaceae includes two genera: Actinomyces and Nocardia.
• Mycobacteriaceae has a single genus Mycobacterium; members of this genus include bacteria that cause leprosy and tuberculosis.
• Streptomyceteae includes species of soil bacteria; several species produce widely used antibiotics.

Growth and Reproduction

Actinomycetes reproduce either by spore formation or hyphae fragmentation. The hyphae of Actinomycetes are much smaller than fungal hyphae, being one-fifth to one-half of fungal hyphae which often bear long chains of spores. These chains can be straight, wavy, hooked, or in tight spirals. The non-sporulating Actinomycetes in the genera Rhodococcus, Nocardia, Gordonia, Mycobacterium, and Dietzia are free-living aerobic bacteria that have broad metabolic capabilities. They can degrade to some extent even the most persistant man-made chemical compounds such as organochlorines, carbamates, and organophosphates.⁴ Most Actinomycetes are not motile.

Pathogenic Actinomycetes

Actinomycetes normally reside in human mouth, throat, gastrointestinal tract, and urogenital tract without producing disease. Since the organisms cannot invade a human or animal body, they must be introduced by a deep puncture wound or trauma such as dental extraction or jaw trauma, aspiration of dental debris, surgery (removal of the appendix), or prolonged use of untrauterine devices. Actinomycetes require dead or devitalized tissue to facilitate their invasion and proliferation into deeper tissues and cause infection and traumas usually result in formation of such tissues. Establishment of human infection by Actinomycetes always require the presence of companion bacteria. These companion bacteria help in initiation of infection by producing a toxin or an enzyme or by inhibiting host immunity. Once the infection by Actinomycetes is established, the immune system of the infected human host stimulates an intense inflammation. Bacteria from the infected site may disseminate to distant organs of the body.²

Infection with Actinomycetes is called actinomycosis which is characterized by formation of abscesses. Lung infection with Nocardia species can quickly spread to the brain and skin. Nocardiosis is most common in persons with impaired immune system. Actinomycosis is not transmitted from person to person. ⁶

Actinomycetes are also the source of numerous antibiotics. Most of actinomycetal antibiotics used to combat plant diseases belong to the genus Streptomyces and are used as direct pesticides. Other genera also demonstrate promising biocontrol properties. Among the antibiotics obtained from actinomycetes which are used in human and veterinary medicine are amphotericin, cyclohexymide, nystatin, and streptomycin.¹

The Actinomycetes are particularly common type of bacteria found on moldy hay. Farmers may be routinely exposed to very high concentrations of Actinomyces and may inhale as many as 750,000 spores per minute. Frequent exposure to Actinomyces is the cause of Farmers Lung respiratory problems.⁵