What Are Actinomycetia and What Do They Do?

Actinomycetia, also known as actinobacteria, are a diverse class of Gram-positive bacteria distinguished by a high percentage of guanine and cytosine in their DNA. This group is varied, encompassing bacteria with different appearances and ecological functions.

Though they often grow in filamentous, branching patterns that resemble fungi, they are fundamentally different. This resemblance is reflected in their name, which comes from the Greek words for “ray” and “fungus”. However, Actinomycetia are prokaryotic organisms, lacking the complex internal structures like a nucleus that define eukaryotic organisms such as fungi.

Unique Traits and Natural Environments

A defining characteristic of many Actinomycetia is their growth in the form of a mycelium, an interwoven network of thread-like filaments called hyphae. This growth pattern is similar to that of fungi, which historically led to their misclassification. These bacterial filaments can fragment, forming rod-shaped or spherical cells, and many species produce spores for reproduction and dispersal. While most are aerobic, requiring oxygen to survive, some are capable of living in anaerobic conditions.

Actinomycetia are found in nearly every environment on Earth, from terrestrial soils to aquatic ecosystems. They are particularly abundant in soil, where their populations can reach millions of cells per gram, and are also found in freshwater and marine environments.

One of the most recognizable contributions of soil-dwelling Actinomycetia is the creation of the “earthy” smell known as petrichor, which follows a rainstorm. This aroma is caused by a compound called geosmin, produced by genera such as Streptomyces. This volatile substance is released into the air, creating a scent linked to a healthy, natural environment.

Beneficial Roles in Nature and Medicine

In their natural habitats, Actinomycetia function as decomposers. They are instrumental in breaking down complex organic materials, such as the cellulose from plants and the chitin that makes up insect exoskeletons. This decomposition is important for nutrient cycling, as it releases molecules back into the soil for plants to absorb. Some species, like those in the genus Frankia, form symbiotic relationships with plants, fixing atmospheric nitrogen into a form the plant can use.

The medical and biotechnological contributions of this bacterial class are significant, primarily through their production of bioactive compounds. Actinomycetia are the source for the majority of naturally derived antibiotics used in medicine. The genus Streptomyces is particularly prolific, responsible for compounds like streptomycin and tetracycline.

Beyond antibiotics, these bacteria synthesize other medically useful substances, including:

• Antifungals and antivirals
• Immunosuppressants used to prevent organ rejection after transplant surgery
• Antitumor drugs
• Enzyme inhibitors

This ability to generate a diverse range of complex molecules makes them a continuing focus of research for new therapeutic agents.

Pathogenic Potential and Human Infections

While the vast majority of Actinomycetia are harmless or beneficial, a small number of species can cause disease in humans. The most well-known pathogen in this class belongs to the genus Actinomyces, which is responsible for an infection called actinomycosis. These bacteria are common, harmless residents of the human mouth, throat, and digestive tract, existing as part of the normal microbiome.

Infections with Actinomyces are opportunistic, meaning they do not cause illness under normal circumstances. An infection occurs when mucosal barriers are broken by dental procedures, physical trauma, or surgery, allowing the bacteria to invade deeper tissues. Because these bacteria are often part of a mixed infection, diagnosis can be complicated.

Actinomycosis is characterized by its slow, progressive nature, often developing over weeks or months. The infection leads to the formation of abscesses, which are swollen, pus-filled areas that can create tunnels, or sinus tracts, that drain to the surface. The appearance of these abscesses containing yellowish sulfur granules, which are colonies of the bacteria, is a distinctive feature. Although these infections can be serious, they are rare and respond well to long-term antibiotic treatment.