Actinomyces are rod-shaped bacteria of the order Actinomycetales, also called ray fungi because of their characteristic appearance under the microscope. The bacteria preferentially colonize vertebrates and appear either parasitically or as commensals. Infection results in actinomycosis of the oral cavity and sometimes the lungs or liver.
What are actinomyces?
Actinomyzetaceae form a family within the bacterial order Actinomycetales, which contains five subgenera. Actinomyces are one genus in this family. They contain the most species within the Actinomyzetaceae. All Actinomycetaceae characteristically consist of elongated branched cells and possess gram-positive properties. Actinomyces possess a slightly curved to straight rod shape and are thus classified as rod-shaped bacteria. The diameter of the cells ranges from 0.2 to 3.0 µm. Although the length can vary, most representatives of the genus are rather long-fibered and reach lengths of more than 50 µm. In some cases they also form small branched mycelia. The bacteria are not capable of active movement. There are numerous representatives within the bacterial genus Actinomyces. Human pathogens are, for example, the species Actinomyces israelii, naeslundii, viscosus and odontolyticus or Actinomyces meyeri and pyogenes. Diseases associated with them include purulent inflammation in addition to actinomycosis. Due to their microscopic appearance and radial-filamentous branching, bacteria of the genus Actinomyces sometimes resemble fungi. In this context, the descriptive generic name ray fungus was introduced.
Occurrence, distribution, and characteristics
Actinomycetes are anaerobic to a large extent. Thus, they do not require oxygen for metabolism and survival. While oxygen is toxic to some anaerobic life forms, this is not true for the Actinomyces. Many of the species are facultatively aerobic and thus can also use oxygen for metabolism. Only a few Actinomyces possess enzymes of catalase. A high CO2 or HCO3 concentration in the culture medium allows most actinomycetes to grow under aerobic conditions. Almost all actinomycetes depend on a complex supply of nutrients for growth. In most cases, their energy metabolism pathway corresponds to a fermentative energy metabolism. Carbohydrates are metabolized to organic acids in this energy metabolism mode. The preferred habitat of most species corresponds to warm-blooded vertebrates, which are colonized by the bacteria as pathogens or as commensals. A commensal is a living organism that lives on food residues and waste products of a host organism and therefore does not harm the host organism. The opposite is the classic parasitic colonization, which deprives the host of substances that it itself needs to survive. Parasitic colonizations harm the host accordingly and are to be classified as pathogenic. Parasitic pathogenic colonizations associated with Actinomyces primarily include infection with the species Actinomyces israelii. The optimum temperature for the growth of Actinomyces is between 30 and 37 degrees Celsius. Living organisms with a constant body temperature in this temperature range provide the bacteria with the best level for this reason. Bacteria from the genus Actinomyces primarily engage in decay for reproduction. This decay corresponds to segmentation into short cells. Endospore formation is not practiced by the bacteria. Actinomyces have a radiate hyphal structure due to their growth starting from a point, and because of this appearance they were confused with fungi in the past before being classified as bacteria. The bacteria are not exclusively species-specific in transmission, but are transmissible from one species to another. This transmission is referred to as zoonosis. Because the bacteria preferentially colonize the gastrointestinal tract and oral cavity of animals, transmission to humans is more specifically zooanthroponosis. In the host’s body, some species of Acitomyces may engage in hematogenous dissemination, reaching the lungs or liver via the blood. However, this spread of the bacteria is a rather rare form.
Diseases and ailments
Actinomyces can cause several diseases.In most cases, the diseases are mixed infections caused by microaerophilic, facultative anaerobic or anaerobic germs that create anaerobic milieu conditions. Since anaerobic bacteria depend on anaerobic environments, they accordingly create the required environment. In this context, pus blisters develop in actinomycosis, which in most cases are associated with the formation of fistulas. The abscesses give off sulfur-yellow drusen. Actinomycosis is a pseudomycosis associated primarily with abscesses in the oral cavity, lungs, and gastrointestinal tract. In actinomycosis, the pus collections usually spread rapidly to the surrounding tissues. The collections are surrounded by connective tissue or granulation tissue with a coarse consistency. In addition to abscess formation, actinomycetes can also cause caries or periodontitis. Actinomycosis is divided into several forms. The cervicofacial form is the most relevant and is predominantly caused by Actinomyces israelii. The infection is often based on an injury within the oral cavity, so that one can speak of an endogenous infection. To be distinguished from this form is thoracic actinomycosis, which can arise from cervicofacial actinomycosis in the context of salivary aspiration. In abdominal actinomycosis, injuries to the intestine or female genital area are considered the origin. Cutaneous actinomycosis occurs after injuries with saliva transmission. In rare cases, the liver is also affected by the infection. Even less common, but possible, is colonization of the tear ducts.
