Index >> Bacterial Taxonomy >> Actinomycetes and Related Organisms

The genus Cellulomonas includes bacteria which have the ability to hydrolyse cellulose. Cells in young cultures are irregular rods about 0.5 µm diameter by 0.7-2 µm or more in length, which may be straight, angular or slightly curved or occasionally club-shaped or beaded. Some cells may be arranged at an angle to each other as in arthrabacter and occasionally cells may show rudimentary branching. True mycelia are not formed and coccoid cells predominate in older

cultures. Motile or non motile. Gram positive to Gram variable. non acid fast, aerobic chemoorganotrophs, with an optimum growth temperature of 30°C. G + C content ranges from 71.7-72.7 moles per­cent. Only one species, C. flavigenum is recognized. which is usuallyfound in the soil. The genus Kurthia is characterized by bacteria which are strict aerobes and chemoorganotrophs. In young cultures the cells are unbranched rods with round ends, occurring in parallel chains. Older cultures consist of coccoid cells formed by fragmentation of rods. Rods are motile by peritrichous flagella and the cells grow well in the presence of 4-6 per cent NaCl. The optimum temperature for growth is 25-30°C. Only one species namely K. zoefi has been described.

The family Propionibacteriaceae includes bacteria which are Gram­ positive, non spore forming, anaerobic to aerotolerant, pleomorphic, branching or regular rods or filamentous. When fermentative, the major end products arc propionic acid, acetic acid. CO2 or a mixture of butyric, formic, lactic or other monocarboxylic acids. Growth is enhanced by CO2. They are normal inhabitants of skin, respiratory and intestinal tracts of most animals. Two genera have been described, namely ,Propiombacterium and Eubacterium.

The genus Propionibacterium consists of cells which are non motile anaerobic to aerotolerant and produce propionic acid and acetic acids. Cells are arranged in ,singles, pairs or 'V' and 'Y' configurations. These are chemoorganotrophs that grow rapidly at 32-37°C. Most strains also grow in 20 per cent bile or 6.5 per cent NaCl glucose broth. Some species may be pathogenic. The genus includes eight species identified based on the basis of end products of metabolism. The other genus Eubacterium consists of cells which may be motile or non motile, obligatory anaerobic and fermentative or non fermentative. Fermentative species produce mixtures of organic acids such as butyric, acetic, formic or lactic, or other monocarboxylic acids. These grow rapidly at 37°C and are found in the cavities of man and other animals, plant products and soil, Some species are pathogenic.

The order Actinomycetales includes members that tend to form branching filaments, which in some cases develop into a mycelium. Filaments may be either extremely short as in Mycobacteriaceae ,or well developed as in the Streptomy­cetaceae  The diameter of the filaments varies from 0.2'-2 µm. In certain families, filaments tend to fragment and fragmentation leads to coccoid, elongate or diptheroid cell In some families true spores are formed on aerial of substrate hyphae. Spores may be produced singly or in chains which may be straight, looped or spiral and such chains arise singly or in a verticillate manner.

Spores may also be borne in sporangia as in Actinoplanaceae and may be motile or non motile. Although Gram positive, this reaction may vary with age. Members of Mycobacteriaceae are acid fast while some members of Nocardiaceae are weakly acid.fast. Except for some genera in the Actinomycetaceae which may be anaerobic, facultatively anaerobic or aerobic, the other members of the order are strictly aerobic. Common inhabitants of soil and some species are pathogenic to man animals or plants. Some are obligate symbionts in plant root nodules and fix nitrogen (Ex. Frankia).

The family Actinomycetaceae has bacteria which are predominantly diptheroid in shape which tend to form branched filaments during some stages of cultural development. Fragmentation of filaments readily occurs to form diptheroid or coccoid cells. Aerial mycelium and spores are not formed. Non motile, usually grow as facultatively anaerboic but some may be anaerobic or aerobic. The family has five genera mainly based on their relationship to oxygen.

The family Mycobacteriaceae contains only one genus Mycobacterium with 30 species. Most of the members are pathogenic. Cells are slightly curved or straight rods sometimes branching. Filamentous or mycelium type growth may occur but usually gets fragmented into rods or coccoid cells. Acid fast, non motile and do not form endospores, conidia or capsules. The cells are characterized by a high lipid content consisting of long, branched chains of mycolic acid. The genus Mycobacterium includes obligate parasites, saprophytes and intermediate forms differing in nutritional requirement. All are aerobic, although growth may occur in depths of the medium. Found in soil, water or warm and cold blooded animals, G +. C content ranges between 62-70 moles per cent.

The family Frankiaceae includes organisms which are symbiotic, filamentous, mycelial bacteria which induce and live in the root nodules of a variety of non leguminous dicotyledonous plants  The bacteria also have a free stage in the soil.

The Family Actinoplanaceae includes bacteria which form distinct., mycelia which is intramatrical or sometimes aerial. Diameter of the filaments varies from 0.2-2.6 µm. Sporangiospores are produced on branched or unbranched hyphae and are of different shapes . Gram positive chemoorganotrophs, with a respiratory metabolism, aerobic and abudant in humus rich soil. The family includes ten genera which fall into two distinct groups, one with large spherical to irregular multisporous sporangia and the second with small filiform or club shaped sporangia containing one to several spores. Spores may be motile or non motile.