Actinomycetes
Actinomycetes are group of bacteria that are characterized by their branched multicellular filaments analogous to the molds. They do not form endospores. Some Actinomycetes reproduce by forming chains of spores(conidia) at the tips of the filaments, a process resembling asexual spore formation in many fungi. Because of their branched filamentous structure and formation of conidia similar to the hyphae and conidia of fungi, they were once considered to be fungi. However, it is now well established that they are bacteria and differ from nearly all true fungi in the fallowing.
1) The mycelium of Actinomycetes is extremely fine and generally is narrower.
2) The cell wall of Actinomycetes consist of peptidoglycan, muramic acid and diaminopimelic acids; where as those true fungi are made up of chitin.
3) Actinomycetes shows typical prokaryotic structure where as the true fungi eukaryotic.
Most of the Actinomycetes are found in the soil. Some members like Streptomyces, are responsible for the characteristic smell of damp soil. Many Actinomycetes like Streptomyces are the sources of many useful antibiotics. Some species are responsible for causing diseases to the plant, animals and human beings.
Mycoplasmas
The first member of this group to be described was the agent of bovine sheep pleuropneumonia and as result mycoplasmas were long designated as the Pleuropneumonia Like Organisms(PPLO).
Bride and Donatien, in 1923-25, for the first time, succeeded in cultivating the similar organisms to be responsible for contagious milk fever in goats. Later on, it was found that similar organisms cause a type of a pneumonia, called primary atypical pneumonia in man. It was in 1967, that Doi et al., and Ishiie et al., reported that yellows, diseases in many plants, which were earlier thought to be caused by viruses, are caused by mycoplasmas.
Since their characteristics have been thoroughly studied in 1956, the ICNB recognized that these organisms are different from bacteria, to be placed in separate class Mollicutes. The class Mollicutes consist of two genera namely Mycoplasma and Acheloplasma, placed in families, Mycoplasmataceae and Acheloplasmataceae, respectively, on the basis of sterol requirements for growth. In the family Mycoplasmataceae, those mycoplasmas are placed which require sterol for their growth, and which do not require sterol are placed in family Acheloplasmataceae.
Fallowing are the characteristics of the mycoplasmas
- Mycoplasmas are the smallest, usually nonmotile, prokaryotic unicellular organisms, capable of independent growth and metabolism.
- Both aerobic and anaerobic species are recognized.
- They can grow in cell free media in the form of small flat colonies, exhibiting a characteristic "fried egg" appearance.
- Like eukaryotes, they usually require sterol for growth.
- They differ from virus in that their cells contain both DNA and RNA.
- They are insensitive to enzymes and penicillin, but are killed by tetracycline and other antibiotics which affect cell metabolism rather than the wall.