Mycoplasmas

INTRODUCTION

• The mycoplasmas are the smallest and simplest self-replicating prokaryotes. They were first discovered by Pasteur in 1843, as a causal agent of pleuropneu­monia of cattle and designated as pleuropneumonia like organism (PPLO).

• Pasteur was unable to isolate them in pure culture. Later, in 1898 Nocard and Roux, successful obtained pure culture of these microorganisms in media containing serum and found that these microorganisms could produce same disease when inoculated in healthy cattle.

• Borrel (1910) named these organisms Asterococcus mycoides. Later, in 1929, Nowak placed the genus Mycoplasma under the class Mollicutes.

• Taxonomically, the lack of a cell wall is used to separate mycoplasmas from other bacteria in a class named Mollicutes.

• Mycoplasma are the smallest known aerobic prokaryotes without cell wall. Hence have no definite shape i.e., pleomorphic and thus are called joker of microbiology.

• The colonies of mycoplasma have a characteristic round form with a thickened centre and a delicate periphery like a fried egg.

• Mycoplasmas are commonly found in soil, hot spring, sewage water and also in humans, plants and animals.

CLASSIFICATION

• Mycoplasmas can be placed in the following three genera on the basis of their nutritional require­ment.

• 1.      Mycoplasma: They need cholesterol for their growth and grow as parasite on animals including man. It causes damage to the mucous membranes and joints of the body.

• 2.      Acholeplasma: They do not need choles­terol for their growth but will incorporate it into the membrane if it gets in the medium. They are found in plants and vertebrates as parasite and also available in sewage water and in soil as saprophytes.

• 3.      Thermoplasma: They do not require cholesterol for their growth. They are strictly aerobic microorganisms showing good growth in acidic pH between 0.96 - 3.0 and optimum temperature of 59 °C.

STRUCTURE

• The cell is devoid of cell wall and thus are highly pleomorphic (showing irregular and variable shapes). They may be ring-like, granular, coccoid, pear-shaped, fila­mentous, etc. The filaments are of two types: unbranched or branched.

• Mycoplasma is currently considered the smallest known cell at about 0.1 micron (µm) in diameter. They can pass through bacteria proof filters and are considered to be intermediate between viruses and bacteria. 

• The mycoplasma cell contains the minimum set of organelles essential for growth and replication: a plasma membrane, ribosomes, RNA, proteins and a genome consisting of a double-stranded circular DNA molecule (Fig 1). The amount of RNA is more than DNA.

• They reproduce by vegetative means i.e., by binary fission and budding.

• They are resistant to the wall attacking antibiotics such as penicillin but sensitive to antibiotics like chlo­ramphenicol, streptomycin, erythromycin that act on metabolic pathway.

• The cell biology of these organisms is interesting not only to Mycoplasmologists but also to the many workers who use mycoplasmas as simple model systems for studying general biological problems, particularly those concerning membrane structure and function.

•  Mycoplasmas are widespread in nature as parasites of humans, mammals, reptiles, fish, arthropods, and plants.

Plant Diseases: §  Little leaf disease of brinjal, §  Bunchy top of papaya, §  Big bud of tomato, §  Witches broom of legumes, §  Yellow dwarf of tobacco, §  Strip disease of sugarcane, §   Clover dwarf, §  Cotton virescence.

Human diseases: §  Pleuropneumonia is caused by Mycoplasma hominis §  Primary atypical pneumonia is caused by Mycoplasma pneumonia §  Mycoplasma hominis and Mycoplasma fermentants cause infertility in man.