The world of bacteria III: bacteria with unusual properties

The world of bacteria III: bacteria with unusual properties

The organisms in volume 3 of Bergey’s manual have properties that are quite different from those of the bacteria described in volumes 1 and 2. Some are distinguished by their unusual type of metabolism. For instance, some of the organisms are phototrophic, able to use light as an energy source. Others are chemolithotrophic, able to obtain energy by oxidizing inorganic compounds such as ammonia, nitrate, hydrogen sulfide, or ferrous iron. Still, others are not distinguished by an unusual metabolism but rather by other features such as the occurrence of gliding motility rather than motility by the action of flagella, reproduction by budding rather than by binary fission, or special morphological structures such as sheaths, prosthesis, and stalks. Most of the organisms are gram-negative eubacteria, but some, such as those that form methane gas, belong to the major bacterial group known as the archaeobacteria, which may strain gram-negative or gram-positive.

Anoxygenic phototrophic bacteria

The bacteria of this group belong to the order Rhodospirillales. They are Gram-negative and are all capable of carrying out a photolithographic and/or photo organotrophic type of metabolism, and they contain bacteriochlorophyll (which differs in structure from chlorophyll such as occurs in cyanobacteria and eukaryotic algae). Also present is various water insoluble carotenoid pigments, which can also trap or absorb light energy and transmit it to the bacteriochlorophyll. The anoxygenic bacteria grow photo tropically only under anaerobic conditions and are incapable of forming O₂ (i.e are anoxygenic) because they possess the only photosystem. They are believed to be more primitive than oxygenic (O₂ evolving) phototrophic organisms: geological studies have provided evidence that the atmosphere of our planet was aerobic at the time life began to develop and that oxygen did not appear in appreciable quantities in the atmosphere until oxygenic bacteria evolved (i.e. bacteria having both photosystems I and photosystem II).

Anoxygenic phototrophic bacteria occurs in aerobic freshwater or marine environment. They may occur beneath the surface of shallow aquatic environment rich in organic matter, they may have a much deeper habit, as the pools or in some instances, they may have a much deeper habitat as the bottom of the lake. The bacteria chlorophyll absorbs light most strongly when the light is of long wavelength about 725-745nm (far red light, at the extreme end of the visible spectrum) to 1035nm (infrared light, invisible to the human eye). This is of longer wavelength than that absorbs by the chlorophyll of oxygenic bacteria or oxygenic eukaryotic algae. Although oxygenic organism may grow on the surface of the shallow, stagnant pond, they do not absorb far red or infrared light and thus do not prevent it from reaching the oxygenic phototrophs below. The bacteriochlorophyll and the carotenoid pigments of anoxygenic bacteria can also absorb some light in blue-green range. This becomes important when anoxygenic bacteria occur in the depth of a lake because blue light can prevent water to the greater distance than red light can.

The color of anoxygenic phototrophic bacteria is determined by the carotenoid pigments rather than by the bacteriochlorophyll, and the anoxygenic phototrophs can be divided into two major groups on the basis of their pigmentation: purple bacteria and green bacteria. Motility, if present in these two groups, is by means of polar flagella, except for the family Chloroflexaceae which exhibits a gliding type of motility. Nitrogen can be purple or green bacteria, but usually only under anaerobic condition illumination.

Purple phototrophic bacteria

These bacteria contain bacteriochlorophyll types a or b. The pigments that harvest the energy of light (i.e. bacteriochlorophyll and auxiliary carotenoid pigments) are located in the cytoplasmic membrane which may be greatly invaginated to form vesicles folded layers or tubules. Two families are recognized as follows:

The family Rhodospirillaceae contains the purple non-sulfur bacteria. Cultures appear orange-brown to purple-red under aerobic conditions. Some may be similarly pigmented under anaerobic conditions, but others may be greenish yellow. The purple non-sulfur bacteria exhibit a diversity of shapes: helical (e.g.,Rhodospirillum), nonpros thecate rod-shaped, ovoid or spherical cells that multiply by the formation of buds at the end of prosthesis. The purple non-sulfur bacteria chemoorganotrophs: organic substances serve both as the carbon source and as the electron donor for the reduction of Carbon dioxide. Some species can grow autotrophically by using H₂S as the electron donor, but if very low concentrations are provided; none can use elemental sulfur as an electron donor. Photosynthesis occurs only under anaerobic condition in the light. Some species can also grow under aerobic or microaerophilic conditions by the dark by respiration with the organic compounds.

The family Chromatiaceae contains the purple sulfur bacteria. Cultures appear brown to purple violet. Purple sulfur bacteria may be ovoid to rod-shaped coccoid or helical. Coccal species may be arranged as diplococci, in cubical packets, or in the flat sheet. Some species contain gas vacuoles.

Green phototrophic bacteria

In contrast to the purple bacteria, these organism contains bacteriochlorophyll types c or d and minor amounts of a. Moreover, cultures are green or brown. The pigments involved in photosynthesis are located in membrane-bound vesicles within the cell; some of these may be attached to the cytoplasmic membrane. The family Chlorobiaceae contains the green sulfur bacteria. The cells are ovoid, bean-shaped or rod-shaped multiply by only binary fission. The family Chloroflexaeceae contains the green non-sulfur bacteria. The main genus, Chloroflexus is thermophilic and occurs in hot spring where it forms green or orange mats.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.