Methanogenic Bacteria [Methanogens]

2.5.1. Methanogenic Bacteria [Methanogens]

The methanogenic bacteria are considered to be the hard-core anaerobes which, invariably possess the capability of deriving energy for their progressive growth by certain particular oxidizing chemical entities, for instance : hydrogen (H 2 ), formic acid (HCOOH) ; and actually exert their ‘action’ by making use of the electrons thus produced to reduce ultimately carbon-dioxide (CO 2 ) to give rise to

the formation of methane (CH 4 ) gas : CO 2 + 4H 2 ⎯⎯⎯→ CH 4 + 2H 2 O

Carbon Hydrogen

It has been observed that certain genera specifically may grow as autotrophs* — thereby utiliz- ing hydrogen and carbon dioxide as exclusive sources of carbon as well as energy ; whereas some others do need several additional components, for instance : organic-sulphur compounds, amino acids, acetic acid, and vitamins. Interestingly, a plethora of species actually grow quite abundantly and aggressively in a complex media viz., comprising of yeast extract in comparison to inorganic-salts containing media.

Coenzymes** : There are at least two uncommon coenzymes that invariably occur in all meth-

anogenic bacteria (methanogens) that have not been noticed in other varieties of microorganisms.

Examples : Following are two typical examples of methanogenic coenzymes :

(a) Coenzyme M — directly involved in methyl transfer reactions, and

(b) Coenzyme F 420 — a flavin-like chemical entity intimately involved in the anaerobic elec-

tron transport system of these microorganisms. It has the ability to fluoresce when exposed to UV light ; and, therefore, its presence may be detected by visualizing the organisms via a fluorescence microscope conveniently (also used for its critical identification and examina- tion).

Differentiation of Methanogens : The genera of methanogens i.e., the methane-producing bac- teria may be clearly differentiated exclusively based upon their morphology*** and Gram reaction****. However, the glaring distinct differences occurring in the cell-wall composition have been duly observed to correlate specifically with these genera.

* Pertaining to green plants and bacteria that essentially form protein and carbohydrate from inorganic salts and CO 2 .

** An enzyme activator ; a diffusible, heat-stable substance of low molecular weight that, when combined with an inactive protein termed as apoenzyme, forms an inactive compound or a complete enzyme called a holoenzyme (e.g., adenylic acid, riboflavin, and coenzymes I and II).

*** The science of structure and form of organisms without regard to function. **** A method of staining bacteria, important in their identification [SYN : Gram Stain ; Gram’s method]. Gram-

negative : Losing the crystal-violet stain and taking the colour of the red counter stain in Gram’s method of staining — a primary characteristic of certain microorganisms ; Gram-positive : Retaining the colour of the crystal-violet stain in Gram’s method of staining.

STRUCTURE AND FUNCTION : BACTERIAL CELLS

Table 2.5 : records the morphology, motility, and wall composition of several methanogenic organisms with specific ‘genus’.

Table 2.5. Differentiation of Methanogens

S.no. (Methano) Genus

Wall Composition 1 Methanobacterium

Morphology

Motility

Gram +ve to Gram-

Pseudomurein

variable long rods

2 Methanobrevibacter

Gram +ve lancet-shaped

—do—

short rods or cocci

3 Methanococcus

Gram –ve pleomor-

+ ; one flagellar tuft

Protein with trace of

phic* cocci

+ ; peritrichous flagella

Protein

5 Methanomicrobium

Gram –ve short-rods

+ ; single polar flagellum —do—

6 Methanosarcina

Gram +ve cocci in

Heteropolysaccharide

clusters

7 Methano spirillum

Gram –ve long wavy

+ ; polar flagella

filaments or curved rods

Importantly, the cell walls of two genera essentially consist of pseudomurein, that prominently differs from eubacterial peptidoglycan by the following two distinct structural features, namely :

(a) substitution of N-acetyltalosaminuronic acid for N-acetylmuramic acid, and (b) presence of tetrapeptide composed totally of L-amino acids, having glutamic acid attached

duly at the C-terminal end. Habitats : Interestingly, the methanogenic bacteria most commonly found in anaerobic habi-

tats that are eventually rich organic matter which ultimately produced by nonmethanogenic bacteria via fermentation to yield H 2 and CO 2 . A few such common as well as vital habitats are, namely : marine

sediments, swamps, marshes, pond and lake mud, intestinal tract of humans (GIT) and animals, rumen of cattle (e.g., cow, buffalow, sheep, pig, goat etc.), and anaerobic sludge digesters in sewage-treatment plants.

Figure. 2.6 [A and B] depicts the diagramatic sketch of the cells commonly observed in various kinds of methanogenic organisms (viz., methane-producing bacteria).

Figure 2.6 [A] evidently shows the typical cells of Methanosarcina barkeri and Methanospirillum hungatei representing ideally the methane-producing bacteria.

Figure 2.6 [B] likewise illustrates the characteristic cells of Methanobacterium thermo- autotrophicum and Methanobacterium ruminantium designating the methanogens.

* Having many shapes.

PHARMACEUTICAL MICROBIOLOGY

Methanosarcina Methanobacterium barkeri

thermoautotrophicum

Methanobacterium ruminantium

Methanospirillum hungatei

[A]

[B]

Fig. 2.2. Diagramatic Sketch of Various Methanogens [Adapted From : Pelczar MH, Jr. et al. Microbiology, Tata McGraw-Hill

Publishing Co. LTD., New York, 5th, edn., 2004]