Bacteria And Actinomycetes

Microorganism in soil and their role:

The microbial population of soil is made up of five groups i.e. Bacteria, Fungi, Actinomycetes, Protozoa, and Algae.

BACTERIA:

Bacteria are the most abundant group usually more numerous than the other four combined. The number of bacterial cells in the soil is large but the individuals are small usually not more than several microns in length because minute sized of bacteria but large cells or extensive filaments of other four groups. The bacteria occupy the less than half of the total microbiological tissue present in the soil. In well-aerated soil, the bacteria and fungi both are dominated where as in environment containing little or no oxygen only bacteria is found and are responsible for all biological and chemical changes. Bacteria carried out most of the biological transaction because of their ability of rapid growth and vigorous decomposition of different types of natural substances or natural substrates.

Winogradsky differentiates bacteria into two broad division:

Autochthonous or Indigenous species

Zymogenous or Fermentation producing organism

Winogradsky^,s ecological scheme is only means of classifying bacteria in the natural environment.

Autochthonous or Indigenous species:

The autochthonous population consists of numerous indigenous bacteria whose abundance is not affected by environment fluctuation. Their nutrient supply is derived from soil organic fraction and no external nutrient or energy source are required. Hence, their number is relatively constant. The autochthonous population is involved in the formation and decomposition of humus. The indigenous flora is relatively stable in its composition and is not influenced by treatment with organic amendment especially common in the environment receiving no plant and animal remains are the bacteria of the genus. Arthrobacter and primitive actinomycetes are classified as Nocardia species. Species of agrobacterium, as well as bacillus spores, are also ubiquitous and indigenous. Bacillus spores are in inactive state and thus make no contribution to the chemical transformation carried out by autochthonous flora when spores are germinated under the favourable condition to vegetative state. Bacillus may be considered as zymogenous flora.

Zymogenous or Fermentation producing organism:

The zymogenous organism is most active in chemical transformation. They are usually scarce but they flourish when organic nutrients are added. These actively fermenting species, therefore, need food in excess quantities then it is actually needed for their growth. The supply of extra nutrients is readily exhausted. So, the zymogenous bacteria respond rapidly to soil amendment, become and remain numerous as long as the nutrients are available then decline once there food source is depleted. Zymogenous flora includes microorganism active in the transformation of added organic matter as well as species whose population responds remarkably after the addition of inorganic matter. Zymogenous microbial flora declines an enter a dormant period as shown as the nutrients have been depleted. Five microbial groups are considered as zymogenous on the basis of their physiological activities:

1. Bacteria utilizing N₂ and those converting NH₃ to nitrates.
2. Fungi that becomes active after the application of organic matter.
3. Actinomycetes in the vegetative state.
4. Vegetative stage of bacillus species which develops only in the presence of suitable carbonaceous nutrients.

Aerobic spore forming rods are found to grow in number when proteinaceous and other amino acid rich materials are supplied or during the late stages of decomposition when protein and growth factors synthesized from the primary flora become available to the secondary population. The growth of pseudomonas and bacillus stimulated on soils treated with crops residue proves their zymogenous nature.

Some other basis of classification:

1. Bergey^,s manual of systematic bacteriology
2. Nutritional and metabolic characters
3. Ability to utilize O₂
4. Morphology
5. Adverse condition

Bacterial genera can be classified into class Schizomycetes. Ten order has been included in this class but two species prominent in soil.

Order I : Pseudomonadales

Family: Pseudomonadaceae e.g. pseudomonas, Xanthomonas, Gluconobacter, Zoogloea.

Short, gram-negative, nonspore forming rods, aerobic, frequently produced blue or

Order II: Eubacteriales

In this order, five class is included. They are:

Family 1 : Rhizobiaceae e.g Rhizobium, Agrobacterium, Chromobacterium.

Rhizobium: Gram-negative, nonspore forming rods that form nodules on legumes, numerically not abundant.

Agrobacterium : Short, Gram-negative, nonspore forming rods facultative anaerobes, nonchromogenic.

Chromobacterium: Short, Gram-negative, nonspore forming motile rods, facultative anaerobes produce the blue-violet pigment called violacein.

Family 2: Achromobacteriaceae eg Achromobacters, Flavobacterium

Achromobacters: Gram-negative, nonspore forming, nonchromogenic rods.

Flavobacterium : Short, gram negative rods produce yellow or orange pigments.

Family 3 : Micrococcaceae eg Micrococcus, sarcina.

Micrococcus : spherical cells gram positive or some time gram negative cells in irregular groups.

Sarcina: Spherical cells in packets usually gram-positive which is yellow-orange or red pigmentation.

Family 4: Corynebacteria eg Corynebacterium, Arthrobacter.

Corynebacterium: Gram-positive, rods, straight or slightly bent, usually aerobic, cells contains irregularly stained segments or granules.

Arthrobacter : Usually rods when young becoming coccoid as their growth. Gram is variables when young gram positive when old.

Family 5 : Bacillaceae eg Bacillus, Clostridium.

Bacillus: Aerobic, facultatively anaerobic rods that formed endo spores.

Clostridium : Anaerobic rod that forms endospore.

Actinomycetes:

They are transitional groups of microorganism which act as the boundary between bacteria and fungi. Numerically they are second abundant microbes in the soil, after bacteria. Besides soils, they are found in other habitats like compost, river, mud and lake bottoms. They are present in the surface soil as well as in the lower horizons. They are the unicellular organism that produced slender, branched mycelium which may undergo fragmentation or may sub divide to form asexual spores. The individual hyphae or filaments appears morphologically similar to fungal filaments but are slender and much less broad usually 0.5-1.2 um in diameter. The dimension is similar to that of the bacterial cell unlike fungi the cell wall of actinomycetes lacks chitin and cellulose. They proliferate or multiply by vegetative means some produced asexual spores called conidia but there is no sexual spore stage. The mycelium of some actinomycetes has extensive branching. They form aerial mycelium and conidia the growth of actinomycetes in liquid culture does not show turbidity like bacteria. But they occur as distinct clumps, pellets. The growth rate is not exponential like that of bacteria but cubic like that of fungi. Unlike fungi, they can be attacked by viruses. The colonies of some genera have firm, consistency on agar medium and adheres tenaciously to the solid substrate. In some genera, the surface of the colonies appears powdery and often become pigmented when the aerial spores are produced. The colonies of organisms have simple mycelium as a more granular or mealy consistency and often crumbles when touched. The incubation period is long because of slow growth characteristic. They are found in the environment of high pH. Optimum pH is 6.5 to 8. The population size is inversely related to the H⁺ ion concentration. Most species cannot tolerate the pH below 5. Therefore, acidification of soil is used to suppressed the plant pathogenic actinomycetes. Generally, the count ranges from 10⁵ to 10⁸ per gram of soil. Even in the c-horizon counts may range from 10³ to 10⁵ per gram. It may be due to the down movement of conidia with water. In spite of the high count, the protoplasmic mass is less as their filaments are always slender, unlike bacteria they are less common in wet than dry areas. The population is greater in grass land than in cultivated land but less in uncultivated than in cultivated one. Peats and water logged areas are unfavorable for their growth. soils in the warmer area are more favorable than in cooler areas. The number of actinomycetes is high in the land rich in organic matter, carbonaceous materials and humus. The optimum temperature ranges further growth of actinomycetes from 28⁰ to 37⁰ C and most are mesophilic in nature. Some are facultative thermophilic and can grow at 50⁰ to 65⁰C as well as 30⁰C but the former range is often more favorable.

Actinomycetes take part in following activities:

1. Decomposition of certain resistance components of plant and animal tissue.
2. Formation of humus to the conversion of raw organic matter into compounds native to soil organic fraction. Many strains produce aromatic molecules which are the major components of humus.
3. Thermophilic actinomycetes take part in rotting and heating of green manures hay compost pile and animal manure.
4. Cause certain soil borne disease like potato scab and sweet potato fox.
5. Liberation of antibiotics like streptomycin, chloramphenicol etc.
6. Streptomyces kanamyceticus – Kanamycin

Streptomyces griseus – streptomycin

Streptomyces oriental – vancomycin

Some Streptomyces species also liberates extracellular protease enzyme which helps to lyze the bacteria. They utilize different carbon sources like organic acids, sugar, polysaccharides, lipid, and proteins.

Actinomycetes have four families and nine genera.

Class : Schizomycetes

order : Actinomycetales

Family I : Mycobacteriaceae

1. Mycobacterium
2. Mycoccus

Family II : Actinomycetaceae

1. Actinomyces
2. Nocardia

Family III : Streptomycetaceae

1. Streptomycetaceae
2. Micromomospora
3. Thermoactinomyces

Family IV : Actinoplanaceae

1. Actinoplanes
2. Streptosporangium

References:

1. Alexander M (1961). Introduction to soil microbiology, Academic press.
2. Rangaswami G and Bagyaraja PT (1993). Agriculture Microbiology (second edition), Prentice Hall of India.