Download actinomycetes-2012

1
22.1 GENERAL PROPERTIES OF THE ACTINOMYCETES
The actinomycetes are a fascinating group of microorganisms. They are the source of most of the antibiotics used in medicine today. They also produce
metabolites that are used as anticancer drugs, antihelminthics (e.g., ivermectin given to dogs to prevent earth worm), and drugs that suppress the immune
system in patients who have received organ transplants. This practical aspect of the actinomycetes is linked very closely to their mode of growth. The
actinomycetes have a complex life cycle. The life cycle of many actinomycetes includes the development of filamentous cells, called hyphae , and spores. When
growing on a solid substratum such as soil or agar, the actinomycetes develop a branching network of hyphae .
An Actinomycete Colony. The cross section of an actinomycete colony with living (blue) and dead(white) hyphae. The substratemycelium and aerial mycelium with chains of spores
are shown.
The hyphae grow both on the surface of the substratum and into it to form a dense hyphal mat
called a substrate mycelium . Septae usually divide the hyphae into long cells (20 μm and
longer) containing several nucleoids. In many actinomycetes, substrate hyphae differentiate
into upwardly growing hyphae to form an aerial mycelium that extends above the substratum.
It is at this time that medically useful compounds are formed. Because the physiology of the
actinomycete has switched from actively growing vegetative cells into this special cell type,
these compounds are often called secondary metabolites. The aerial hyphae septate to form
thin-walled spores. These spores are considered exospores because they do not develop within
a mother cell like the endospores of Bacillus and Clostridium. If the spores are located in a
sporangium, they may be called sporangiospores .The spores can vary greatly in shape.
Examples of Actinomycete Spores as Seen in the Scanning Electron Microscope. (a) Spores of Pilimelia columellifera on mouse hair . (b) Micromonospora echinosp.
(c) A chain of hairy streptomycete spores. (d) Microbispora rosea, paired spores on hyphae.(e) Spore chain of Kitasatosporia setae.
Like spore formation in other bacteria, actinomycete sporulation is usually in response to nutrient deprivation. In general, actinomycete spores are not
particularly heat resistant but with stand desiccation well, so they have considerable adaptive value. Most actinomycetes are not motile, and spores are dispersed
by wind or adhering to animals; in this way, they may find a new habitat to provide needed nutrients. In the few motile genera, motility is confined to flagellated
spores. Actinomycetes also have great ecological significance. They are primarily soil inhabitants and are widely distributed. They can degrade an enormous variety
of organic compounds, and are extremely important in the mineralization of organic matter. Although most actinomycetes are free-living microorganisms, a few
are pathogens of humans, other animals, and some plants. Actinomycete cell wall composition varies greatly among different groups and is of considerable
taxonomic importance. Four major cell wall types can be distinguished according to three features of peptidoglycan composition and structure: the amino acid in
the tetrapeptide side chain position 3, the presence of glycine ininterpeptide bridges, and peptidoglycan sugar content ( table 22.1 ).
2
Whole cell extracts of actinomycetes with wall types II, III, and IV also contain characteristic sugars that are useful in identification. Some other taxonomically
valuable properties are the morphology and color of the mycelium and sporangia, the surface features and arrangement of spores, the percent G /C in DNA, the
phospholipid composition of cell membranes, and spore heat resistance. Several actinomycete genomes have been sequenced, including Mycobacterium
tuberculosis, M.leprae, Streptomyces coelicolor, and S. avermitilis. Based on 16SrRNA sequence data, all are placed in the phylum Actinobacteria and classified. The
phylum is large and very complex; it contains 1 class ( Actinobacteria ), 5 subclasses,6 orders, 14 suborders, and 44 families. In this system, the actinobacteria are
composed of the actinomycetes and their high G /C relatives.
Actinomycetes belonging to the order of Actinomycetales are grouped under four families viz Mycobacteriaceae, Actinomycetaceae, Streptomycetaceae and
Actinoplanaceae. Actinomycetous genera which are agriculturally and industrially important are present in only two families of Actinomycetaceae and
Strepotmycetaceae.
Functions / Role of actinomycetes:
1. Degrade/decompose all sorts of organic substances like cellulose, polysaccharides, protein fats, organic-acids etc.
2. Organic residues / substances added soil are first attacked by bacteria and fungi and later by actinomycetes, because they are slow in activity and growth than
bacteria and fungi.
3. They decompose / degrade the more resistant and indecomposable organic substance/matter and produce a number of dark black to brown pigments which
contribute to the dark colour of soil humus.
4. They are also responsible for subsequent further decomposition of humus (resistant material) in soil.
3
5. They are responsible for earthy / musty odor / smell of freshly ploughed soils.
6. Many genera species and strains (eg. Streptomyces if actinomycetes produce/synthesize number of antibiotics like Streptomycin, Terramycin, Aureomycin etc.
7. One of the species of actinomycetes Streptomyces scabies causes disease "Potato scab" in potato.
Differences between Actinomycetes and Fungi
Actinomycetes
Fungi
4
1. Filaments are 1- 5 μm in diameter and few
mm in length.
10-20 μm in diameter and mycelia are
often several inches in length.
2. Structures is prokaryotic in nature.
Eukaryotic in nature
3. Cell wall contains peptidoglycan , muramic
acid diaminopimellic acid and variation in
murein occurs.
Contains chitin
4. Sexual phenomenon is absent
Sexual phenomenon is present
m
5
Major Group
Characteristics
Other
Substrate
Aerial
Spore
Motile
Murein DAP
Wall
G+C % Mycelium Mycelium Formation Spores
Components
Actinobacteria
60-77
-
-
-
-
Variable
51-71
V
V
-
-
Arabinogalactose
Nocardiform
Variable
Meso- DAP
Mycolic acid
Dermatophilus
66-78
V
+
-
+
Meso-DAP
Variable
Group
Streptomycetes
66-78
+
+
+
Actinoplanetes
71-73
+
-
+
V
None
LL-DAP
Xylose,arabinose
Meso-DAP Or 3hydroxl-DAP
@@@@@@@@@@@@@@@@
6