The Actinomycetes
[s., actinomycete], according to the latest edition of Bergey’s Manual
(
Volume 4), represent an aerobic, Gram-positive bacteria which predominantly and essentially give rise
to specific
branching filaments* or asexual spores** or hyphae***. It has been duly observed that the
elaborated morphology, arrangement of spores, explicit cell-wall chemistry, and above all the various
kinds of carbohydrates critically present in the cell extracts are specifically vital and equally important
requirement for the exhaustive taxonomy of the
actinomycetes. Consequently, these informations are
utilized meticulously to carry out the articulated division of these bacteria into different well-defined
categories with great ease and fervour. It is quite pertinent to state at this juncture, that the
actinomycetes
do possess and exert an
appreciable practical impact by virtue of the fact that they invariably play an
apparent
major role in the following two highly specialized and particular aspects, namely:
(
a) Mineralization of organic matter in the soil, and
(
b) Primary source of most naturally synthesized antibiotics.
3.5.1.1. General Characteristics
The general characteristics of the
actinomycetes are as stated under :
(
a) The branching network of hyphae usually developed by the actinomycetes, grows critically
both on the surface of the
solid substratum (e.g., agar) as well as into it to give rise to the
formation of
substrate mycelium. However, the septate**** mostly divide the hyphae into
specific elongated cells (
viz., 20 μm and even longer) essentially consisting of a plethora of
nucleoids*****.
(
b) Invariably, the actinomycetes afford the development of thallus. Noticeably, a large crosssectionof the
actinomycetes do possess an aerial mycelium that extends above the solid
subtratum,
and produces articulately asexual, thin-walled spores known as conidia
[
s., conidium] or conidiospores at the terminal ends of filaments. In an event, when the
spores are located strategically in a
sporangium, they are termed as sporangiospores.
(
c) The spores present in the actinomycetes not only vary widely in terms of shape and size, but
also
develop them (spores) by the help of septal formation at the tips of the filaments, invariably
in response to
nutrient deprivation. Besides, a larger segment of these spores are
specifically devoid of any thermal resistance; however, they do
withstand dessication quite
satisfactorily
, and thus exhibit considerable adaptive value.
(
d) Generally, most actinomycetes are not found to be motile,* and the motility is particularly
confined to the flagellated spores exclusively.
In the recent past, several taxonomically characteristic features and useful techniques are of
immense value and worth, such as:
• Morphological features and the colour of
mycelia and sporangia
• Surface properties and arrangement of
conidiospores
•
% (G + C) in DNA
•
Phospholipid content and composition of cell membranes
•
Thermal resistance encountered in spores
• Comparison of
16S rRNA sequences and their values
• Production of relatively
larger DNA fragments by means of restriction enzyme digestion,
and
• Ultimate separation and comparison of
‘larger DNA fragments’ by the aid of Pulsed FieldElectrophoresis.
Significance of Actinomycetes
There are, in actual practice,
three most important practical significances of the actinomycetes,
as mentioned below:
(1)
Actinomycetes are predominantly the inhabitants of soil and are distributed widely.
(2) They are able to degrade a large variety and an enormous quantum of organic chemical
entities. However, these are of immense significance in the mineralization of organic matter.
(3) They invariably and critically give rise to a large excess of extremely vital
‘natural antibiotics’
that are used extensively in the therapeutic armamentarium
e.g., actinomycetin. Importantly,
a plethora of
actinomycetes represent free-living microbes, whereas a few are
pathogens to human beings, animals, and even certain plants.
Fig. 3.5. illustrates the cross-section of an actinomycete colony with
living and dead hyphae.The substrate and aerial mycelium having chains of conidiospores have been depicted evidently.
Classification
The
actinomycetes have been duly classified into three major divisions based upon the following
characteristic features:
(
a) Whole cell carbohydrate patterns of aerobic actinomycetes
(
b) Major constituents of cell wall types of actinomycetes, and
(
c) Groups of actinomycetes based on whole cell carbohydrate pattern and cell wall type.The aforesaid three major divisions shall now be dealt with separately in the sections that follows.
Actinomycetes with Multiocular** Sporangia***
The latest version of
Bergey’s Manual has explicitly described the actinomycetes occurring as
the
‘clusters of spores’ in a specific situation when a hypha undergoes division both transversely and
logitudinally.
In reality, all the three genera critically present in this section essentially possess chemotype
III cell walls,
whereas the cell extract carbohydrate patterns differ prominently.
Salient Features:
The salient features of the actinomycetes with multiocular sporangia are as
follows :
(1) The mole % (G + C) values varies from 57 to 75.
(2)
Chemotype III C Cell Walls****: Geodermatophillus belonging to this category has motilespores and is specifically an aerobic soil organism.
(3)
Chemotype III B Cell Walls : Dermatophillus invariably gives rise to pockets of motile spores
having tufts of
flagella. It is a facultative anaerobe and also a parasite of mammals actually
responsible for the skin infection
streptothricosis.
(4)
Chemotype III D Cell Walls: Frankia usually produces non-motile sporangiospores evidently
located in a sporogenous body. It is found to extend its normal growth in a symbiotic
association particularly with the roots of
eight distinct families of higher non-leguminous
plant sources
viz., alder trees. These organisms are observed to be extremely efficient
microaerophilic nitrogen-fixers
which frequently take place very much within the root
nodules
of the plants. Furthermore, the roots of the infected plants usually develop nodules
that would eventually cause fixation of nitrogen so efficiently that a plant, for instance : an
alder tree,
may grow quite effectively even in the absence of combined N2, when nodulated
respectively. It has been duly observed that very much inside the
nodule cells, Frankia invariably
gives rise to
branching hyphae having globular vesicles strategically located at
their ends. Consequently, these vesicles could be the most preferred sites of the N
2 fixation
ultimately. However, the entire phenomenon of N
2 fixation is quite similar to that of Rhizobium
wherein it is both O
2 sensitive and essentially and predominantly needs two elements, namely :molybdenum (Mo), and cobalt (Co).
Actinomycetes and Related Organisms
This particular section essentially comprises of a relatively heterogenous division of a large crosssection
of microorganisms having altogether diverse characters including:
group, genus, order, and
family,
as outlined below :
(
a) Group: Coryneform
(
b) Genus: Arthrobacter, Cellulomonas, Kurthia, Propionibacterium
(
c) Order: Actinomycetales, and
(
d) Family: Actinomycetaceae, Mycobacteriaceae, Frankiaceae, Actinoplanaceae, Nocardiaceae,Streptomycetaceae, Micromonosporaceae.
Salient Features :
The salient features of coryneform bacteria are as follows:
(1) They are usually non-motile, Gram-positive, and non-acid fast.
(2) They are mostly chemoorganotrophs, aerobic, and also facultatively anaerobic.
(3) They are widely distributed in nature with % (G + C) values ranging between 52 to 68 moles
per cent.
(4) The type species belonging to this class is represented by
C. diphtheriae which is particularly
known to produce a highly
lethal exotoxin and causes the dreadly disease in humans
called
diphtheria.
(
b) Plant pathogenic corynebacteria: Interestingly, the bacteria belonging to this particular class
is closely akin to those present in section (
a) above; however, these are essentially characterized by three
prominent features, namely: (
i) less pleomorphic, (ii) strictly aerobic in nature, and (iii) possess % (G + C)
values ranging between 65–75 moles per cent.
Based on ample scientific evidences, this particular section is further sub-divided into
four
categories, such as: (
i) types of polysaccharide antigens, (ii) composition of amino acids present duly in
cell walls, (
iii) minimal nutritional requirements, and (iv) etiology of the disease caused in plants.
(
c) Non-pathogenic corynebacteria: This particular section essentially consists of non-pathogenic
corynebacteria quite commonly derived and isolated from soil, water, air, and are invariably described in
the literature very scantily by virtue of their
morphological similarities and hence, the virtual scope ofany possible distinct differentiation.
Genus
The
four prominent genus shall be treated individually in the sections that follows:
(
a) Arthrobacter: The genus Arthrobacter essentially consists of such organisms that undergo a
marked and pronounced change in form particularly in the course of their respective growth on the
complex media.
It has been duly observed that the relatively ‘older cultures’ do comprise of coccoid
cells*
very much resembling to micrococci in their appearance. In certain specific instances, the cells
could be either
spherical to ovoid or slightly elongated. Importantly, when these are carefully transferred
to the
‘fresh culture media’, consequently the ultimate growth takes place by two distinct modes,
namely : (
a) due to swelling, and (b) due to elongation of the coccoid cells, to produce rods thatessentially have a diameter precisely much less in comparison to the corresponding enlarged cells.
Arthrobacter’s subsequent growth and followed up divisions usually yields irregular rods that
vary appreciably both in size and shape.
Importantly, a small segment of the rods are invariably arranged at an
‘angle’ to each other
thereby causing deformation. However, in richer media, cells may exhibit preliminary (rudimentary)
branching, whereas the formation of
true mycelia cease to form. Besides, along with the passage of the
‘exponential phase’,
the rods turn out to be much shorter and get converted to the corresponding coccoid
cells.
A few other prevalent characteristics are as follows:
• Rods are either non-motile completely or motile by one sub-polar or a few lateral flagella.
• Coccoid cells are Gram-positive in nature, chemoorganotrophic, aerobic soil organisms having
a distinct
respiratory metabolism.
• Species present within the genus are invariably categorized and differentiated solely depending
on the composition of cell wall; hydrolysis of gelatin, starch etc.; and the ultimate
growth-factor requirement.
It is, however, pertinent to state here the
two other genera although whose actual and precise
affiliation is still
‘uncertain’, yet they are quite related to the Arthrobacter, namely: Brevibacterium and
Microbacterium.
(
b) Cellulomonas: The genus Cellulomonas essentially comprises of bacteria that have the competence
and ability to hydrolyse the
cellulose particularly.
Salient Features :
The various vital and important salient features are as stated below:
(1) The
cells usually observed in young cultures are irregular rods having a diameter nearly 0.5 μm
and a length ranging either between 0.7 to 2
μm or even slightly in excess.
(2) The appearance of the
cells could be straight, slightly curved, or angular or beaded or occasionally
club-shaped.
(3) Importantly, certain
cells may be arranged strategically at an angle to each other as could be
observed in the case of
Arthrobacter [see section 5.1.4.2(a)]; besides, they (cells) may infrequently
exhibit rudimentary branching as well.
(4)
Older cultures are invariably devoid of ‘true mycelia’ but the ‘coccoid cells’ do predominate
in number.
(5) The
cells may be Gram-positive to Gram-negative variable, motile to non-motile variable,
non acid-fast, aerobic chemo-organotrophos, having an
optimum growth temperature at 30°C.(6) The % (G + C) values ranges between 71.7 to 72.7 moles.
Interestingly, there exists only one species,
Cellulomonas flavigenum, which is exclusively known
and recognized; and found commonly in the soil.
(
c) Kurthia: The genus Kurthia is specifically characterized by organisms that are prominently
and rigidly
aerobic in nature; besides, they happen to be chemoorganotrophs. Young cultures essentially
comprise of cells that are mostly unbranched rods having round ends, and occurring as distinct
parallel chains. Older cultures
normally comprise of coccoid cells that are critically obtained by the
fragmentation of rods.
Salient Features:
The salient features of the organisms belonging to the genus Kurthia are as
given under:
(1) The rods are rendered
motile by the presence of peritrichous flagella*.
(2) The
cells predominantly grow in abundance, particularly in the presence of sodium chloride
(NaCl) solution [4 to 6% (w/v)] prepared in sterilized distilled water.
(3) The optimum temperature required for the healthy growth of the cells usually varies between
25 to 30°C.
Interestingly, there prevails only one species,
Kurthia zoefi, that has been duly recognized anddescribed in the literature.
(
d) Propionibacterium: The family Propionibacteriaceae invariably consists microbes that have
the following characteristic features :
(
i) They are all Gram-positive, non-spore forming, anaerobic to aerotolerant, pleomorphic,
branching or filamentous or regular rods.
(
ii) On being subjected to ‘fermentative procedures’ it has been duly observed that the major
end-products ultimately generated are, namely :
propionic acid, acetic acid, carbon dioxide,
or a
mixture of butyric, formic, lactic together with other monocarboxylic acids.
(
iii) Growth: Their normal growth is usually enhanced by the very presence of carbon dioxide,
and
(
iv) Habitat: These microbes are normally inhabitants of skin, respiratory, and the intestinal
tracts of a large cross-section of animals.
A survey of literature would reveal the description of
two genera, namely : Propionibacteriumand Eubacterium.
Propionibacterium
: The genus Propionibacterium predominantly comprises of such bacterial
cells
that happen to be virtually non-motile, anaerobic to aerotolerant, and essentially give rise to propionic
acid
as well as acetic acid.
Salient Features:
The bacterial cells do have the following salient features, such as :
(1) They are quite often arranged in pairs, singles or ‘V’ and ‘Y’ configurations.
(2) These are actually
chemoorganotrophs which eventually attain growth very rapidly between
a temperature ranging between 32–37°C.
(3) A large and appreciable quantum of strains do grow either in 20% (w/v) bile salts or 6.5%
(w/v) sodium-chloride/glucose broth.
(4) Certain species are observed to be pathogenic in nature.
However, the genus
Propionibacterium essentially includes eight species that have been duly
identified, characterized, and recognized entirely based upon their
end products derived from their
respective metabolism.
Eubacterium
: The genus Eubacterium comprises prominently of such bacterial cells that could
be either motile or non-motile, obligatory anaerobic, and lastly either non-fermentative or fermentative
in nature. It has been adequately demonstrated that particularly the
fermentative species give rise to
mixtures of organic acids,
viz., butyric, acetic, formic or lactic, or even other monocarboxylic organic
acids. Besides, these bacterial cells undergo both profuse and rapid growth at 37°C, and are invariably
observed to be located strategically in the various marked and pronounced cavities in
humans, animals,
soil
, and plant products.Interestingly, there are certain species belonging to this genus which exhibit distinct pathogenicity.