Actinomycetes and Related Organisms

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Chapter: Pharmaceutical Microbiology : Characterization, Classification and Taxonomy of Microbes

This particular section essentially comprises of a relatively heterogenous division of a large cross-section of microorganisms having altogether diverse characters including: group, genus, order, and family, as outlined below :

Actinomycetes and Related Organisms


This particular section essentially comprises of a relatively heterogenous division of a large cross-section 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.


All the aforesaid divisions shall now be treated individually and briefly in the sections that follows.


Group [Example : Coryneform]


The coryneform group essentially comprises of organisms that have the following three charac-teristic properties, namely :


·        These are Gram-positive in nature

·        These are non-spore forming rods of irregular outline, and

·        These are represented by diverse species.


Species: The species belonging to the coryneform group includes microbes three individual sections which would be treated separately in the sections that follows.


(a) Human and animal parasites and pathogens : Importantly, the bacteria which are intimately associated with this section are observed to be straight to slightly curved rods, and invariably appear as club-shaped swellings, as shown in Fig. 3.6.


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 particu-larly 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 of any possible distinct differentiation.




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 trans-ferred 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 that essentially have a diameter precisely much less in comparison to the corresponding enlarged cells.


Eventually, there may be predominant ‘outgrowths’ occurring at more than one segment of the cell as depicted in Fig. 3.7.


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 hav-ing a distinct respiratory metabolism.


·        Species present within the genus are invariably categorized and differentiated solely de-pending 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 com-petence 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 occa-sionally 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]; besides, they (cells) may infre-quently exhibit rudimentary branching as well.


(4) Older cultures are invariably devoid of ‘true mycelia’ but the ‘coccoid cells’ do predomi-nate 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 essen-tially 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 and described in the literature.


There are certain characteristic features of the genera Corynebacterium, Arthrobacter, Cellulomonas, and Kurthia that have been duly summarized in Table 3.8.


(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 diox-ide, 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 : Propionibacterium and Eubacterium. These two genera shall now be dealt with briefly and separately in the sections that follows:


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.




The order Actinomycetales shall be treated at length in this particular section.


Importantly, Actinomycetales do contain such members that necessarily have a typical and prominent tendency to produce the ‘branching filaments’ in particular, which in certain instances ultimately develop into a full-fledged mycelium. Interestingly, the family: Mycobacteriaceae — does possess extremely short filaments ; whereas, the family : Streptomycetaceae — does exhibit distinctly well-developed filaments. Fig. 3.8. illustrates the filaments duly formed in the specific case of streptomyces.


Salient Features: The salient features of the filaments/spores occurring in various families are as stated under:


(1) The diameter of the filaments in Streptomyces ranges between 0.2 to 2 μm.


(2) A few families do possess such filaments that usually tend to fragment; and subsequently the ensuing fragmentation gives rise to coccoid, elongate, or diploid bacterial cells.


(3) In certain families, one may observe the formation of ‘true spores’ occurring specifically either on aerial or substrate hyphae.


(4) Invariably, spores may be produced either singly or in chains that could be straight, looped, or spiral in appearance; and such chains usually come into being either singly or in a verticillate* manner.


(5) It may be seen that the spores are duly borne in sporangia as in the particular instance of the family : Actinoplanaceae, which could be either motile or non-motile. Importantly, the or-ganism though is Gram-positive in character, but the aforesaid reaction might change with aging.


(6) The characteristic features of certain other family members of the order Actinomycetales are as given under:


(a) Mycobacteriaceae: are acid fast in character


(b) Nocardiaceae: are found to be weakly acid-fast in nature


Family :


There are in fact, seven prominent families belonging to the category of Actinomycetes and Related Organisms, which shall be treated individually in the sections that fol-lows:


(a) Actinomycetaceae : The cardinal characteristic features of the family Actinomycetaceae are as follows:

(1) Bacteria are predominantly ‘diploid’ in shape that have been observed to exhibit a clear tendency to give rise to the formation of branched filaments during certain stages of their ‘cultural development’.

(2) Evidently, the fragmentation of filaments invariably takes place quite rapidly to produce diploid as well as coccoid cells.

(3) The formation of ‘aerial mycelium’ and ‘spores’ do not take place at all.

(4) The bacterial cells are non-motile that invariably extend their growth as anaerobic facultatively, whereas quite a few may turn out to be either absolutely anaerobic or aerobic in nature.

It has been duly observed that the family Actinomycetaceae has five distinct genera exclusively based upon their intimate (direct) relationship to oxygen.


(b) Mycobacteriaceae: The salient features of a large segment of the members belonging to the family Mycobacteriaceae are as stated under :

(1) Invariably most of its members are pathogenic in nature.

(2) The bacterial cells are slightly curved or straight rods which are occasionally exhibited in a ‘branching mode’.

(3) Importantly, both mycelium and filamentous type growths are generally found ; and even-tually they get fragmented into the corresponding rods or coccoid cells.

(4) The bacterial cells are usually found to be acid-fast, non-motile, and failed to give rise to the formation of endospores,* conidia,** and capsules.***

(5) The bacterial cells are usually characterized specifically by a relatively much higher lipid content and are also comprised of long, branched chains of mycolic acids.****


Importantly, the genus Mycobacterium includes prominently the host of such critical and vital components as: obligate parasites, saprophytes, and intermediate forms which do vary in their nutri-tional requirements appreciably. Besides, all microbes are usually aerobic in nature, and a possible growth may take place very much in depths of the ensuing medium. Generally, they are found in warm/ cold blooded animals, soil and water; whereas, the % (G + C) values range between 62–70 moles per cent.


(c) Frankiaceae: The family Frankiaceae predominantly comprises of such organisms that are symbiotic, mycelial, and filamentous in nature. Besides, they are capable of inducing and residing particularly in the root modules of a large cross-section of non-leguminous dicotyledonous plants as summarized in Table 3.9.


Table 3.9. Non-leguminous Nodule-bearing Dicotyledonous Plants with Frankia Species as Endophyte


(d) Actinoplanaceae: The family Actinoplanaceae consists of microorganisms which do pos-sess the following characteristic features, such as :

(1) They give rise to distinct mycelia that may be either intramatrical or occasionally aerial in nature.

(2) The filaments have a diameter ranging between 0.2 to 2.6 μm mostly.

(3) Importantly, the sporangiospores are usually produced either on branched or unbranched hyphae. These are of two distinctly different shapes, namely :

(i) Having large spherical to specific irregular multisporous sporangia, and

(ii) Having small club-shaped or filiform sporangia consisting of one to several spores.

However, the spores could be either motile or non-motile. These two different fruiting structures are vividly illustrated in Fig. 3.9(a) and (b).

(4) These are invariably Gram-positive chemoorganotrophs having a respiratory metabolism which being aerobic in nature and available abundantly in particular humus rich soil.

(5) The family exclusively comprises of ten (10) genera that may be grouped into two broad divisions as described in section (3) above.


(e) Nocardiaceae: The family Nocardiaceae essentially and solely comprises of aerobic actinomycetes wherein the mycelium could be present either in the rudimentary (elementary) or in an extensive form. It has been duly observed that ‘sporogenesis’ i.e., the production of spores significantly varies with the genus. It is, however, pertinent to state here that Nocardiaceae possesses prominently two genera, namely: Nocardia, and Pseudonocardia.

Nocardia — It has the following characteristic features, such as:

(1) Possesses specific spores not produced on differentiated hyphae.

(2) Essentially the reproduction bodies are typical mycelial fragments that are produced quite irregularly either in the aerial hyphae or in the substrate.

(3) The genus Nocardia is usually further categorized into three distinct morphological groups that are solely based upon the critical extent of the specific mycelial development.

(4) The % (G + C) values ranges between 60 to 72 moles per cent.

(5) Importantly, the carotenoid pigments (viz., β-carotene) are usually produced by various species.


Pseudonocardia : This particular genus essentially comprises of two distinct species, namely: P. thermophilia and P. spinosa. Importantly, both aerial as well as substrate hyphae are duly gener-ated. The spores may be formed suitably either on substrate mycelium or on aerial mycelium. The colonies of pseudonocardia are duly obtained either as colourless or may vary from slightly yellow to orange. However, the genus pseudonocardia is usually found in soil and manure ; and even some may grow at ~ 60°C.


(f) Streptomycetaceae : Incidentally, this particular family, streptomycetaceae has gotten the cognizance of being one of the most vital and important families belonging to the natural order Actinomycetales.


Salient Features: The various salient features of the family streptomycetaceae are as enumer-ated under:


(1) The vegetative hyphae ranges between 0.5 to 2 μm in diameter.


(2) One of the most common apparent features being the presence of a well-branched mycelium which fails to undergo rapid fragmentation.


(3) The phenomenon of reproduction predominantly takes place either due to spores or occa-sionally by the aid of simultaneous growth of mycelial fragments.


(4) They invariably behave as Gram-positive microbes, and also are aerobic having Type-I Cell Walls.


(5) The % (G + C) values of the DNA in the specific genera so far examined ranges between 69 to 73 moles per cent.


The Streptomycetaceae family has essentially four distinctly well-recognized genera that are obviously segregated based entirely upon the typical sporulation characteristic features, as given below:


(i) Streptomyces — Importantly, the genus streptomyces received a well-deserved world wide recognition by virtue of its critical role in the production of antibiotic. In fact, there are several strains identified and examined, which precisely gave rise to either one specific or a plethora of antibiotics.

·        The bacterial cells are found to be heterotrophic, aerobic, and also extremely oxidative.

·        Various other members of this family, Streptomyces, do give rise to a broad spectrum of pigments.

·        Bergey’s Manual include at least 463 species of the specific genus, and surprisingly a good number of them do possess even ‘uncertain taxonomic status’.


(ii) Streptoverticillium : Interestingly, the genus Streptoverticillium vividly consists of forty species. The characteristic features of this particular genus are as follows:

·        Aerial mycelium and substrate are both present.

·        The branching ‘aerial mycelium’ more or less looks very much similar to the ‘barbed-wire’.

·        Reproduction is accomplished by means of either spores or by fragmentation of the corre-sponding mycelium.

·        The specific genus, Streptoverticillium, critically responsible for the production of a large cross-section of vital and important ‘antibiotics’* and ‘pigments’.**


(iii) Sporichthya : The genus, Sporichthya, possesses such vital members that essentially give rise to the formation of hyphae which are found to be not only branched, but also reason-ably short in structure. Its characteristic features are as stated below:

·        The aerial mycelium is found to be strategically attached to the solid medium critically with the help of hold-fasts that actually originate from the very wall of the hyphae base.

·        The aerial hyphae are observed to be articulately split up into smooth walled spores that essentially possess a collar-like structure which in turn gives rise to the origination of a flagellum.

·        The spores are motile in water.

·        The genus forms Gram-positive/Gram-negative strains, heterotrophic in nature, grows on rich media, and lastly bacteria-like growth is observed explicitely.


(iv) Microellobospora: The genus, Microellobospora, critically comprises of such organisms having slender hyphae with a diameter of 1 μm. It has been duly observed that the substrate mycelium usually grows into a compact layer. Besides, the aerial mycelia and the substrate mycelia predominantly form sporangia, strategically located on short sporangiophores.


Sporangia do contain a single longitudinal row consisting of non-motile sporangiospores.


Spores are observed to come into being by virtue of the simultaneous division of the specific intrasporangia hyphae.


Antibiotics : Certain typical strains belonging to this genus produce useful antibiotics as well.


Cell-wall is found to be typically of type-I and also aerobic and heterotrophic in character ; whereas, the formation of arthrospores* is not observed.


(g) Micromonosporaceae: The family Micromonosporaceae necessarily comprises of such members that cause the production of aerial mycelium as well as substrate mycelium except in the genus micromonospora.


The various characteristic features of the family Micromonosporaceae are as stated under :


(i) They are devoid of the sporophores or are sometimes quite short in structure ; and also in certain specific instances do exhibit dichotomous branching.


(ii) In a broader perspective, these are aerobic in nature, largely mesophilic; and certain species are thermophilic, besides being primarily saprophytic in the environment of the soil.


In fact, the family Micromonosporaceae comprises of six distinct genera that are exclusively classified based upon either the presence or the absence of aerial mycelia together with other corre-sponding typical sporulation characteristics.


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