Actinomycetes

Filamentous, gram positive bacteriafilamentous

These are the organisms with characteristics common to bothbacteria and fungi but yet possessing distinctive features to delimitthem into a distinct category. In the strict taxonomic sense,actinomycetes are clubbed with bacteria the same class ofSchizomycetes and confined to the order Actinomycetales.

They are unicellular like bacteria, but produce a mycelium which isnon-septate (coenocytic) and more slender, like true bacteria they donot have distinct cell-wall and their cell wall is without chitin andcellulose (commonly found in the cell wall of fungi). On culturemedia unlike slimy distinct colonies of true bacteria which growquickly, actinomycetes colonies grow slowly, show powderyconsistency and stick firmly to agar surface. They produce hyphaeand conidia / sporangia like fungi. Certain actinomycetes whosehyphae undergo segmentation resemble bacteria, bothmorphologically and physiologically.

Actinomycetes are numerous and widely distributedin soil and are next to bacteria in abundance. Theyare widely distributed in the soil, compost etc. Platecount estimates give values ranging from 10^4 to10^8 per gram of soil. They are sensitive to acidity /low PH (optimum PH range 6.5 to 8.0) andwaterlogged soil conditions. The population ofactinomycetes increases with depth of soil. They areheterotrophic, aerobic and mesophilic (25-30 ^c)organisms and some species are commonly presentin compost and manures are thermophilic growing at55-65° c temperature (eg. Thermoatinomycetes,Streptomyces).

In the order of abundance in soils, the commongenera of actinomycetes are Streptomyces (nearly70%), Nocardia and Micromonospora althoughActinomycetes, Actinoplanes, Micromonospora andStreptosporangium are also generally encountered.

Functions / Role of actinomycetes:

1. Degrade/decompose all sorts of organic substances likecellulose, polysaccharides, protein fats, organic-acids etc.

2. Organic residues / substances added soil are firstattacked by bacteria and fungi and later by actinomycetes,because they are slow in activity and growth than bacteriaand fungi.

3. They decompose / degrade the more resistant andindecomposable organic substance/matter and produce anumber of dark black to brown pigments which contributeto the dark colour of soil humus.

4. They are also responsible for subsequent furtherdecomposition of humus (resistant material) in soil.

5. They are responsible for earthy / musty odor / smell offreshly ploughed soils.

6. Many genera species and strains (eg. Streptomyces ifactinomycetes produce/synthesize number of antibioticslike Streptomycin, Terramycin, Aureomycin etc.

7. One of the species of actinomycetes Streptomycesscabies causes disease "Potato scab" in potato.

Characteristics• Gram positive bacteria• Filamentous forming mycelial structure like fungi• Form spores• Egs. Streptomyces and Nocardia

Streptomyces• Over 500 sp are known• Branching seen• Growth is always at the tips of the filaments• Vegetative phase- a complex, tightly woven matrix,

resulting in a compact, convoluted mycelium andsubsequent colony

• As the colony ages, characteristic aerial filamentscalled sporophore are formed

• They project above the colony and give rise tospores distinct from endospores

Spore formation in Streptomyces

Morphology of spore bearing structureMonoverticillate with nospiral

Monoverticillate withspiral

biverticillate with nospiral

biverticillate with spiral

Closed spiral

Open spiral straight

flexous

Ecology of Streptomyces• Aquatic habitat• Primarily soil organisms(found in well-drained soils)

preferably aerobic envt• Produce metabolite called geosmins responsible for

earthy smell• They prefer alkaline to neutral soils• Nutritional requirement – versatile (wide variety of

carbon sources such as sugars, alcohol etc.)• They produce extracellular enzymes for utilizing

starch, cellulose and hemicellulose, proteins and fatseven lignin, tannin or even rubber

• 50 distinct carbon sources can be utilized so veryimportant for recycling

Importance of Streptomyces• Antibiotic producers. Over 500 distinct antibiotics

have been produced by streptomycesMore than 60 antibiotics have been producedwhich an important application in human andveterinary medicine, agriculture and industry

• Egs- streptomycin, neomycin, tetracycline, nystatin

Nocardia• Mycelial forms fragment to form coccoid or

elongate elements• Aerial spore are occasionally produced• Sometimes they are acid-fast• Lipid content in cell and cell wall is very high• Common soil organisms, found everywhere• Obligate aerobes• Many utilize hydrocarbon

Nocardia is a genus of weakly staining Gram-positive,catalase-positive, rod-shaped bacteria. It formspartially acid-fast beaded branching filaments (actingas fungi, but being truly bacteria). It contains a totalof 85 species. Some species are nonpathogenic,while others are responsible for nocardiosis.Nocardia species are found worldwide in soil rich inorganic matter. In addition, they are oral microflorafound in healthy gingiva, as well as periodontalpockets. Most Nocardia infections are acquired byinhalation of the bacteria or through traumaticintroduction.

The most common form of human nocardial diseaseis a slowly progressive pneumonia, the commonsymptoms of which include cough, dyspnea(shortness of breath), and fever. It is common inimmunocompromised persons. Every organ can beaffected if a systemic spread takes place.Nocardia species are deeply involved in the processof endocarditis as one of its main pathogenic effects.In about 25–33% of people Nocardia infection takesthe form of encephalitis and/or brain abscessformation. Nocardia may also cause a variety ofcutaneous infections such as actinomycetoma(especially N. brasiliensis), cellulitis, andsubcutaneous abscesses.