Antibiotics Production In Actinobacteria

Published Date: 02 Nov 2017

Most antibiotics are products of the secondary metabolism of three main groups of microorganisms which are: eubacteria, actinobacteria and filamentous fungi. The actinobacteria group produces the largest number and greatest variety of antibiotics (Waksman, 1950). The actinobacteria consist of a group of branching unicellular gram-positive bacterial organisms, with the DNA rich in Guanine and Cytosine (70%). They are extensively spread in nature, occurring typically in soil, composts, and aquatic habitats. Most species are free-living and saprotrophic, but some may form symbiotic associations, while others are pathogenic in man, animals and plants.

The growth of actinobacteria is filamentous, on solid or liquid medium they form a mass of growth generally referred to as "colony". This mass is branching filaments that initiated from a single spore or from a small part of vegetative mycelium. The vegetative growth of the actinobacteria, or stroma is usually shiny, gel like, or lichenoid in appearance and varies in shape, size and thickness. Actinobacteria reproduce by either binary fission or special conidia.

Actinobacteria are often characterized by the production of a variety of pigments, both on biological and on artificial media. The variation of color depends upon many factors, such as the nature and age of the culture. Acids and alkalis are known to have a clear effect upon the nature and integrity of the pigment produced (Waksman, 1950). The color of the pigment produced varies from one strain to another. Some may be whitish or cream colored; others may appear yellow, red, pink, orange, green, violet or brown.

The actinobacteria vary greatly in their nutritional requirements. They are able to use a great variety of simple and complex organic compounds as sources of carbon and energy and carbon. Among These compounds are sugars, hemicelluloses, organic acids, celluloses, proteins, polypeptides, amino acids, nitrogen base and others. Certain actinomycetes can also use, to a more limited extent, fats, hydrocarbons, benzene ring compounds, and even more resistant substances, such as lignin, tannin and rubber.

Streptomyces

Streptomyces is one of the actinobacteria genuses that have become well-known for genetic research. This could be attributed not only to the ability of this microorganism to produce a huge number and wide variety of antibiotics but also to the simplicity of isolating the organism from the soil and the convenience of cultivating them in the laboratory.

Streptomyces are aerobic gram-positive soil bacteria that grow vegetatively as a branching and generally non-fragmenting mycelium. The individual Streptomyces branches are called hyphae. Occasional cross walls are formed in the hypha, with unequal spacing. After a certain amount of growth, aerial branches arise from the ‘vegetative’ substrate mycelium of surface grown colonies. The aerial mycelial branches finally differentiate into chains of spores. Aerial hyphae seem to grow partially by utilizing the degraded substrate mycelium.

Streptomyces colonies grown in laboratory conditions are sometimes visible as colonies with alternating surface color which is related to that of the spores and the white hairiness typical of aerial mycelium. This is caused by multiple rounds of germination and sporulation in the laboratory culture. (Dowding, 1973).

Germinated spores, vegetative hyphal fragments, aerial hyphal fragments produced by mutants blocked at any stage of variation are all capable of initiating a new colony.

Ecology of Streptomyces

Streptomyces exist significantly in nature. Their ability to colonize the soil is greatly supported by the growth of a vegetative mass which can differentiate into spores that encourage the expansion and stability. The semi-dormant stage of the spores in the life cycle enable them to survive in soil for long periods of time (Mayfield et al., 1972; Ensign, 1978); viable Streptomyces cultures were retrieved from 70 year old soil samples (Morita, 1985). The spores show resistance to low nutrient and water availability, while the mycelial stage is sensitive to drought (Karagouni et al., 1993). The relatively high numbers of Streptomyces in soil exist largely as inactive spores for most of the time. When laboratory -grown spores were added to nonsterile soil, they exhibited very low germination efficiencies, probably because of competition with the domestic microorganisms, but pre-germinated spores grew for a short time and then re sporulate (Lloyd, 1969). Germination can be partially density-dependent, but the interaction did not cross species boundaries (Triger et al., 1991), suggesting special signaling factors between spores of the same strain, causing inhibition of germination above a certain concentration. The advantage would be to limit the number of germinating propagules in accordance with available resources. Spore germination requires exogenous nutrients, water and Ca2+ (Ensign, 1978). Nutrient status of the germination area limits the spread of hyphal growth and the time needed to differentiate into aerial hyphae and ultimately spores (Wellington et al., 1990).

Actinobacteria produce many extracellular enzymes in the soil. Because they decompose complex mixtures of polymers in dead plant tissues, animal and fungal material (McCarthy, 1987; Crawford, 1988; Wang et al., 1989), they are important in soil biodegradation for recycling nutrients associated with recalcitrant polymers (McCarthy and Williams, 1992). Moreover the addition of chitin to acidic soil directed to chitinolytic activity of acidophilic Streptomyces, followed by ammonification of the soil and eventual colonization by neutrophiles (Williams and Robinson, 1981). The enzymes required to catabolize such a common material as straw are very complex and recently much effort has been contributed to characterizing the cellulases, xylanases, amylases, maltases and other enzymes involved.

In addition to their ability to colonize bulk soil, many Streptomyces successfully were able to colonize the rhizosphere (Watson and Williams, 1974). This ability may be partly due to the antagonistic characteristics of Streptomyces against other rhizosphere bacteria such as and bacillus and Pseudomonads. An added advantage over Gram-negative soil bacteria is their ability to spread through moderately dry soil thru hyphal growth; in wetter soil, motile bacteria such as Pseudomonas fluorescens show more extensive colonization of rhizospheres than Streptomyces (Elliott-Juhnke et al., 1987; Karagouni et al., 1993; Milus and Rothrock, 1993).

Streptomyces secondary metabolism & differentiation

Most Streptomyces do not produce antibiotics throughout the period of vegetative growth. Instead, their antibiotics production begins as their growth rate slows down. Therefore the production of the secondary metabolites is considered unnecessary for vegetative growth of the producing organisms. In Streptomyces colonies that are grown on solid surface, this slowdown occurs as the aerial mycelium starts to develop from the substrate mycelium. In liquid grown culture, it takes place at a ‘transition stage’ as biomass changes from the quasi-exponential toward the stationary phase.

It has been suggested that such timing of antibiotic production and differentiation is adaptive in assisting to avoid invasion of microorganisms that could else steal the nutrients liberated by the lysis of the substrate mycelium, which are intended to supply nutrients for the development of the aerial mycelium.