ABSTRACT

A total of 106 actinomycetes isolated from the rhizosphere of plants in abbatoir and refuse dumps in Awka and Onitsha were investigated for the production of antimicrobial substances. Five of them were found to show antimicrobial activity against Gram positive and Gram negative bacteria as well as fungi on solid media. Three of the very active isolates designated MP-75, SP -76 and QP-100 were further investigated in submerged medium in a shake-flask experiment using glucose and soy bean as carbon and nitrogen sources respectively. Isolates MP-75 and SP-76 were found to produce antimicrobial substances. The antimicrobial substance produced by isolates SP-76 showed the highest antimicrobial activity against Escherichia coli, Pseudomonas aeruginosa, Klebsiella sp, Staphylococcus aureus and Bacillus sp. The isolates were identified as Streptomyces species based on their characteristic features. Activity of the antimicrobial substance produced by Streptomyces SP-76 was maximum when 4% (w/v) glucose and 2% (w/v) soybean were used in the fermentation process. Influence of surfactants on accumulation of antimicrobial substance by Streptomyces SP-76 in the fermentation broth showed that Tween 80, oleic acid, palmitic acid and stearic acid enhanced antimicrobial activity. The effect of growth promoters on antibiotic production by Streptomyces SP-76 indicated that peptone, casein and yeast extract stimulated antimicrobial activity against the test organisms. The effect of varying pH on antibiotic production by Streptomyces SP-76 showed that there was maximum antimicrobial activity at pH 7. In a time course for antibiotic production, maximum antimicrobial activity was obtained at 120h. Paper chromatography of the culture filtrate of Streptomyces SP-76 indicated that it contains more than one antimicrobial substance. From this, it can be seen that the growth and subsequent production of bioactive metabolites by Streptomyces SP-76 isolated from the soil

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INTRODUCTION

For centuries, preparations derived from living matter were applied to wounds to destroy infection. The fact that a microorganism is capable of destroying one another

was not established until the latter half of the 19th century, when Pasteur noted the antagonistic effect of other bacteria on the anthrax organism and pointed out that this action might be put to therapeutic use. Meanwhile, the German Chemist, Paul Ehrlich developed the idea of selective toxicity; that certain chemicals that would be toxic to some organisms like infectious bacteria, would be harmless to other organisms e.g humans (Limbird, 2004).

In 1928, Sir Alexander Fleming, a Scottish biologist, observed that Penicillium notatum, a common mold, had destroyed Staphylococcus bacteria in culture and in 1939, the American microbiologist Rene Dubois demonstrated that a soil bacterium was capable of decomposing the starchlike capsule of the Pneumococcus bacterium, without which the Pneumococcus is harmless and does not cause pneumonia.

Dubois then found in the soil a microbe, Bacillus brevis, from which he obtained a product, tyrothricin, that was highly toxic to a wide range of bacteria (Limbird, 2004). Tyrothricin, a mixture of the two peptides, gramicidin and tyrocidine, was also found to be toxic to red blood and reproductive cells in humans but could be used to good effect when applied as an ointment on body surfaces. Penicillin was finally isolated in 1939, and in 1994 Selman Waksman and Albert Schatz, American microbiologists, isolated streptomycin and a number of other antibiotics from Streptomyces griseus (Calderon and Sabundayo, 2007).

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Discovery of new antibiotics produced by Streptomyces still continues. Today, due to the increasing resistance of pathogenic bacteria to our current arsenal of antibiotics, a great need exist for the isolation and discovery of new antibiotics and other drug agents (Jarroff, 1994; Rice, 2003; Wenzel, 2004). Fifty years ago, it was easy to discover new antibiotics by simply screening the fermentation broths of actinomycetes and fungi. Today, it is much more challenging, but there are much better tools to address this problem. Discovering new antibiotics, pharmacophores is a long-term endeavor that requires deft orchestration and support of many innovative sciences (Cuatrecasas, 2006). There are three approaches that can be used to improve our chances of finding new antibiotic substances: new test methods, new organisms, and variation of culture conditions. None of these three options guarantees success alone and the chances are best if the three are combined. There is need for long-term basic microbiological research, which should cover the following areas: methods of isolating and cultivating microorganisms that have not yet been accessed or only with great difficulty, studies on the transportation of antibiotics into the bacterial cell, comparative biochemistry of prokaryotes and eukaryotes, mode of action of antibiotics and pathogenicity factors. In addition to search for new antibiotics, long-term strategies to prevent the development and spreading of resistant bacteria must be developed (Fiedler and Zanher, 1995). Therefore, it is time to define natural product discovery from actinomycetes and other microbes as a major priority for medical sciences and to engage the most creative scientists in academia (Cuatrecasas, 2006) in close collaboration with biotechnology and pharmaceutical companies, which would elevate the science to a new level of achievement so as to be commensurate with past successes and present demonstrated potential.