INTRODUCTION

The soil being the outermost layer of the earth crust is a natural reservoir for microorganisms and their antimicrobial products. The numbers and species of microbes in the soil is dependent on environmental conditions like nutrient availability, soil texture, presence of moisture in soil and type of vegetation cover, and their number varies according to the type of environmental condition. The soil is a favourable habitat for the growth of microorganisms including bacteria, algae, fungi, viruses and protozoa. Microorganisms are found in large numbers in soil, usually several millions are present per gram of soil with bacteria and fungi being the most prevalent.

An antibiotic was in previous times defined as chemical substances produced by various species of microorganisms that is capable, in low concentrations, of inhibiting the growth of or killing other microorganisms. Today, this definition has been replaced by any substance produced by a microorganism or a similar product produced wholly (synthetic) or partially (semi-synthetic) by chemical synthesis and in low concentrations inhibits the growth of or kills microorganisms. Antibiotics are one of the most important commercially exploited secondary metabolites produced by bacteria and employed in a wide range use in Medicine and Agriculture (Musliu and Salawudeen, 2012).

However, soil microorganisms with known antibiotic properties are mainly bacteria and fungi. A group of Gram-posititve bacteria distinguished from other bacteria by their morphology called actinomycetes have been identified as the major producers of antibiotics (Abebe et al., 2013). More than 70% of naturally occurring antibiotics have been isolated from different genera of actinomycetes especially Streptomyces spp (Khanna et al., 2011). Other bacterial species that have the ability to produce antibiotics are Bacillus species and Lactobacillus species (Musliu and Salawudeen, 2012) while Penicillium and Cephalosporium, terrestrial and marine molds respectively are antibiotic-producing fungi.

The worldwide use of antibiotics has rapidly increased since the accidental discovery of Penicillin by Alexander Fleming in 1928. Since then, thousands of useful secondary metabolites of microbial origin have been discovered (Nordenfjäll, 2014). While many antibiotics are known to exist, efforts to discover new antibiotics still continue. Therefore many species such a Streptomyces, Bacillus and Penicillium have been studied continuously for their ability to produce antibiotics (Sandeepta et al., 2015). Some antibiotics like Penicillin, Erythromycin and Methicillin which used to be one-time effective treatment against infectious diseases are now less effective because bacteria have become more resistant to such antibiotics (Raja et al., 2010).

Pathogenic bacteria are acquiring resistance to existing antibiotics, most of which are expensive and have been associated with side effects like nephrotoxicity. Bacteria have evolved numerous strategies for resisting the action of antibiotics and antibacterial agents (Sandeepta et al., 2015). Antibiotic resistant pathogens such as Methicillin and Vancomycin resistant strains of Staphylococcus aureus and others are enormous threat to the treatment of serious infections and to avoid this happening, immediate replacement of the existing antibiotic is necessary.

The problem of resistance against the present antibiotics in bacteria increases day by day. So there is an urgent need to search new antibiotics or the sources of new antibiotics. A lot of work has been done during last few decades that has witnessed the production of novel antibiotics from different microorganisms (Mashoria et al., 2014). Considerable research is being done in order to find new antimicrobial producing bacteria isolated from soil. One of the possible ways to increase the chance of finding novel antibiotics is to find new approaches for isolating interesting bacteria and fungi or at least make the existing methods more efficient (Nordenfjäll, 2014).