Antimicrobial activity of rare actinomycetes isolated from natural habitats in KwaZulu-Natal, South Africa

Abstract

This paper reports the presence of new antibiotic-producing organisms in the relatively underinvestigated region of KwaZulu-Natal, South Africa. Using a modified agar-streak method and selective isolation media during the primary screening phase, eighty isolates showing antimicrobial activity were isolated from soil samples of various habitats in the KwaZulu-Natal Midlands. The use of selective isolation media, with antibiotic incorporation and/or heat pretreatment, enhanced the isolation of certain rare actinomycete colonies. The number of culturable antibiotic-producing microorganisms constituted about 3% (on average) of the total microbial population in the different samples studied. The highest percentage of antimicrobially active isolates came from a forest soil site whereas the lowest percentage was present in a riparian soil. One of the isolates, N8, tentatively identified as an Intrasporangium species, was isolated from barnyard soil at a poultry farm. It produced at least one broad-spectrum antibiotic active against both Gram-positive and Gram-negative bacteria and fungi and also inhibited the growth of all seven test organisms, especially Pseudomonas fluorescens and Xanthomonas campestris pv campestris at minimum inhibitory concentrations (MIC) of 0.0625 μg/ml and 0.0025 μg/ml, respectively. To our knowledge, members of this actinomycete genus have not been associated previously with antibiotic production. These data confirm that KwaZulu-Natal soils harbour rare actinomycetes that inhibit the growth of Pseudomonas fluorescens and Xanthomonas campestris pv campestris, representatives of two genera which are notoriously difficult to contain in the field. Such antibiotic producers should become more commercially important if the current trend towards the use of biocontrol agents rather than chemical treatments of plant diseases persists.