Isolation and identification of antibiotic producing microorganisms from natural habitats in the KwaZulu-Natal midlands.
The search for new antibiotics continues in a rather overlooked hunting ground. In the course of screening for new antibiotic-producing microorganisms, seventy-nine isolates showing antimicrobial activity were isolated from soil samples from various habitats in the KwaZulu-Natal midlands, South Africa. Existing methods of screening for antibiotic producers together with some novel procedures were reviewed. Both modified agar-streak and agar-plug methods were used in the primary screens. The use of selective isolation media, with or without antibiotic incorporation and/or heat pretreatment, enhanced the development of certain actinomycete colonies on the isolation plates. Winogradsky's nitrite medium (Winogradsky, 1949), M3 agar (Rowbotham and Cross, 1977), and Kosmachev's medium (Kosmachev, 1960), were found to be selective for actinomycetes. Statistical analysis showed highly significant interactions between isolates, assay media and the test organisms. The diameters of inhibition zones were found to be larger on Iso-sensitest agar (ISTA)[Oxoid, England] than in nutrient agar plates. Of the 79 isolates that showed antimicrobial activity, 44 isolates were selected for confirmatory screening. Of these, 13 were selected for secondary screening. Criteria for selection were based on significant inhibition of at least two test organisms and/or the inhibition of the specifically targeted organisms, Pseudomonas and Xanthomonas species. Following secondary screening eight isolates were considered for further investigation. The isolates were tentatively identified . on the basis of morphological features, using both light microscopy and scanning electron microscopy(SEM); their ability to utilize various carbon sources; and selected physiological and staining tests. Suspected actinomycetes were further characterized on the basis of selected chemical properties using thin layer chromatography (TLC) and high pressure liquid chromatography (HPLC) techniques. High pressure liquid chromatography analysis (Beckman 6300 analyzer) detected the presence of diaminopimelic acid (DAP) in whole-cell hydrolysates of six of the isolates while TLC analysis confirmed the type ofDAP present. The isolates N2, N12, N16, N19 and N35 were tentatively identified as Thermomonospora, Saccharopolyspora, Nocardiodes, Corynebacterium and Promicromonospora, respectively. Isolate N30 was identified as belonging to the coryneform group ofbacteria, possibly an Arthrobacter species. Isolate, N8, tentatively identified as Actinosynnema, was unique among the isolates tested as it showed good antimicrobial activity against all the Gram- positive and Gram-negative bacteria, and yeasts used as test organisms in the present investigation.