Antibiotic producing potentials of three freshwater actinomycetes isolated from the Eastern Cape Province of South Africa

Crude extracts of three actinomycetes species belonging to Saccharopolyspora (TR 046 and TR 039) and Actinosynnema (TR 024) genera were screened for antibacterial activities against a panel of several bacterial strains. The extracts showed antibacterial activities against both gram-negative and gram-positive test bacteria with inhibition zones ranging from 8 to 28 mm (TR 046); 8 to15 mm (TR 039); and 10 to 13 mm (TR 024). The minimum inhibitory concentrations ranged from 0.078 to 10 mg/mL (TR 046); 5 to >10 mg/mL (TR 039); and 1.25 to 5 mg/mL (TR 024). Time-kill studies revealed that crude extract of TR 046 showed strong bactericidal activity against Bacillus pumilus (ATCC14884), reducing the bacterial load by 10(4) cfu/mL and 10(2) cfu/mL at 4x MIC and 2x MIC, respectively, after 6 h of exposure. Similarly, against Proteus vulgaris (CSIR 0030), crude extract of TR 046 achieved a 0.9log(10) and 0.13log(10) cfu/mL reduction at 5 mg/mL (4x MIC) and 1.25 mg/mL (2x MIC) after 12 h of exposure. The extract was however weakly bactericidal against two environmental bacterial strains (Klebsiella pneumoniae and Staphylococcus epidermidis); and against Pseudomonas aeruginosa (ATCC 19582): the extract showed bacteriostatic activities at all concentrations tested. These freshwater actinomycetes appear to have immense potential as a source of new antibacterial compound(s).