Determination of In vitro lethality of actinomycetes antibiotic extracts from Menengai crater geothermal vents, Kenya

Introduction: Drug resistance is the biggest threat to human existence today. This study was conducted to investigate the sensitivity of selected pathogenic microorganisms to antibiotic extracts from soils of Menengai crater geothermal vents in Kenya. Method: Antibiotic-producing actinomycetes were isolated from geothermal vents of the crater. The isolates were identified using colony characteristics and biochemical means. Sensitivity of the selected pathogenic microorganism was carried out using primary and secondary screening techniques. Four actinomycetes coded PAN 9, PAN 18, PAN 117, and PAN 138 were selected for further analysis based on the size of the zone of inhibition and the broad spectrum of activity. Extraction of antibiotics from the selected actinomycetes was carried out using ethyl acetate. A sensitivity test of the pathogens to the extracts was done using Karby Bauer disk diffusion technique. Cytotoxicity of the antibiotic extracts was carried out using Brine shrimp lethality test and sheep blood hemolytic bioassays. The isolates presented typical actinomycetes characteristics. Results: A total of 20 actinomycete isolates showed antagonism against the test pathogenic microorganisms. The selected actinomycetes inhibited growth of Gram-positive and negative bacterial and fungal pathogens in both primary and secondary sensitivity tests. There was a significant difference in the diameters of zones of inhibition produced by the test pathogens when subjected to the antibiotic metabolites from the selected actinomycetes (F = 6.6046 P = 0.001338). Discussion: The minimum inhibitory concentration and minimum bactericidal concentration/minimum fungicidal concentration did not vary significantly (F=0.4503, P = 0.7187). The LC50 values of the antibiotic extracts varied from 2.438±0.47µg in PAN 9 to 9.3455±0.41µg in PAN 138. PAN 18 and PAN 117 showed partial hemolysis while PAN 9 and PAN 138 exhibited complete hemolysis of sheep blood erythrocytes. Soils from Menengai crater geothermal vents have actinomycetes that produce antibiotics. The antibiotics are cytotoxic to brine shrimps. In addition, the antibiotics cause hemolysis of sheep blood erythrocytes. There is a need to purify the antibiotics and structure elucidate them.  The active antibiotic extract needs to be evaluated to determine whether the hemolysis is due to the compound itself or some other chemical constituent.