Objectives: The indiscriminate use of antibiotics has led to an upsurge in antibiotic resistance in many parts of the world. This study aimed at determining the seasonal variation of antibiotic-producing actinomycetes from Menengai crater, Kenya.
Methods: Soil samples were collected in February which receives 52mm of rainfall, July (100mm), and May (194mm). Actinomycetes were isolated using serial dilution and spread plate bioassays. The antibiotic property of the isolates against Staphylococcus aureus (ATCC 25923), Streptococcus pneumoniae (ATCC 49617), Enterococcus faecalis (ATCC 29212), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and Proteus vulgaris (ATCC 49990) was determined using primary and secondary screening methods.  Based on the size of the zone of inhibition and a broad spectrum of activity, four actinomyces coded PAN 25, 41, 75, and 110 were selected for further analysis.
Results: The physicochemical properties of the soil samples did not vary significantly (P=0.42). However, the number of actinomycetes isolated varied significantly (P=0.035) between February, May, and July. The mean zone of inhibition varied between 22±0.3 in PAN 110 to 27±0.3mm in PAN 75. Isolate PAN 110 produced an 829bp fragment, PAN 75 (971bp), PAN 41 (1250bp), and PAN 110 (829bp). The 16S rRNA gene sequence and nucleotide BLAST from NCBI revealed that isolate PAN 110 had 99% similarity with S. acrimycini strain K30, PAN 75 (S. luteogriseus strain ZG728), PAN 41 (S. indiaensis), and   PAN 25 (Streptomyces variabilis).
Conclusion: Very dry and wet conditions reduced the number of actinomycetes isolated from the soils of the Menengai crater.