Bioassay and Antibiotic Activity of Jamaican Actinomycetes Isolates
The growth in antibiotic resistance has resulted in a constant need for novel antimicrobials. To slow down the acquisition of resistance many pharmaceutical companies are now focusing on the development of novel, narrow-spectrum therapeutics with specificity, potency and decreased side effects. Antibiotic production is a feature of several soil microbes and may represent a survival mechanism whereby organisms can eliminate competition and colonize a niche. Since many of the original antibiotics were harvested from tropical soil Actinomycete species, we collected soil samples from Jamaica as the island’s soil microbiology is rich and poorly understood. From sixteen sites twenty- nine colonies of Actinomyces and Bacillus spp. were obtained and twenty isolates were identified to have antibiotic activity against a panel of clinically important human bacterial pathogens. Eight were effective against one or two of the test bank of microbes and Actinomyces were selected for further study. Antimicrobial activity was assessed in methylene chloride:methanol (1:1) extracts of 1 L broth cultures using a 96-well microdilution assay against Escherichia coli (ATCC 25922), Mycobacterium smegmatis (ATCC 14468; model for M. tuberculosis), Pseudomonas aeruginosa (ATCC 27853), methicillin-resistant Staphylococcus aureus (ATCC 43300), Candida albicans (ATCC 90028), and Cryptococcus albidus (ATCC 34140; model for C. neoformans). Three of the five isolates produced activity which was highly effective against M. smegmatis with growth inhibition comparable to 3.2 µg/ml of the antibiotic isoniazid). Two isolates inhibited the yeast C. albicans to degree similar to amphotericin B at a concentration of 0.25 µg/mL. All three isolates tested for cytotoxicity using the brine shrimp assay were found to be non-lethal. The bacterial source of these compounds with antimicrobial activity were identified commercially as Actinomyces using PCR analysis, but had no homology to known species by BLAST analysis. Given their activity against M. smegmatis these isolates may have potential for future drug development.