Studies on the Kinetics of Antibacterial Agent Production in Two Actinomycete Strains, F9 and Is5, Isolated from Soil Samples, Iran

The main purpose of this research was to search various collections of soil samples for isolating antibacterial agent-producing actinomycetes against drug-resistant pathogens and find the relationship between growth and antibacterial agent production in broth culture. Primary screening for antibacterial agent production was done by overlay agar technique. Production kinetics was assessed by minimum inhibitory concentration (MIC) determination using dilution method for 10 days and compared to growth in broth culture. The Hypothetical growth cycle in solid medium was studied using slide culture technique. Among 110 isolated strains, 17% were active against Staphylococcus aureus (ATCC 6538), 17% against Bacillus subtilis (ATCC 12711), 8% against methicillin-resistant Staphylococcus aureus (MRSA), 4.5% against vancomycin-resistant enterococci (VRE), and 0.9% against Escherichia coli (ATCC 1399). Two of the best strains, F9 and Is5, were selected for assessing production kinetics. They showed an irregular growth in liquid medium. Fluctuations are the continuous growth/death processes. Antibacterial agent production reached the highest expression level in the end and middle of the first descending phase in growth curve for strains F9 and Is5, respectively. The best MIC values of strain F9 extract were 0.97, 0.97, 1.37, and 1.1 mg/L against S. aureus, B. subtilis, MRSA, and VRE, while the best MIC value of Is5 extract was 1.92 and 1.67 mg/L against S. aureus and B. subtilis, respectively. No change was seen in growth of E. coli in the presence of nearly 1500 mg/L strain Is5 extract during 10 days. Comparing the 16S rDNA gene sequences of isolates and type strains retrieved from GenBank/EzTaxon showed that strains F9 and Is5 have 99.88 and 95.68% similarity with Streptomyces chryseus and S. anulatus, respectively. 16S rDNA gene sequence of the isolates was deposited in the NCBI GenBank database, under the accession numbers KX417085 and KX417086. Based on the results obtained in the present study, it can be concluded that the separation possibility of the antibacterial agent-producing actinomycetes against Gram-negative and drug resistance bacteria is very low. However, new places can be a promising source for isolation of strains with the ability to produce new, diverse and effective antibacterial agents.