ISOLATION AND CHARACTERIZATION OF ANTIMICROBIAL METABOLITES PRODUCING BACTERIA FROM SOILS IN AL-AHSA, SAUDI ARABIA

The current study aims to isolate and test microorganisms from soils in Al-Ahsa, Saudi Arabia, for their antibiotic-producing ability. A total of fourteen bacterial isolates were obtained from six locations, including poultry house soil, Al-Asfar Lake soil, Nada dairy products farm soil, the rhizosphere of Prosopis farcta in Ayen Al-Harrah, Circular Road desert soil, and Al-Uqair beach soil. Screening revealed antibacterial activity in six of the fourteen isolates against specific pathogens. Notably, Salmonella enterica was inhibited by isolates NP1 and NP2. Additionally, NY1 inhibited NY2 and NP2, NN1 inhibited NN2, NH1 inhibited NH2, and NC1 inhibited NC2. Morphological and biochemical differences were observed among the isolates. The disc diffusion method indicated a minimum inhibitory concentration (MIC) range of 0.78 – 12.5 µg/μl for the antibiotic-producing isolates. Gas chromatography- mass spectrometry (GC-MS) analysis of these isolates revealed several secondary metabolites from various chemical classes, including cyclohexasiloxane, dodecamethyl-, cycloheptasiloxane, tetracadmethyl, and phenol 2,2'-methylenebis[6-(1,1-dimethylethyl)-4-methyl-], which may inhibit harmful microorganisms. Phylogenetic analysis based on 16S ribosomal ribonucleic acid (16S rRNA) gene sequencing showed that most isolates belonged to the genera Bacillus and Alcaligenes. NP1 exhibited 98.76% sequence similarity to Bacillus subtilis strain HAU427, while NP2 showed 99.65% homology with Bacillus amyloliquefaciens. Isolates NY1, NN1, and NC2 were closely related to Bacillus thuringiensis, whereas NY2, NN2, and NH2 resembled Bacillus pumilus. NH1 was similar to Bacillus cereus (97.41%), and NC1 closely matched Alcaligenes sp.. The ability of these soil-derived bacterial isolates to inhibit multiple bacterial strains at low concentrations suggests their potential as a source of antibacterial agents against antibiotic-resistant pathogens.