Evaluation of the Antimicrobial Activity of cellulolytic Bacillus spp. Isolated from Rice Paddy Fields in the Philippines

Abstract

Drug-resistant strains are becoming a public health problem as antibiotic-resistant diseases become more prevalent in clinical settings throughout the world. Multidrug-resistant microorganisms are constantly adapting to the effects of currently produced antibiotics by inactivating them via a variety of mechanisms. To address this issue, microbial natural sources isolated from various settings are being used to meet the demand for new antimicrobials. On this study, Bacillus species were isolated from rice field soils and their enzyme production and antibacterial activity were determined using plate techniques and agar-well diffusion assays. The results revealed that all isolates synthesized cellulase, with strain BI-AI.1 exhibiting the largest hydrolysis zone of 2.07±1.07 mm. Bacillus cell-free supernatant (CFS) from strains NE-AIII.10, BI-BI.7, and BI-CI.2 also demonstrated antimicrobial activity against Enterococcus faecalis, Micrococcus luteus, and Candida albicans. Only NE-AIII.10 and BI-BI.7 inhibited Gram-negative Klebsiella pneumoniae (8.25±0.12 mm) and Aeromonas hydrophila (2.91±0.72 mm) with p-values of 0.00002 and 0.00006, respectively. Interestingly, Bacillus BI.CI.2 inhibited methicillin-resistant Staphylococcus aureus (MRSA) with a zone of inhibition (ZOI) of 9.83±0.44 mm (p = 0.0005). Bacillus CFS Minimum Inhibitory Concentration (MIC) assays revealed the inhibition of test pathogens at varied concentration ratios. Molecular identification of the top strains NE-AIII-10 and BI-BI.7 using 16s rRNA gene sequence analysis showed similarities of the isolates to Bacillus subtilis. The findings established the presence of cellulolytic Bacillus species in the soils of Philippine rice paddy fields. Additional investigation of the Bacillus CFS is required to confirm the presence of the active components responsible for the inhibitory effects against test pathogens.