Antibacterial evaluation of methanol extract of Azadirachta indica leaves and its partitioned fractions against multiple antibiotic-resistant oral pathogens

Abstract

A promising solution to multiple antibiotic resistance is the development of more efficient and efficacious antimicrobial agents from the medicinal plant source. This study aimed at evaluating the antibacterial activity of methanol crude extract of Azadirachta indica (Neem tree) leaves and its fractions against multiple antibiotic resistant oral pathogens. Oral pathogens were isolated from the buccal cavity of 100 apparently healthy individuals using standard microbiological procedures. The isolated bacteria were characterized using conventional biochemical tests. Antibiotic susceptibility profiles of the isolates to 12 commonly used antibiotics were determined using the Kirby-Bauer disk diffusion method. The minimum inhibitory concentrations of methanol crude extract, N-Hexane, Dichloromethane and Ethyl acetate partitioned fractions against thirty-one multiple antibiotic-resistant isolates, selected based on their multiple antibiotic resistance index ≥ 0.5, were determined using the broth micro-dilution method. The bacteria used comprised of Escherichia coli (2), Klebsiella pneumoniae (12), K. oxytoca (3), K. planticola (1), K. ornithinolytica (1), Bacillus subtilis (8), B. lentus, Proteus mirabilis (1) and Micrococcus luteus (1). The phytochemical constituents of the extract and its fractions were also determined using standard techniques. Results revealed the presence of 6 genera namely Klebsiella (40%), Bacillus (34%), Staphylococcus (13%), Escherichia (10%), Micrococcus (2%) and Proteus (1%). Majority of the isolates were susceptible to levofloxacin, ciprofloxacin, amikacin and gentamicin. The results showed that ethyl acetate fraction was the most effective against all the isolates with MIC50 and MIC90 of 6.25 and 12.5 mg/mL, respectively and N-Hexane fraction as the least effective with MIC50 and MIC90 of 50 and 100 mg/mL, respectively. However, while Bacillus spp was the most susceptible to ethyl acetate fraction with MIC range of 12.5–0.78 mg/mL among the Gram-positive isolates, Klebsiella spp was the most susceptible with MIC range of 12.5 - 0.58 mg/mL among the Gram-negative bacterial isolates. The study concluded that all the partitioned fractions tested displayed activity at varying concentrations against all the oral pathogens tested with ethyl acetate fraction showing potentially interesting activity and could be useful in the management of multiple antibiotic-resistant oral pathogens.