Determination of antibacterial activity of some plant extracts against antibiotic resistant clinical isolates

Acinetobacter baumannii is an aerobic, pleomorphic and immobile Gram negative bacillus. A. baumannii, an opportunistic pathogen, has a high incidence among immunocompromised individuals, especially those who are hospitalized for a long time (> 90 days). The organism causes outbreaks of infections, including bacteremia, pneumonia, meningitis, urinary tract infection, and wound infection. Antimicrobial resistance greatly limits therapeutic options for patients infected with this organism, particularly if the isolates are resistant to the carbapenem class of antimicrobial agents. K. pneumoniae is an opportunistic pathogen that mostly affects people with weakened immune systems and tends to cause nosocomial infections. K. pneumoniae strains have become increasingly resistant to antibiotics. The discovery of new agents is important as therapeutic options for multidrug-resistant Acinetobacter and Klebsiella infections are limited. Plants are known to produce synergistic secondary metabolites that overcome the resistance expressed by bacteria. Therefore, the antimicrobial effect of plant metabolites and their combinations with antibiotics are studied for the discovery of new agents. In this thesis study, cinnamon, thyme, nettle, white tea and rosehip herbs were obtained from the herbalist and methanol extracts were prepared. Antibacterial activity of plant methanol extracts was investigated against antibiotic resistant A. baumannii and K. pneumoniae isolated from Trabzon Fatih State Hospital by liquid microdilution method. The antibacterial activity of methanol extract of nettle against A. baumannii was not observed in the tested concentration range. The MIC values of rosehip, cinnamon, thyme, white tea methanol extracts for A. baumannii were determined as 0.07825 g/mol, 0.015125 g/mol, 0.030625 g/mol, 0.00796875 g/mol, respectively. Antibacterial activity was not observed against K. pneumoniae in the concentration ranges of thyme, nettle, white tea and rosehip methanol extracts. The MIC value of cinnamon methanol extract against K. pneumoniae was found to be 0.0605 g/mol. The in vitro effects of plant methanol extracts showing antibacterial activity and antibiotic combinations were investigated by the checkerboard method. As a result, combinations of imipenem with cinnamon, white tea and rosehip methanol extracts showed additive/indifferent effects for A.baumannii, while the combination of imipenem with methanol extract of thyme showed antagonistic effect. It was observed that the combination of imipenem with cinnamon methanol extract had an antagonistic effect for K. pneumoniae. The combination of cinnamon methanol extract with imipenem showed antagonistic effect for K. pneumoniae and an additive/indifferent effect for A.baumannii. As a result, it was found that plant extracts have antibacterial activity, plant methanol extracts used did not show synergistic effect with imipenem. The potential of epigallocatechin 3-gallate, quercetin, cinnamaldehyde, carvacrol and thymol phytocompounds found in plants with antibacterial effects to be inhibitor candidates for OXA-23 β-lactamase in A. baumannii was evaluated as in silico. Epigallocatechin 3-gallate, quercetin, cinnamaldehyde, carvacrol, thymol, imipenem (substrate), avibactam (inhibitor) were attached to the active site of OXA-23 by molecular docking method. It was determined that epigallocatechin 3-gallate had the closest binding energy to the inhibitor (avibactam) and this phytochemical made four hydrogen bonds with the residues Lys82, Lys216, Arg259, Asp222. It was also observed that this phytochemical had higher binding energy (-5.82 kcal/mol) than imipenem binding energy (-5.40 kcal/ mol). When phytochemicals are compared in terms of binding energies, the highest binding energies are -5.82 kcal/mol, -5.77 kcal/mol, -4.98 kcal/mol, -4.18 kcal/mol -4.00 kcal/mol for epigallocatechin-3-gallate, quercetin, cinnamaldehyde, carvacrol, and thymol, respectively. Results from in silico studies suggested that epigallocatechin 3-gallate, quercetin, cinnamaldehyde, carvacrol, thymol plant compounds could be non-covalent inhibitor candidates for OXA-23 β-lactamase. These results may be supported by experimental studies in the future and may allow the discovery of new inhibitors against OXA-23 β-lactamase.