Antibacterial and antibiofilm efficacy of eugenol, carvacrol, and cinnamaldehyde against colistin-resistant Klebsiella pneumoniae.

BACKGROUND

Colistin-resistant Klebsiella pneumoniae (CR K. pneumoniae) is considered one of the bacteria with the highest levels of antibiotic resistance, necessitating the discovery of alternative therapeutic strategies. This study aimed to evaluate the ability of eugenol, carvacrol, and cinnamaldehyde to inhibit CR K. pneumoniae and the biofilm community of this bacterium.

METHOD

The minimum inhibitory concentration (MIC) and antibiofilm effects of compounds were assessed using broth macrodilution and microtiter plate assays, respectively. Time-kill assays determined the bactericidal effects of natural compounds on CR K. pneumoniae isolates. Protein and nucleic acid leakage were examined to assess metabolic disruption and cell membrane integrity. Real-time PCR was used to evaluate the effect of natural compounds on the expression of biofilm-related genes (mrkA, treC, and luxS).

RESULTS

In the disk diffusion test, the inhibition zones of eugenol, carvacrol, and cinnamaldehyde were 15 ± 1, 29 ± 2, and 30 ± 1 mm, respectively. The MIC of eugenol, carvacrol, and cinnamaldehyde were 260, 119, and 128 µg/mL, respectively. The time-kill assay demonstrated the rapid bactericidal effects of eugenol, carvacrol, and cinnamaldehyde at 2× MIC, with kill times of 6, 2, and 3 h, respectively. At the MIC, the kill times were 12, 10, and 12 h, respectively. These compounds significantly released proteins and nucleic acids from the treated bacteria. They also inhibited biofilm formation and disrupted mature biofilms. Furthermore, mrkA and treC expression levels were significantly reduced in the presence of eugenol and cinnamaldehyde.

CONCLUSION

These natural compounds demonstrated significant antibacterial and antibiofilm activity against CR K. pneumoniae, emerging as a promising natural alternative for treatment.