Cinnamaldehyde: a compound with antimicrobial and synergistic activity against ESBL-producing quinolone-resistant pathogenic Enterobacteriaceae
Usage of cephalosporin and quinolone antibiotics has aggravated the development of extended-spectrum beta-lactamase (ESBL)–producing quinolone-resistant (QR) pathogenic Enterobacteriaceae. The present study aims to determine antimicrobial activity of cinnamaldehyde alone or in combination with cefotaxime/ciprofloxacin to reverse the drug resistance and evaluations of efficacy, and possible molecular mechanism of action of the combination was also evaluated using in vitro assays. Broth microdilution assay was used to determine minimum inhibitory concentrations (MICs) of cinnamaldehyde and antibiotics against ESBL-QR Enterobacteriaceae. Synergistic effect and dynamic interaction with antibiotics were further examined by checkerboard assay, isobologram analysis, and time-kill assay, respectively. Cellular morphology of bacteria was viewed with scanning electron microscopy (SEM). Effects of cinnamaldehyde and its combination on the expression of gene encoding—porins (ompC, ompF, ompK35, and ompK36), efflux pump genes (acrB–E. coli, acrB–K. pneumoniae), and antibiotic-resistant genes (blaTEM, blaSHV, blaCTXM, and QnrB) were evaluated using real-time quantitative PCR (RT-qPCR). Majority of the E. coli (32.1%) and K. pneumoniae (24.2%) isolates demonstrated MIC of cinnamaldehyde at 7.34 μg/mL and 0.91 g/mL, respectively. Synergism between cinnamaldehyde and cefotaxime was noted among 75% E. coli and 60.6% K. pneumoniae. Similarly, synergism with ciprofloxacin was observed among 39.6% and 42.4% of the bacteria, respectively. Thus, cinnamaldehyde reduced MIC of cefotaxime and ciprofloxacin 2–1024-fold with bactericidal and/synergistic effect after 24 h. Cinnamaldehyde and its combination altered gene expression by ~ 1.6 to ~ 400-fold. Distorted bacterial cell structures were visible after treatment with cinnamaldehyde and/or with cefotaxime/ciprofloxacin. The results indicated the potential efficacy and mode of action of cinnamaldehyde alone and in combination with antibiotics against pathogenic ESBL-QR bacteria.
KeywordsCinnamaldehyde ESBL Quinolone Synergy Scanning electron microscopy Real-time quantitative PCR
The authors are extremely grateful to the Director, Calcutta School of Tropical Medicine, Kolkata, India, for providing necessary facilities for this study.
This study is funded by the Indian Council of Medical Research (Grant No. -58/67/BMS-2012).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
Ethical statement and informed consent
The study was approved by the ethical research committee (reference number: CREC-STM/53 dated 23/09/2011). Informed consent was obtained from the patients for participating in the study.
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