In the current study A. hydrophila was isolated from the well water near to Kozhikode, Kerala. Isolation was followed by biochemical characterization and molecular identification by using 16s rRNa sequencing methods. On the basis of 16s rRNA it had been confirmed that the isolate was A. hydrophila and it had shown 97.6% similarity with closely related species (Fig. 1). Hence, aligned sequence similarity with isolate depicted that the isolated bacteria was novel strain and have had 97.6% similarity with A. hydrophila. Sequence had been submitted to genbank and submitted sequence no. is MW590617.
Antibiotic resistant pattern and pathogenic characterization of A. hydrohila:
Antibiotic susceptibility test was performed to determine the resistant profile of A. hydrophila by using cefoxitin(CXT), erythromycin(ERT), Teicoplanin(TEI), Clindamycin(CLD), Linzolid(LZD), Azythromycin(AZM), cefpirome(CFP), ceftazidime(CAZ), imipenem(IPM), ceftriaxone(CTR) piperacillin (PIT), cefetaxime(CTX), colistin(CLS), and cefepime(CPM) antibiotics. A. hydrophila was resistant to cefoxitin, tiecoplanin, clindamycin, linzolid, imipenem, ceftriaxone, piperacillin, cefetaxime, and colistine, while sensitive to erythromycin, azythromycin, cefpiron, ceftazidime and cefepime (table.1). The antibiotic resistant profile of bacteria illustrated that A. hydrophila was a multiple drug resistant bacteria. Further, their pathogenic characteristics were explored by using congo red binding assay, blood hemolytic test and ability to form biofilm.
Table-1 Characterization of A. hydrophila for identification of antibiotic resistance profile and pathogenic characteristics
Name of bacteria
|
Antibiotic resistance profile
|
Congo red Binding assay
|
Blood Agar test
|
Biofilm formation by CV assay
|
A. hydrophila
|
CXTR ERTS TEIR CLDR LZDR AZMS CFPS CAZS IPMR CTRR PITR CTXR CLSR CPMS
|
+++
|
+++
|
± 0.732
|
Note− R= resistant, S= sensitive, +=positive, − = negative |
Congo red binding assay was performed to determine the presence of curli (amyloid) fiber on the bacteria surface. Herein, the dark red colony was observed that is basically indicating the presence of curli fibers (table.1). Bacteria, generally utilizing this accessory structure for the biofilm formation. Furthermore, blood agar test was performed to determine the presence or absence of beta hemolytic activity in isolated A. hydrophila, formation of clear zone around the streak zone on blood agar plate clearly indicated that isolated strain had beta hemolytic activity (table.1). Adding to this, biofilm forming efficiency was examined by using crystal violet assay and it was observed that isolated A. hydrophila is a high biofilm forming bacteria. By determining these pathogenic characteristics, it had been confirmed that the isolated A. hydrophila was a pathogenic bacteria with multiple drug resistant characteristic.
Minimum inhibitory concentration of eucalyptus, peppermint and cinnamon essential oils against A. hydrophila:
To determine the minimum inhibitory concentration (MIC) of essential oils (eucalyptus, peppermint and cinnamon), we had used seven different concentrations (0.1 to 0.7%) against A. hydrophila by using well agar diffusion method. After analyzing the inhibition zone of each concentration of essential oils, it had been clear that the MIC of cinnamon essential was 0.3%, while peppermint had shown inhibition activity at 0.5%, and eucalyptus at 0.7% (Fig. 2). Herein, cinnamon essential had shown highest killing activity as compare to peppermint and eucalyptus essential oil. In addition to agar well diffusion test, vapor phase test was also performed with o.7% concentration only, since essential oils are enriched with volatile substances as well, hence to examine the volatile properties of used essential oils, vapor phase test was performed. The vapor phase test indicated that cinnamon and peppermint have had given high content of volatile substances as compare to eucalyptus (Fig. 2). On the basis of vapor phase experiment, it had been confirmed that cinnamon and peppermint essential oil were enriched with volatile substances. Hence, further study was conducted only on cinnamon essential oil.
Determination of bioactive functional compound in cinnamon essential oil:
FTIR spectra of Fig. 3 depicted about the presence of functional group of cinnamon essential oil. Graph had shown two major peaks in between 300–1200 cm− 1 and 1700 − 1600 cm− 1. As per Li et al. 2016, absorbance at 1666.90 cm− depicting about stretching of aldehyde group that indicating the presence of cinnamaldehyde. However, the absorbance peak at 1271 cm− 1 corresponds to presence of phenol group which is indicating about the presence of eugenol.
For further identification of main bioactive component of cinnamon essential oil, HPLC was performed by using cinnamaldehyde as a standard. In HPLC of cinnamon essential oil, there was a peak observed in chromatograph that was corresponding to cinnamaldehyde at same retention time of standard cinnamaldehyde (Fig. 4). It confirmed about the presence of cinnamaldehyde in cinnamon essential oil.
Exploration of killing mechanism of cinnamaldehyde for A. hydrophila:
Bactericidal effect of cinnamon essential oil was performed by determining the colony forming unit after the treatment of bacteria with 0.5, 0.6 and 0.7% of cinnamon essential for 5 hrs. At every 1 hr, total viable count was performed for 5 hrs. There was 4.6 log10 cfu/ml was observed after 1 hr with 0.5 % treatment of cinnamon essential oil. Similarly, there was significant reduction in cfu observed on increasing the concentration of cinnamon essential oil along with increasing the exposure time (Fig. 5). There was no colony observed at 5hrs, hence, the results indicated that cinnamon essential oil has potential to kill the cells within 5hrs.
Cell wall integrity test:
Effect of cinnamon essential oil on cell wall integrity of bacteria was determined by measuring the concentration of nucleic acid in cell suspension. In Fig. 6 the graph depicted about the graduation increase in concentration of DNA in supernatant on increasing the incubation time with cinnamon essential oil. Herein, we had used 0.7% of cinnamon essential oil. The result indicated about the leakage of cell on exposure to cinnamon essential oil.
Electrical conductivity test:
Electrical conductivity test was performed to measure the concentrations of electrolytes leaked from the cell membrane. Herein, we had used 0.7% essential oil treatment for 5hrs and we observed that on gradually increasing the incubation time the electrical conductivity was also increased (Fig. 6). The result indicated that cinnamon essential targeting the cell membrane.
Effect of essential oil on motility of A. hydrophila (By examine motility on slide: A new concept):
Herein, the effect of cinnamon essential was examined against A. aeromonas motility (swimming). Here, concentration dependent inhibition activity of cinnamon essential was examined against the swimming motility of A. hydrophila. We had designed the experiment to performed on slide, despite of using petridishes, it is new concept as well(Fig. 7). After observing the motility results, it was cleared that swimming motility was affected by the cinnamon oil; however at 0.7% the motility was completely inhibited. In addition to this, twitching motility was also performed (result has not shown), and there was no such significant effect was observed. Furthermore, the molecular docking was also performed to see the impact at molecular level and it has been found that the cinnamaldehyde has potential to bind with flagellar gene (flgH), which is responsible for the motility in A. hydrophila (table.2). Cinnamaldehyde interacted with ARG and LEU amino acid of flagellar protein. Herein, we had performed molecular docking by using cinnamaldehyde against flgn, flij and flha. These are the gene responsible for the motility of bacteria.
Cytotoxic test:
In addition to this, the cytotoxicity of cinnamon essential oil was determined by using MTT test. MTT is a redox dye. It is yellow in color, however, the metabolically active cells have tendency turn the color from yellow to purple while metabolically inactive cells it is remain yellow in color. Herein, when 0.5, 0.6 and 0.7% concentrations of cinnamon essential oil applied on the A. hydrophila, than it has been observed that cells without cinnamon essential turned yellow color to purple while treated cells were remain yellow, the result indicated that at all the three concentration of cinnamon essential oil; there were no live cells (Fig. 8).
Table.2 Molecular Docking:
Compound
|
|
Cinnamaldehyde
|
Docking Score(DS)
(kcal.mol-1)
|
Interacted amino acids
(≤5Å)
|
Residue
chain
|
Image of ligand protein interaction
|
|
Flgn
|
|
-3.7
|
ARG
LEU
LEU
LEU
LYS
|
89B
110A
110A
110A
114A
|
|
Flij
|
-4.8
|
TYR
LEU
TYR
|
49A
53A
69A
|
|
Flha
|
-4.8
|
PRO
PRO
ALA
GLN
|
550A
550A
585A
589A
|
|