Materials
P. aeruginosastrain was obtained from IMTech, Chandigarh. Whatman filter papers were obtained from Whatman International Ltd Maidstone England. LB Broth, LB Agar and serine was procured from HiMedia Laboratories Pvt. Ltd. Honey (Dabur Ltd, India ) was purchased locally. Safranin dye was procured from CDH. The water used in this study was double distilled water.
Preparation of Serine and Honey strips
The Whatman filter paper was cut into strips (60 mm X 10 mm). Serine solution (1mM) was prepared with autoclaved distilled water and sterilized using a syringe filter (0.22µm nylon membrane). The solution was subsequently stored in flat bottom vials. The autoclaved strip was loaded with 2ml of serine solution. The strip, after an incubation of 1 hour, was removed and air dried in sterile conditions. Serine strips (SS) thus prepared, were used in further experiments. Sterilized strips were similarly used for the preparation of honey strips (HS). About 2 ml of sterilized stock solution (25% honey V/V) was prepared and paper strips were likewise incubated for an hour to generate HS. Control strips (C) were also designed containing SS on one end and water imbibed on another end.
HS and SS prepared were annexed to each other via biologically friendly glue to attain a serine-honey strip (SHS) containing an attractant on one end and an inhibitor on another end(Dasgupta et al., 2016).
Preparation of artificially spiked water
Pseudomonas was grown in nutrient broth overnight at 37ºC with proper aeration in the rotating incubator. The overnight culture was subsequently inoculated in 25 ml fresh broth and OD600 was monitored till 0.123 corresponding to 3 x 108 CFU/ml according to the McFarland scale. The culture was centrifuged at 8000g for 15 min and the cell pellet was resuspended in 25 ml distilled filtered water (spiked water). This spiked water was hereafter used for testing the efficacy of honeytrap for removal of Pseudomonal cells.
Chemoattractant potential of SS
To assess the trapping mechanism ofP. aeruginosacells by SHS, glass slides loaded with 200 µl of ‘spiked’ water were horizontally exposed to the SHS (Serine end) for different incubation periods (15min, 30 min, 45min, 1hr, 1.5hr, 2hr). Bacterial chemotaxis was monitored intermittently via microscope (Magnus Opto Systems India Pvt. Ltd.) at various time durations. Subsequently, strips were placed on nutrient agar to assess the possible adsorption of P. aeruginosaby honeytrap and incubated overnight at 37 ºC.
Trapping efficacy of honeytrap
‘Spiked’ water sample (2ml) containing P. aeruginosawas taken and approximately 1 cm2 of honeytrap was dipped into the vials at the serine side. Vials were exposed to different incubation times (15min, 30 min, 45min, 1hr, 1.5hr, 2hr) with SHS and absorbance at 600nm was monitored. The change in OD was attributed to the fact that there was a change in the bacterial concentration. Spectroscopic monitoring for the residual bacterial cells (Bt) enabled calculation of the percentage removal of bacterial cells after each incubation interval using the following formula:-
Percentage removal = B0 – Bt/ B0 *100
where B0 = number of viable microorganisms before treatment
The logarithmic scale is normally used for convenient calculation of cell number alteration mainly due to some form of antimicrobial treatment(Lee et al., 2014; Müller et al., 2011).Interestingly, the log reductions are converted to percent reduction which allows us to compare and correlate the efficiency of the treatment. Log reduction was calculated as follows:-
Log reduction = log10 (B0) – log10 (B2hr)
where B2hr = number of viable microorganisms after treatment
Electrical conductivity has been considered as a criterion to measure the concentration of bacteria in suspension (HARRIS and PATTY, 1949).Hence, serine as a trapping constituent of P. aeruginosawas assessed by measuring the change in electrical conductivity of the suspension post incubation periods. Briefly, 2 ml ‘spiked’ water samples were incubated with honeytrap and change in electrical conductivity of the culture was monitored using a digital 3-in-1 TDS/EC/Temp meter (Labcare,India) post incubation periods. The alteration in the electrical conductivity of suspension was plotted against the incubation time and correlation studied with the change in percentage of bacterial cells.
Effect of HS on adsorbed Pseudomonas
Two sets of 2 ml ‘spiked’ water samples were taken and exposed to control strips and honeytrap respectively and incubated for different time periods. Subsequently, strips were incubated overnight at 37ºC on nutrient agar to assess the possible adsorption and inhibition of P. aeruginosa by control and SHS strips.
Imaging and characterization
The functionality of serine as an attractant was confirmed using an optical microscope (40X magnification). 200µl of overnight bacteria culture was placed on the glass slide with a honeytrap placed at one end of the glass slide. Bacterial adsorption on the device was observed via Field emission scanning electron microscopy (Fe-SEM) (Nova Nano FE-SEM 450) after exposing the strip (1cm2) to vacuum gold sputter coater and the images were observed with 5.00 kV accelerating voltage of electrons and at a magnification of 10,000X.