Materials and methods.
The effect of manuka honey on the structure of Pseudomonas aeruginosa.
Materials and methods.
General A culture of Ps. aeruginosa ATCC 27853 was tested with a sample of manuka honey (M109) that was a gift from Prof. Molan of the University of Waikato, New Zealand. Antibacterial potency of the sample was equivalent to 18% (wlv) phenol using a bioassay developed in New Zealand [15]. Minimum inhibitory concentrations and minimum bactericidal concentrations The minimum inhibitory concentration (MIC) was determined in 96-well, flat bottomed microtitre plates (Nuno, Roskilde, Denmark) using double strength nutrient broth (oxoid,of 200 HL overnight broth cultures of the test organism (1 was used as an inocu without dilution, and total viable cell counts were performed to check retrospectively that each well had received approximately 100 cells. Plates were incubated at 37 C for 24 hours and turbidity measured at 400 nm in a plate reader (Anthos btec Instruments) Positive (broth and inoculum) and negative (broth and honey) controls were included Wells with the lowest concentration without growth were recorded as MIC. MBC was determined by plating 20 HL from wells without growth onto nutrient agar (oxoid. Basingstoke, UK) and incubating at 37 c for 24 hours to find the lowest concentration without viable bacteria. Experiments were performed in duplicate on each of three occasions.
Time-kill curve
A time-kill study was performed by inoculating 40 uL of an overnight culture of Ps. aeruginosa ATCC 27853 into 20 mL nutrient broth with and without 20% (wlv) M109 and incubating at 37°c for 24 hours in a shaking water bath (120 cycles min (the honey concentration was approximately twice the MIC value). Samples were removed at known intervals and Miles and Misra surface drop counts were performed by serial decimal dilution in quarter-strength Ringers solution, plating onto nutrient agar and incubating at 37°C for 24 hours
Electron microscopy
Electron microscopy was performed using the test organism in either the exponential or stationary phase of growth following cultivation in isosensitest broth (oxoid, Basingstoke, UK at 37ec in a shaking water bath for either 3 hours or overnight, respectively. Cells were harvested by centrifugation at 3000 g for 30 minutes (MSE harrier 15/80 centrifuge, Sanyo) at room temperature and suspended in MoPs buffer (pH 7.2) with and without 20% (wlv) manuka honey for 8 hours, or in MoPS buffer containing 20% (wlv) artificial
honey solution to determine the effect of sugars in honey in ce structure (Cooper, Halas & Molan, 2002: Cooper, Molan & Harding, 2002). cells were examined in scanning (SEM) (520OLv Jeol, Herts, Uky and transmission electron microscopy mEM) (1210 Jeol. H uk by the method of Lemar, Turner & Lloyd 16), except that harvested cell pellets for TEM were embedded in Araldite resin. not Spurr.
Analysis of images
Electron micrographs of untreated and treated cells were examined to identify structural changes such as altered shape, modified surface layers, the presence of electron dense material, and cellular debris. Typically at least six photographs, each with approximately 160 cells were observed, so that more than 1000 cells were counted in total for each sample. Data was analysed for statistically significant differences by the Mann-Whitney test using Minitab (version 15)
Results.
Inhibition studies MIC and MBC were found to be 9.5 and 12 (wlv) manuka honey, respectively. The close proximity of these two values indicates a bactericidal mode of inhibition. This was confirmed by time-kill studies (Fig. 1) where cells exposed to manuka honey were found to lose viability with time yet numbers of untreated cells increased. The time estimated to achieve a 5 log reduction of test organism incubated with nutrient broth containing 20% (wlv) manuka honey was 257 minutes.
Structural studies
The effect of manuka oney on cell structure was investigated in both exponential and stationary phase cultures because stationary phase cells are often less susceptible to antimicrobial agents than exponential cells. However the structural changes observed in both of these stages of growth were similar and therefore only electron micrographs of exponential cells are presented here. Using scanning electron microscopy the smooth surface layers of untreated cells (Fig 2a) and cells exposed to 20% (w/v) artificial honey (Fig 2b) contrasted with those of honey treated Ps aeruginosa cells, which exhibited marked cell surface changes as furrows and blebs (Fig. 2c). Honey-treated cells also appeared to be shortened and to have distorted shapes (Fig. 2c). In untreated samples 2% of cells were found to have structural irregularities, whereas 80 and 60 cells of exponential and stationary cultures, respectively exhibited irregular cell structure. These differences were statistically significant Table 1). For exponential phase cells exposed to 20% (wlv) artificial honey, 7% cells were found to exhibit structural irregularities. This suggests that the effect of manuka honey on Ps aeruginosa is not due exclusively to the sugars contained in honey. Using TEM, untreated cells (Fig. 3a) and cells incubated in MoPs containing 20% (wlv) artificial honey (Fig. 3b) were entire cells with relatively densely stained contents. In TEM images of honey-treated P aeruginosa (Fig. 3c) cellular debris was clearfy evident and whole cells with evacuated areas were observed.
