After rinsing, the biofilm was soaked in a diluent containing NAC (0, 0.5, 1, 2.5, 5, 10 mg/ml) for 24 h at 37°C. After rinsing with PBS, the samples were examined for the degree of biofilm removal by observation under a confocal laser scanning microscopy (CLSM). To analyze the effects of NAC on biofilms, 2 independent biofilm experiments were performed. From each cover slip, 5 image stacks were acquired at different Rabusertib positions; thus, 10 image stacks were analyzed for each concentration of NAC. Images were acquired at 1 μm
intervals down through the biofilm and, therefore, the number of images in each stack varied according to the thickness of the biofilm. All microscopic observations and image acquisitions used CLSM (Olympus FV1000, Japan). Images were obtained with a 60× objective lens and laser excitation at 488 nm. Z-series of optical sections were reconstructed into three-dimensional images by Olympus FV10-ASM 1.7 Software. Fluorescence intensity in each fixed scanning area was measured. The biofilm structure was quantified from the confocal stacks using the image analysis software package COMSTAT (kindly donated by A. Heydorn, Technical University
of Denmark, Lyngby) . This software can interface with Matlab and utilizes Matlab’s image analysis software toolbox. COMSTAT offers an array of functions and is capable of generating up to 10 different statistical parameters for quantifying the 3-dimensional biofilm structure. For this study, 7 COMSTAT parameters were used to determine the differences between biofilms selleck chemical grown under each of the 5 NAC concentrations. These parameters were biomass, substratum coverage, maximum thickness, average thickness, surface area of biomass, surface to volume ratio and roughness coefficient. Detection of viable cells in biofilms using MTT assay Dimethylthiazol diphenyltetrazolium bromide (MTT) and extraction buffer were prepared as selleck chemicals previously described . In brief, MTT was dissolved at a concentration of 5 mg/ml
in PBS. Extraction buffer was prepared by dissolving 20% (wt/vol) sodium dodecyl sulfate (SDS) at 37°C in a solution of 50% each of N,N-dimethylformamide (DMF) and demineralized water; the pH was adjusted to 4.7. MTT assay. Twenty N-acetylglucosamine-1-phosphate transferase μl of the 5-mg/ml MTT stock solution was added to each well of a 96-well microtiter plate (Costar, USA) containing 190 μl of bacteria. After incubation for 2 h at 37°C, 90 μl of extraction buffer was added to each well. After thorough extraction, optical densities were measured at 595 nm using a microplate reader (Pulang New Technology Corporation, China). MHB (incubated with MTT and extraction buffer) was used as a blank control. The assay was calibrated using series dilutions of P. aeruginosa ATCC 27853 as standards, which had been subjected to the same procedure.