A novel image-analysis technique for measurement of bacterial cell surface tension

Cell-surface hydrophobicity is different for Staphylococcus epidermidis cells grown under different environmental conditions; this might influence attachment and colonization of surfaces. Although a wide variety of techniques has been employed to measure bacterial surface hydrophobicity, including contact angle determinations, adherence to hydrocarbons, hydrophobic-interaction chromatography and salt aggregation, many of these either require large numbers of cells or do not yield comparable quantitative data. This study describes a novel, quantitative method for the determination of bacterial surface tension on the basis of image analysis of cell-cell interactions. S. epidermidis (strains 900 and 901) were suspended in different concentrations of propanol of known surface tension and examined by bright-field microscopy linked via a charge-couple device (CCD) camera to an image analyser. Frames were chosen randomly and the data recorded as a ratio of count/percentage coverage for each frame. The results showed that for strains 900 and 901 this ratio was maximum at surface tensions of 67 and 61 mN m(-1) respectively. At these values of minimal interaction the surface tension of the liquid was equal to the bacterial cell surface tension. The results were in close agreement with those obtained from contact angles. The advantage of surface tension measurements is that, irrespective of the method used, the results generated are quantitative values and are therefore directly comparable. The method reported is reliable, reproducible and is of particular value because the number of cells required is, typically, at least two orders of magnitude lower than is required for commonly used alternative methods.