Abstract
Serratia marcescens is an important pathogen with noteworthy hydrophobicity characteristics as assessed by microbial adhesion to hydrocarbons. However, the present knowledge on the surface characteristics of S. marcescens strains does not include physicochemical properties relevant for adhesion such as surface free energy and zeta potential. Also, little attention has been paid hitherto to the structural features and chemical composition of the cell surface. Therefore, as a primary aim of this paper, we characterized S. marcescens strains by means of contact angle and zeta potential measurements, X-ray photoelectron spectroscopy, and infrared spectroscopy. In addition, transmission electron microscopy on negatively stained (methylamine tungstate) and ruthenium red stained cells was employed to study structural features on the cell surface. Furthermore, as a secondary aim of this paper, the power of the various techniques to discriminate between strains was evaluated. Negative staining showed that S. marcescens RZ almost completely lost its surface fibrils upon increasing the growth temperature from 30 to 37 °C. This loss of surface fibrils was accompanied by a decrease in hydrophobicity, as measured by water contact angles on bacterial lawns. No significant differences in hexadecane contact angles were observed. Zeta potentials were only different for S. marcescens 3164, showing a considerably higher isoelectric point (IEP = 3.9) than the other strains involved (IEP about 2.5). X-ray photoelectron spectroscopy yielded differences in O/C, N/C, and P/C surface concentration ratios, which related with the IEPs of the strains, despite the fact that X-ray photoelectron spectroscopy is done on fully dehydrated cells, whereas zeta potentials are measured on cells in their physiological state. Infrared spectroscopy was not sufficiently surface sensitive to discriminate between these strains. N/C surface concentration ratios by X-ray photoelectron spectroscopy, which probes approximately 5 nm deep from the surface, were slightly higher for the pigmented, prodigiosin-containing strains RZ30 and 3164, although the presence of prodigiosin did not influence the cell surface hydrophobicity. Thus the prodigiosin is probably confined in deeper layers than probed by contact angles (approximately 0.3–0.5 nm). Key words: Serratia marcescens, fibrils, surface properties, hydrophobicity, zeta potential.