Dynamics in bacterial cell surface properties assessed by fluorescent stains and confocal laser scanning microscopy

We investigated the dynamics of hydrophobic and hydrophilic properties of the bacterioplankton cell surface in different life stages, as they may play a crucial role in the uptake of nutrients and in regulating the grazing pressure of potential predators. Bacterial strains were grown in artificial seawater under controlled nutrient conditions and the dynamics of surface properties were assessed by staining living bacteria concurrently with a hydrophobic, a polar and a nucleic acid stain. Mean fluorescence intensity of the individual stain for each single cell was determined using a confocal laser scanning microscope and advanced image analysis. No proportional changes of the hydrophobic or hydrophilic properties with cell size were detectable, indicating their independence of cell size. While hydrophobic properties remained fairly constant and exhibited species-specificity, the polarity of the surface was more dependent on the life stage of free-living cells. Bacterial cells assembling in aggregates, however, exhibited a small but distinct elevated hydrophobicity compared to free-living cells. Based on their hydrophobicity, 2 distinctly different groups of bacteria could be distinguished. One group of bacterial strains exhibited a continuous increase in the ratio of hydrophilicity to hydrophobicity, whereas the less hydrophobic group exhibited 2 peaks in the ratio hydrophilicity to hydrophobicity. This ratio was significantly lower for the less hydrophobic group, indicating that less hydrophobic bacteria are also less hydrophilic. Increased growth rates were found with higher cell surface hydrophobicity. This might be interpreted as an expression of 2 different life strategies in bacterioplankton. Increased hydrophobicity and concomitant increase of polar moieties facilitate nutrient assimilation and seem to be advantageous for bacteria adapted to high nutrient environments maximizing growth at higher grazing losses. In contrast, bacteria with lower hydrophobicity exhibit a lower assimilation efficiency with reduced grazing pressure.