Abstract
Planktonic cells and those grown on surfaces (or as colony biofilm) are known to show significant differences regarding growth behavior, cell physiology, gene expression and stress tolerance. In order to compare stress behavior of different growth forms, shake cultures for planktonic growth and agar plate cultivation for colony growth, were carried out with the well investigated model organism, Pseudomonas putida mt-2. Cells were exposed to sodium chloride to cause osmotic stress as one main environmental stressor bacteria have to cope with when growing in soil. Planktonic cells were more tolerant with a complete inhibition of growth at 0.7 M NaCl, compared to 0.5 M for agar-grown cells. Cell surface hydrophobicity, measured as water contact angles, was significantly higher for agar-grown cells (92°) than for planktonic cells (40°), and increased in the presence of NaCl. Agar-grown cells also showed a significantly higher degree of saturation of membrane fatty acids that increased in the presence of NaCl. These results demonstrate that planktonic and colony grown bacteria show different responses when confronted with osmotic stress suggesting that the tolerance and adaptive mechanisms are dependent on the environmental conditions as well as the initial physiological state.
Highlights
Planktonic cells and those grown in form of microcolonies or biofilms show significant differences regarding growth behavior, cell physiology, gene expression and stress tolerance
With the experimental approach used in our studies we could show that planktonic and colony grown cells of the well-investigated strain P. putida mt-2 show different responses when confronted with osmotic stress which suggests that the tolerance is linked to the environmental conditions and the initial physiological state
Despite about 3.5 times lower growth rates of agar grown cells, a dose depended growth inhibition was observed for both forms of growth with identical effective concentration 50% (EC50) values but different minimal inhibitory concentration (MIC) values for sodium chloride
Summary
Planktonic cells and those grown in form of microcolonies or biofilms show significant differences regarding growth behavior, cell physiology, gene expression and stress tolerance. Physiological studies and genome analyses highlight the impressive potential of stress adaptation and morphological or phenotypic variety of bacteria. Phenotypic variations of bacteria lead to enormous differences in cell physiology and biochemistry as well as in tolerance towards environmental stresses Regular stress tolerance studies with bacteria test the adaptive potential under ideal planktonic growth conditions (Unell et al 2007; de Carvalho et al 2014). Except for some very specific studies on pathogenic bacteria (Garcia et al 2013) our knowledge of bacterial adaptation to environmental stressors when cultivated in different growth conditions is still only partial
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