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Bacterial Water Relations Using Polyethylene Glycol 4000¹

ABSTRACT

The effect of polyethylene glycol (PEG) 4000 on the growth of Arthrobacter crystallopoietes and Escherichia coli was evaluated in succinate-limited batch culture. Arthrobacter crystallopoietes was little affected by PEG down to –1.5 MPa, and showed optima in both growth rate and cell nitrogen yield around –1.0 MPa, consistent with loss of turgor as the passive mechanistic response of this organism to matric water potential stress. The apparent growth rate (difference between the growth and death rates) of E. coli decreased 70% from 0 to –0.75 MPa, with extinction at –0.8 MPa compared to extinction at –4.0 MPa under salt stress. A relatively constant level of dissimilatory CO₂ production independent of PEG (inconsistent with a substantially increased energy requirement for growth under PEG stress), together with microscopic and nitrogen balance evidence of increasing cell death and lysis under increasing PEG stress, indicated that accelerated cell death was a major factor contributing to the reduction in growth rate of E. coli caused by decreasing PEG matric water potential. Volume changes in the cell wall under PEG stress are postulated as interfering with turgor pressure modulation and maintenance of cell integrity by E. coli.

To the extent that PEG is acting purely as an impermeant osmoticum, then over the 0- to –1.5-MPa range, any effect of soil matric water potential on the growth of A. crystallopoietes may be attributed to a solute-diffusion problem. However, reductions in the soil matric water potential in the < –0.25-MPa range are predicted to severely repress the growth of E. coli, with growth extinction at about –0.8 MPa, independent of water content-solute transport considerations.

¹ This research was supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison (Project WIS 02192), the Agricultural Experimental Station, University of Arizona-Tucson, and a grant from the National Science Foundation (BSM 75-18582).

² Research Assistant and Professor, Dep. of Soil Science and Bacteriology, Univ. of Wisconsin-Madison; and Head, Dep. of Soils, Water and Engineering, Univ. of Arizona. The permanent address of the Senior Author is Ruakura Agricultural Research Centre, Private Bag, Hamilton, New Zealand.

Received for publication September 15, 1981. Accepted for publication December 30, 1981.

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