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Survival of Pseudomonas fluorescens 2-79RN₁₀ in Clay Powders Undergoing Drying

Soil Science Division, Univ. of Idaho, Moscow, ID 83844-2339

*Corresponding author (mmorra{at}uidaho.edu).

ABSTRACT

Although clay minerals differentially alter bacterial survival during desiccation, the mechanism responsible is speculative. Our objective was to determine how clay water content might control the enhanced survival of Pseudomonas fluorescens (Migula) 2-79RN₁₀ in the presence of various clays. Pseudomonas fluorescens 2-79RN₁₀ was suspended in phosphate buffer, mixed into Ca²⁺-saturated pyrophyllite, kaolinite, clinoptilolite, and montmorillonite (0.5 mL of cell suspension g^–1 clay), and air dried at 20°C. Bacterial colony-forming units (CFU), gravimetric water, and water activity (a_w) were determined when mixed and at 9, 14, 19, 28, and 48 h after mixing. Bacterial survival was greatest in air-dried montmorillonite and lowest in kaolinite. The rate of change in a_w, final a_w values, and rate of gravimetric water loss were similar for all clays. However, a_w values during the initial stage of drying varied because the same cell-suspension volume was amended to equivalent weights of clays possessing different surface areas. The length of time prior to decreases in a_w varied in the order: kaolinite > clinoptilolite > pyrophyllite > montmorillonite, a sequence inversely proportional to survival. Alteration of clay particle size and amendment with different cell-suspension volumes showed that increased bacterial survival could be achieved by shortening the time a_w remained elevated. Increased survival of 2-79RN₁₀ is thus promoted by cell-clay interactions resulting from more rapid decreases in a_w values during the initial stages of drying. Our results have relevance to bacterial survival in clay formulations and soil.

Received for publication July 31, 1997.