Improved method for colony counts of the microaerophile Spirillum volutans

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Abstract
Studies of adaptive responses of Spirillum volutans to various stresses such as heat shock have been hampered by an inability to obtain reliable colony counts of the organism by the spread plate method, due in part to differences among various lots of the casein hydrolysate component of the medium. Colony counts approaching direct microscopic counts (DMCs) were obtained by inoculating culture dilutions into a semisolid version of the medium and using this to overlay a thicker layer of sterile medium, and by supplementing the medium with pyruvate, which destroys hydrogen peroxide. Both the pyruvate and the overlay were necessary for optimal results. Use of the new overlay–pyruvate method for colony counts revealed that exposure of S. volutans to 40 °C for 100 min results in a greater survival at 45 °C compared with cells having no prior exposure to 40 °C.Key words: pyruvate, microaerophile, Spirillum volutans, heat shock.