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Potassium Requirement of Oral Streptococci

Potassium (K+) is considered to be an essential mineral for all living cells, including microorganisms. Unstimulated, mixed human saliva contains approximately 800 Mug K+/ml and about 230 Mug Na+/ml; whereas stimulated, mixed saliva has about 570 ug K+/ml and 570 ,Mg Na+/ml (ALTERMAN and DITrMER (eds), Metabolism, 1968). Most lactic acid bacteria have requirements of up to 1,000 Mug K+/ml of medium for maximal growth (McLEOD and SNELL, J Biol Chem 170: 351-365, 1948). In the present report, the specific requirements for two species of oral streptococci that colonize the dentition were determined. Streptococcus mutans and S sanguis cultures were grown in a chemically defined medium (COWMAN, PERRELLA, and FITZGERALD, Appl Microbiol 27: 86-92, 1974) made K+-deficient by neutralizing and buffering with sodium salts (final concentration of Na+ was 805 ,ug/ml) and to which increasing amounts of potassium chloride (KCl) were added. The final concentration of K+ in the media was verified by flame photometry. Inoculated tubes of medium were incubated under 90% N2-10% CO2 for 16 hours at 35 C. After the third consecutive transfer, the growth density attained by each culture was measured at 600 nm. Stock cultures were maintained in Trypticasea-GlucoFe Broth (COWMAN, PERRELLA, and FITZGERALD, Appl Microbiol 27: 86-92, 1974) that contained approximately 200 Mug K4/ml of medium. In the absence of a sufficient external K+ concentration, S mutans and S sanguis were unable to maintain an adequate level of growth in the defined medium, as shown in the illustration, for a representative number of strains. When as