Production of Small-Colony Variants of Staphylococcus aureus.

We have previously reported the production of small-colony variants, or G forms, after prolonged serial transfer of S. aureus in lithium chloride broth or plain broth media. 1 , 2 Swingle 3 subsequently confirmed these results and also obtained these forms after extended growth in nutrient broth or casein-digest medium. Under these conditions the variants occur only rarely and in small numbers, seldom exceeding 1.0% of the colonies on a plate. Chinn 4 in a study of the metabolic activity of the small colony variants of S. paradysenterió Sonne and S. aureus found them to be metabolically far less active than the normal forms, having a reduced generation-time, reduced fermentative powers, lessened ability to reduce methylene blue and a slower cataphoretic velocity. This lowered metabolic activity and the ability to revert to forms identical with the parent culture furnishes some evidence that small-colony variants may be forms whose metabolism has been greatly reduced by some unknown factor.

Quastel and Woolridge 5 studying the enzymes of B. coli found that among many inorganic salts barium, chloride was the most effective in inhibiting the metabolic activity of washed suspensions of these organisms. These results, and the known lowered metabolic activity of the small-colony variants, indicated that barium chloride might be effective as an incitant of their production.

Concentrations of barium chloride of 0.5 to 5.0% were made up accurately in 1.0% peptone solution. Since S. aureus produces only slight growth in plain peptone medium the acetone-soluble portion of meat extract containing Hughes' 6 growth factor was added. This was prepared by extracting 50 gm. of meat extract dissolved in 100 cc. distilled water with 6 to 8 volumes of acetone.