A rapid bile solubility test for pneumococci.

The observation that pneumococci when grown in an anaerobic atmosphere with 10% CO2 produce characteristically large colonies (Howden, 1976) called for a simple rapid confirmatory test; in this way a precise report could be issued on the morning of isolation. Hawn and Beebe (1965) described a solubility test for aerobic pneumococcal colonies using a sodium deoxycholate solution (pH 70). Aerobic colonies rarely exceed 1 mm in diameter; consequently, we found the results with this method difficult to interpret. With the large anaerobic colonies, however, that problem did not occur. The size of the colony permitted the careful placing of a loopful of bile in such a way that one part of the colony could be seen to dissolve, the other part serving as a negative control. It was found that a 2% solution of sodium deoxycholate (pH 8-2) (SD) gave the most rapid reaction with the minimal effect on the blood in the culture plate. After several comparisons the following proved to be the optimum method for routine cultures: (1) Samples were inoculated on to horse blood Columbia agar and incubated overnight at 370C in an anaerobic environment containing 10% added carbon dioxide (Fig. 1). (2) A loopful of bile salt (SD) was placed on the suspected pneumococcal colonies, and the plate was returned to the incubator. (3) After 15 minutes, the dissolution of treated colonies compared with the adjacent ones could be clearly distinguished (Fig. 2). (4) If, after a second period of 15 minutes' incubation, the colony had not dissolved, the test was considered to be negative. Initially, results were confirmed by the optochin sensitivity and standard bile solubility methods