What is the primary purpose of the bile solubility test?

The primary purpose of the bile solubility test is indeed to identify Streptococcus pneumoniae. This test exploits the unique property of S. pneumoniae, which is lysed by bile salts or sodium deoxycholate. When a colony of S. pneumoniae is exposed to bile salts, it will undergo lysis, leading to a clearing around the colony on a solid culture medium. This reaction is a critical characteristic that distinguishes S. pneumoniae from other species of streptococci.

In contrast, testing antibiotic susceptibility involves assessing the effectiveness of antibiotics against a particular organism, which is not the focus of the bile solubility test. Isolating Gram-negative bacteria typically requires different selective media and different testing methods altogether, as the bile solubility test specifically relates to a Gram-positive organism. Differentiating between Streptococcus species generally involves a variety of tests, such as hemolysis patterns and biochemical tests, rather than just the bile solubility test, which is particularly targeted toward S. pneumoniae. Therefore, the most accurate application of the bile solubility test is in the identification of S. pneumoniae.