What is the primary characteristic of S. pneumoniae when subjected to the bile solubility test?

The primary characteristic of Streptococcus pneumoniae when subjected to the bile solubility test is the lysis of colonies. This test exploits the unique feature of S. pneumoniae to be lysed by bile salts or other surfactants, which leads to the breakdown of the bacterial cell wall. When S. pneumoniae is exposed to bile salts, the normally smooth and mucoid colonies become less prominent or completely disappear, indicating lysis.

This characteristic is particularly useful for differentiating S. pneumoniae from other viridans streptococci, which do not exhibit lysis in the presence of bile. The bile solubility test is based on the fact that S. pneumoniae has a specific enzyme, autolysin, that becomes more active in the presence of bile salts, leading to cell lysis and the disappearance of colonies on the agar plate.

In contrast, other choices do not correctly describe the behavior of S. pneumoniae in this test; for instance, the inhibition of growth or no effect on colonies would be indicative of non-lytic bacteria. Therefore, the ability for S. pneumoniae to undergo lysis in the presence of