Lysogeny of Streptococcus pneumoniae with MM1 phage: improved adherence and other phenotypic changes

Pneumococcal prophages are extremely frequent, but no role in pathogenesis has so far been attributed to them. We isolated a variant of phage MM1, named MM1-1998, from a serotype 24 strain of Streptococcus pneumoniae. We created three isogenic strain pairs (serotypes 3, 4, and 24) that differed only by the lysogenic presence of the MM1-1998 phage and did a phenotypic comparison. Lysogeny led to improved adherence to inert surfaces and pharyngeal cells compared to that with the cured variants of the strains. We found that lysogeny with MM1-1998 coincided with a more transparent phenotype and phage curing with more opaque colonies in all strain pairs, and we discovered that transparency was associated with more successful and stable lysogeny. Since transparency alone was possibly responsible for the adherence difference, we further compared the TIGR4 lysogen with an equally transparent variant of TIGR4 in order to reassess the role of phage or transparency separately. The results revealed that improved adherence was independently associated with lysogeny with the MM1-1998 phage. Other phenotypic differences such as faster growth, increased autolysis, and decreased intracellular hemolytic activity were more likely due to transparency. By improving the adherence of pneumococci, this prophage may contribute to their fitness and possibly to their persistence in humans.