GSE103364
Streptococcus pneumoniae TIGR4 phase-locked opacity variants differ in virulence phenotypes
Streptococcus pneumoniae (pneumococcus) is a leading human pathogen that can cause serious localized and invasive diseases. Pneumococci can undergo a spontaneous and reversible phase variation that is reflected in colony opacity, which allows the population to adapt to different host environments. Generally, transparent variants are adapted for nasopharyngeal colonization whereas opaque variants are associated with invasive disease. In recent work, colony phase variation was shown to occur by means of recombination events to generate multiple alleles of the hsdS targeting domain of a DNA methylase complex, which mediates epigenetic changes in gene expression. A panel of isogenic strains were created in the well-studied S. pneumoniae TIGR4 background that are “locked” in the transparent (n=4) or opaque (n=2) colony phenotype. The strains had significant differences in colony size which were stable over multiple passages in vitro and in vivo. While there were no significant differences in adherence for the phase-locked mutant strains to immortalized epithelial cells, biofilm formation and viability was reduced for opaque variants in static assays.  Nasopharyngeal colonization was stable for all strains, but the mortality rate differed between them.  Transcript profiling by RNA-seq analyses revealed that expression of certain virulence factors were increased in a phase-specific manner.  As epigenetic regulation of phase variation (often referred to as phasevarion) is emerging as a common theme for mucosal pathogens, these studies serve as a model for future studies of host-pathogen interactions. Overall design: RNA from pneumococcal strains TIGR4, MBO15, MBO20, MBO21, MBO22, MBO23, MBO24, and MBO25 were analyzed. TIGR4 was the parent strain. MBO15 was the hsdS deletion strain. MBO20 through MBO25 were the isogenic TIGR4 strains containing alleles A through F, respectively. RNA was extracted from a total of 8 strains, with three replicates per strain, resulting in 24 total samples analyzed.