Comprehensive Investigation into the Molecular Mechanisms Driving the Emergence of emm4 Group A Streptococcus

Advisor: Samuel Shelburne, MD, PhD

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Meeting ID: 943 9024 7626
Passcode: 962969

The major gram-positive bacterium group A Streptococcus (GAS) is a model organism for studying microbial epidemics as it is well known to cause infections that occur in waves. GAS strains are grouped based on the composition of the N-terminal sequence of the M protein which is encoded by the emm gene.  With the advent of large-scale whole genome sequencing, GAS clonal emergence events in emm1, emm3, and emm89 types have been identified and subsequently attributed to augmented infection severity due to increased virulence factor production.  Recently, we characterized a clonal emergence, expansion, and replacement involving emm4 GAS strains in the United States and the United Kingdom.  Using strains collected as part of active surveillance, we estimated that a new emm4 clone emerged around 1996 and, by 2017, had completely replaced the existing 'historic' emm4 strains.  My thesis sought to identify mechanisms underlying this temporal clonal emergence amongst emm4 GAS given that the 'emergent' strains did not produce augmented levels of virulence factors relative to the replaced or 'historic' strains.  Through the creation and analysis of isoallelic strains, I characterized the impact of several key genetic changes separating the 'historic' and 'emergent' strains, including an emm gene fusion event. Specfifically, we determined that a conserved mutation in a previously undescribed gene encoding a putative carbonic anhydrase was responsible for the defective in vitro growth observed in the 'emergent' strains.  We also identified that the 'emergent' strains survived better inside macrophages and killed macrophages at lower rates relative to the 'historic' strains.  Via the creation of isogenic mutant strains, we linked the 'emergent' strain 'survival' phenotype to the downregulation of the genes encoding the potent toxins NAD-glycohydrolase and streptolysin O (Nga/Slo) and upregulation of the msrAB operon, which encodes proteins involved in defense against extracellular oxidative stress.  Our findings are in accord with recent surveillance studies, which found a high ratio of mucosal (i.e., pharyngeal) relative to invasive infections amongst emm4 GAS. Inasmuch as ever-increasing virulence is unlikely to be evolutionarily advantageous for a microbial pathogen, our data add increased understanding to the well-described oscillating patterns of virulent GAS infections by demonstrating mechanisms by which emergent strains adapt a 'survival' strategy to outcompete previously circulating isolates.

Advisory Committee:
Samuel Shelburne, MD, PhD, Chair
Anthony Flores, MD, PhD
Blake Hanson, PhD
Ana Konovalova, PhD
Steve Norris, PhD