Necrotising fasciitis (NF) due to invasive Group A streptococcus (iGAS) is frequently
associated with emm1 isolates, with an attendant mortality of approximately 40%. Cases
occur in previously healthy individuals with a history of upper respiratory tract infection,
soft tissue contusion and no obvious portal of entry. Using a new model of mild contusion
injury, we set out to determine the impact of contusion on iGAS bacterial burden,
phenotype and host cytokine response.
Firstly invasive bioluminescent GAS strains were developed using a replicative plasmid
pTHLK but these were unstable in vivo. A stably bioluminescent M89 iGAS strain was
developed using a plasmid pICL18Lux and characterised in vitro and in vivo models of
invasive infection. Bioluminescence measured in vivo correlated with bacterial quantity
but practical application to produce a lower respiratory tract infection or prolonged soft
tissue infection was limited by detection of light production during infection. The new
isolate was attenuated in growth and virulence and it could not be used to study the
interaction of contusion and virulence.
A new model of soft tissue contusion was developed that resulted in local soft tissue
neutrophilic inflammation, no bony injury, systemic cytokine production or weight loss. The
model of mild contusion was used to determine the impact of trauma on emm1 iGAS.
Surprisingly, mild contusion did not provide a focus for initiation or seeding of bacteraemic
GAS infection, but instead provided an environment that enhanced GAS dissemination to
local lymph nodes. Dissemination was linked to a phenotypic change with the emergence
of mucoid GAS colonies in lymphoid tissue, bloodstream and spleen. The mucoid GAS
colonies in the lymph node demonstrated a significant increase in production of capsular
hyaluronan and were not associated with covRS mutations. Whole genome sequencing
was applied to understand the genetic change responsible, a selection of mutations
occurring in the spleen and lymph node were discovered. The exact role of some of the
mutations in the pathogenesis of dissemination of GAS and the influence of trauma is not
entirely known and the subject of on-going investigation.