INTRODUCTION
Staphylococcus aureus (S. aureus ), a gram-positive
commensal bacterium, is associated with a wide spectrum of pathology
ranging from asymptomatic colonisation of the nares to being the leading
cause of nosocomial bacteraemia with an associated mortality of
15-60%/.1 Due to the diverse phenotypic behaviour ofS. aureus it has been difficult to characterise its involvement
in diseases such as chronic rhinosinusitis (CRS). S. aureuscolonises the nasal cavity in 64% of patients with nasal polyps
(CRSwNP) compared with 33% of those without polyps (CRSsNP) and 20% in
those without CRS.2,3 A higher proportion of patients
with CRSwNP demonstrate IgE towards S. aureus enterotoxins in
their serum than those without CRS (22.6-32.5 % vs 6.7-14.3% in
controls).4 Culture of S. aureus pre- and
post-operatively in patients with CRS is a poor prognostic indicator for
disease recurrence and recalcitrance.5 However, the
factors responsible for the enhanced pathogenicity of S. aureusstrains prevalent in difficult-to-treat CRS disease remain poorly
understood.
S. aureus can persist in the nasal cavity of CRS patients,
evading the immune system and the effects of
antimicrobials.6 It can achieve this through
internalisation within host cells by localising within the intracellular
space or creating extracellular biofilms.7, 8 In 2015,
our group made the novel observation that S. aureus internalises
within mast cells in nasal polyps which could act as a reservoir of
bacteria seeding into the extracellular space and driving chronic
inflammation in CRSwNP patients.9 To transform from a
free-floating planktonic S. aureus phenotype into a biofilm or
intracellular bacteria requires the expression of virulence
genes.10 These fall into three main categories
including pore forming toxins known as exfoliative toxins, enzymatic
toxins and superantigens.11 Exfoliative toxins
including α haemolysin and bi-component leukocidins are involved in
lysing phagocytes and have been shown to be essential for intracellular
survival and phagosome escape.12,13 Exoenzymes promote
biofilm formation and disruption of cell function.14Superantigens such as Staphylococcus enterotoxin B (SEB) have been shown
to promote S. aureus uptake and degranulation in mast cells
within CRS sinonasal tissue.15 Most exert their
toxicity by activation of T and B cells via binding human leukocyte
antigen molecules which communicate with the variable β chain of the T
cell receptor causing widespread inflammatory, type 2 cytokine
release.11
Notably, S. aureus has limited ability to survive
intracellularly. It has been reported that S. aureus from only
seven out of twenty-three clinical isolates from patients’ nares were
able to survive intracellularly in a keratinocyte cell
line.16 Furthermore, recent evidence demonstrates an
increased number of prophages (bacterial viruses) within the genome ofS. aureus cultured from CRSwNP patients, providing additional
virulence genes.17 Given these findings, we
prospectively set out to investigate the differences in virulence factor
gene carriage in S. aureus isolates cultured from controls,
CRSsNP and CRSwNP patients using short read genome sequencing and
bioinformatics techniques. We then used a representative strain from the
control and CRSwNP group to compare intracellular localisation in a mast
cell model and determine which genes may be linked with intracellular
survival.