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.