Paracellular sugar flux enhances apical Staphylococcus aureus growth in an airway epithelial-bacterial co-culture model

D Braun, JP Garnett, A McCarthy, JA Lindsay, EH Baker, DL Baines. St George's University of London, London, UK

Sugars (glucose and fructose) are known to promote the growth of S. aureus, a key bacterium in respiratory infections. Glucose concentration in the airway surface liquid (ASL) is normally lower than blood (~0.4mM compared to ~5mM). Previously, we developed an in vitro model of airway glucose homeostasis using human airway epithelium (H441), in which we demonstrated that glucose diffuses from blood/interstitial fluid across the respiratory epithelium into the ASL via paracellular pathways, and this is limited by epithelial permeability, glucose uptake and metabolism. Furthermore, increasing the diffusion gradient for glucose across the epithelium (e.g. hyperglycaemia) elevated glucose in ASL in this in vitro model. Here we used our established airway epithelial-bacterial co-culture model to examine the effect of sugar diffusion into the ASL on the growth of S. aureus.

S. aureus (JE2) addition on the apical surface of H441 monolayers led to enhanced paracellular 14C-L-glucose flux across the epithelium. Apical S. aureus growth was significantly increased by elevating basolateral glucose or fructose concentrations (p < 0.05, n = 5). To confirm that S. aureus utilised sugars that diffused across the epithelium into ASL, we obtained a S. aureus strain in which the fructose specific permease had been disrupted (NE768), which exhibited significantly inhibited growth in the presence of fructose (from 0.000 ± 0.1 to 6.86 ± 0.1 log10 cfu) compared to glucose (from 4.9 ± 0.3 to 8.4 ± 0.03 log10 CFU at 8 hours; p < 0.001, n = 3) consistent with an inability to utilise fructose to support its growth. In epithelial-bacterial co-culture, elevation of basolateral fructose increased apical JE2 growth to a greater extent than NE678 after 7 hours (p < 0.05, n = 5). Pre-treatment of H441 monolayers with metformin (1 mM; 18 hours) prior to S. aureus addition, significantly inhibited both glucose- and fructose-induced bacterial growth (p < 0.05, n = 4), which corresponded to reduced paracellular flux of sugar across metformin-treated monolayers (p < 0.001, n = 8). Metformin had no significant effect on S. aureus growth alone in culture or H441 sugar uptake (p > 0.05, n = 3).

These data provide new evidence that the diffusion of both fructose and glucose across the airway epithelium can be utilised by S. aureus to support its growth and that S. aureus growth can be reduced by limiting the paracellular leak of sugar using metformin.