Metformin reduces airway glucose permeability and hyperglycaemia-induced Staphylococcus aureus load independently of effects on blood glucose

Background Diabetes is a risk factor for respiratory infection, and hyperglycaemia is associated with increased glucose in airway surface liquid and risk of Staphylococcus aureus infection. Objectives To investigate whether elevation of basolateral/blood glucose concentration promotes airway Staphylococcus aureus growth and whether pretreatment with the antidiabetic drug metformin affects this relationship. Methods Human airway epithelial cells grown at air–liquid interface (±18 h pre-treatment, 30 μM–1 mM metformin) were inoculated with 5×105 colony-forming units (CFU)/cm2 S aureus 8325-4 or JE2 or Pseudomonas aeruginosa PA01 on the apical surface and incubated for 7 h. Wild-type C57BL/6 or db/db (leptin receptor-deficient) mice, 6–10 weeks old, were treated with intraperitoneal phosphate-buffered saline or 40 mg/kg metformin for 2 days before intranasal inoculation with 1×107 CFU S aureus. Mice were culled 24 h after infection and bronchoalveolar lavage fluid collected. Results Apical S aureus growth increased with basolateral glucose concentration in an in vitro airway epithelia–bacteria co-culture model. S aureus reduced transepithelial electrical resistance (RT) and increased paracellular glucose flux. Metformin inhibited the glucose-induced growth of S aureus, increased RT and decreased glucose flux. Diabetic (db/db) mice infected with S aureus exhibited a higher bacterial load in their airways than control mice after 2 days and metformin treatment reversed this effect. Metformin did not decrease blood glucose but reduced paracellular flux across ex vivo murine tracheas. Conclusions Hyperglycaemia promotes respiratory S aureus infection, and metformin modifies glucose flux across the airway epithelium to limit hyperglycaemia-induced bacterial growth. Metformin might, therefore, be of additional benefit in the prevention and treatment of respiratory infection

Contribution of glucose to increased respiratory bacterial burden in hyperglycaemia

It has recently been proposed that uncontrolled hyperglycaemia in people with diabetes increases lung glucose, so providing a richer growth medium facilitating bacterial infection. Where diabetes is controlled, there is a reduced associated risk of bacterial infection. The aim of this thesis was to determine the effect of glucose on bacterial growth in vivo and directly link bacterial glucose metabolism with increased respiratory tract bacterial load in hyperglycaemia. P. aeruginosa, a Gram-negative opportunistic pathogen, which is a major cause of respiratory infections, was used as chronic P. aeruginosa infections are most commonly associated with people with cystic fibrosis (CF) and chronic obstructive pulmonary diseases (COPD), but importantly P. aeruginosa is increasingly diagnosed in diabetic patients with pneumonia/lung infection. To test the hypothesis, P. aeruginosa mutants were generated by deleting oprB, gltK, gtrS and glk genes and these mutant strains were used in in vitro and in vivo infection models developed during this thesis. The mutants had drastically reduced growth in minimal medium containing glucose as the sole carbon source, whereas they were unaltered when grown in rich medium. In order to explore the effect of elevated glucose with minimal effects on the immune response, streptozocin was used to induce diabetes, instead of genetically obese mice, which have a complex phenotype and impaired immune responses. Streptozocin induced hyperglycaemia also led to increased bacterial load in the airways when mice were infected with wild type PAO1 but not with the glucose uptake and metabolism mutants. To further support this hypothesis when metformin was used to lower glucose levels it resulted in a reduced bacterial burden.Open Acces