IL6 and chronic obstructive pulmonary disease: When pulmonary epithelial cells are simultaneously infected with P. aeruginosa and F. nucleatum, F. nucleatum coaggregates with P. aeruginosa to synergistically invade into pulmonary epithelial cells and transiently resists P. aeruginosa-induced cytotoxic damage, and induces over-release of IL-6 and TNF-α, which may cause a chronic persistent pulmonary infection and increase the susceptibility of COPD exacerbation.