Inhibitory effects of mycobacterial cell wall lipids on bovine lung surfactant extract: An in vitro study at the air-aqueous interface

Lung surfactant is a lipoprotein mixture synthesized by the type II alveolar cells in the lungs and lines the air-aqueous interface. It is essential for stabilizing the pulmonary alveoli. Deficiency or dysfunction of the surfactant due to chemicals or toxins results in alveolar collapse and if untreated leads to respiratory distress. In pulmonary tuberculosis, Mycobacterium tuberculosis bacteria reside in the alveoli, in close physical proximity with the alveolar surfactant. The surface active mycobacterial cell wall lipids may easily detach from bacterial cell wall and interact with surfactant components altering their surface activity. In this study, effects of mycolic acid and cord factor (the abundant surface lipids of mycobacterial cell wall) on the surface activity of modified bovine surfactant extract (Survanta™) were characterized in vitro using the Wilhelmy surface balance at physiological temperature (37 °C) and pH 7.4. Inhibitory effects of mycobacterial cell wall lipids on the terminal airway surfactant were evaluated using a capillary surfactometer. Addition of graded amounts of mycolic acid and cord factor significantly increased the minimum surface tension of Survanta™ monolayer from <1 mN/m to ∼11-20 mN/m. Presence of mycobacterial cell wall lipids decreased the rate and magnitude of Survanta™ adsorption and fluidized the surfactant monolayer. Capillary surfactometer study exhibited 100% capillary closure on addition of mycobacterial cell wall lipids to the modified bovine surfactant extract. AFM imaging revealed alteration of surface topography and presence of aggregates on addition of mycobacterial cell wall lipids to Survanta™ monolayer. This study suggests a biophysical inhibition of lung surfactant which may play a role in the pathogenesis of pulmonary tuberculosis.