Abstract
INTRODUCTION: Pseudomonas aeruginosa biofilms contribute to the persistent presence of this bacterium in the cystic fibrosis airways. P. aeruginosa produces histamine in vitro and expresses histamine receptors. We investigated whether histamine regulated P. aeruginosa biofilm formation in vitro and contributed to bacterial virulence in Galleria mellonella. METHODS: P. aeruginosa biofilms were measured by staining bacteria adhered on polystyrene with crystal violet. Histamine concentrations were measured by ELISA. G. mellonella survival upon inoculation with P. aeruginosa was measured in the absence or presence of histamine. RESULTS: The concentration of histamine in the BHI broth was 140 ng/mL (1.3 μm ). Addition to the broth of diamine oxidase (DAO), an enzyme that catabolizes histamine, reduced by ∼3-fold the concentration of histamine and by 2-fold PAO1 strain biofilms. Addition of histamine (10-9m -10-4m ) to the LB medium augmented P. aeruginosa biofilms. Maximum effects were observed with concentrations of 10-5m and 10-8m for the mucoid NH57388A strain and the PAO1 strain, respectively. DAO reduced mucoid NH57388A biofilms induced by histamine (10-4m ) added to the LB medium. Addition of histamine to 48 h formed biofilms reduced anti-biofilm activities of gentamicin and azithromycin. Inoculation of G. mellonella with the PAO1 strain led to augmented histamine concentration in the haemolymph. Inoculation of histamine (10-8m ) reduced the survival rate of G. mellonella infected with the PAO1 strain. CONCLUSION: Histamine produced during periods of infection may augment P. aeruginosa virulence by promoting the biofilm mode of life of this bacterium.