<i>Pseudomonas aeruginosa</i> and Its Unsuspected Ally <i>Lactiplantibacillus plantarum</i> : Enhanced Pathogenicity in A Combined Cystic Fibrosis Sputum – Epithelial Cell Model

<i>Pseudomonas aeruginosa</i> is a notorious bacterial pathogen causing chronic pulmonary infections in people with cystic fibrosis (CF) due to its high tolerance to antibiotics and ability to form recalcitrant biofilms. A newer approach to attenuate the virulence of <i>P. aeruginosa</i> in CF could be the local reinforcement of a resilient community of competing bacteria in the lung. Lactobacilli can mediate antagonistic effects against <i>P. aeruginosa</i> by production of organic acids, but it is not entirely clear if they can exert this beneficial effect locally at the site of infection. While the nutritional environment of the airways in CF promotes <i>P. aeruginosa</i>, it does not support robust growth of lactic acid bacteria, thus attenuating their probiotic potential. To overcome this obstacle, we hypothesized that prebiotic fructooligosaccharides (FOS) could selectively stimulate <i>Lactiplantibacillus plantarum</i> during culture in synthetic cystic fibrosis sputum medium (SCFM2). Indeed, FOS supported the growth of <i>L. plantarum</i> and led to increased acid production. Co-cultivation of <i>L. plantarum</i> and <i>P. aeruginosa</i> reduced biofilm formation and FOS enabled <i>L. plantarum</i> to grow to higher densities in dual-species biofilms. However, this came at the cost of an increased production of the cytotoxic metabolite pyocyanin by <i>P. aeruginosa</i>. To examine whether <i>L. plantarum</i> would influence the pathogenicity of <i>P. aeruginosa</i>, we developed a dual-bacterial species infection model using a CF - relevant airway cell line exposed to the nutritional environment of SCFM2. <i>L. plantarum</i>, grown in SCFM2 or SCFM-FOS, did not inhibit the adhesion of <i>P. aeruginosa</i>. In contrast, the presence of live as well as heat-inactivated <i>L. plantarum</i>, or sterile <i>L. plantarum</i> supernatants drastically enhanced the cell damage during coinfection with <i>P. aeruginosa</i>. This effect was not exclusively dependent on differences in the proliferation of <i>P. aeruginosa</i> or addition of SCFM2 to the cell culture medium. Our data indicate that a potential benefit of bacteriotherapy is determined by the nutritional environment of the diseased body site and that the use of <i>L. plantarum</i> in the context of chronic pulmonary infections must be carefully evaluated.