Polymicrobial airway infections in persons with cystic fibrosis (pwCF) can positively or negatively impact the course of disease. A major CF pathogen,
Pseudomonas aeruginosa,
often establishes a chronic infection leading to lung deterioration. Interestingly, the presence of certain oral commensal streptococci is correlated with improved outcomes for pwCF. We previously reported that hydrogen peroxide production by these commensals combined with nitrite generates reactive nitrogen intermediates (RNI), which inhibit
P. aeruginosa in vitro
. In this study, we utilized a rat co-infection lung model to assess whether oral commensal-generated RNI can restrict the pathogenesis of a CF isolate of
P. aeruginosa
. We report that the oral commensal
Streptococcus parasanguinis
and nitrite reduce
P. aeruginosa
-induced host inflammation in wild-type rats. To better recapitulate CF-specific airway conditions, we used a bronchial epithelial cell culture model to gain a better understanding of how
S. parasanguinis
and nitrite may influence
P. aeruginosa
burden during a CF infection. Hence, we co-infected wild-type and cystic fibrosis transconductance regulator (CFTR) channel-deficient bronchial epithelial cells with
P. aeruginosa
and
S. parasanguinis
with or without nitrite. Strikingly,
S. parasanguinis
reduced the bacterial burden of
P. aeruginosa
without nitrite, promoted epithelial cell viability, and stimulated nitrite production in the wild-type and CFTR-deficient epithelial cells, where nitrite induction was more apparent in the CFTR mutant cells. Taken together, our study demonstrates that the commensal
S. parasanguinis
may provide protection against
P. aeruginosa
-induced inflammation and cell death, as well as modulate nitrite flux in airway epithelial cells.
IMPORTANCE
Respiratory infections are a leading cause of morbidity and mortality in people with cystic fibrosis (CF). These infections are polymicrobial in nature with overt pathogens and other colonizing microbes present. Microbiome data have indicated that the presence of oral commensal bacteria in the lungs is correlated with improved outcomes. We hypothesize that one oral commensal,
Streptococcus parasanguinis,
inhibits CF pathogens and modulates the host immune response. One major CF pathogen is
Pseudomonas aeruginosa
, a Gram-negative, opportunistic bacterium with intrinsic drug resistance and an arsenal of virulence factors. We have previously shown that
S. parasanguinis
inhibits
P. aeruginosa in vitro
in a nitrite-dependent manner through the production of reactive nitrogen intermediates. In this study, we demonstrate that while this mechanism is evident in a cell culture model of the CF airway, an alternative mechanism by which
S. parasanguinis
may improve outcomes for people with CF is through immunomodulation.