Moraxella catarrhalis-produced nitric oxide has dual roles in pathogenicity and clearance of infection in bacterial-host cell co-cultures.

In humans, the free radical nitric oxide (NO) is a concentration-dependent multifunctional signaling or toxic molecule that modulates various physiological and pathological processes, and innate immunity against bacterial infections. Because the expression of bacterial genes encoding nitrite reductase (AniA) and NO reductase (NorB) is highly upregulated in biofilms in vitro, it is important to investigate whether bacterial NO-metabolism might subvert host NO signaling and play pathogenic roles during infection. The Moraxella catarrhalis AniA and NorB directly function in production and reduction of NO. Using M. catarrhalis-human bronchial epithelial cell (HBEC) co-cultures, we recently reported AniA/nitrite-dependent cytotoxic effects on HBECs, including altered protein profiles of HBECs and induced HBEC apoptosis, suggesting bacterial nitrite reduction likely dysregulates host cell gene expression. To further clarify whether nitrite reduction-derived NO or nitrite-dependent stimulation of bacterial growth was responsible for adverse effects on HBECs, we monitored bacterial nitrite reduction, levels of NO in co-cultures and resulted dynamic effects on HBEC proliferation and bacterial viability. This study demonstrated that M. catarrhalis nitrite reduction-derived NO was responsible for observed adverse effects on HBECs at mid-to-late stages of infection. More importantly, our data showed that while nitrite promoted bacterial growth and biofilm formation at early hours of infection, nitrite reduction-derived NO was toxic towards M. catarrhalis in maturing biofilms, suggesting nitrite reduction-derived NO might be a possible dualistic mechanism by which M. catarrhalis promotes diseases and spontaneous resolutions.

ModM DNA methyltransferase methylome analysis reveals a potential role for Moraxella catarrhalis phasevarions in otitis media.

Moraxella catarrhalis is a significant cause of otitis media and exacerbations of chronic obstructive pulmonary disease. Here, we characterize a phase-variable DNA methyltransferase (ModM), which contains 5'-CAAC-3' repeats in its open reading frame that mediate high-frequency mutation resulting in reversible on/off switching of ModM expression. Three modM alleles have been identified (modM1-3), with modM2 being the most commonly found allele. Using single-molecule, real-time (SMRT) genome sequencing and methylome analysis, we have determined that the ModM2 methylation target is 5'-GAR(m6)AC-3', and 100% of these sites are methylated in the genome of the M. catarrhalis 25239 ModM2 on strain. Proteomic analysis of ModM2 on and off variants revealed that ModM2 regulates expression of multiple genes that have potential roles in colonization, infection, and protection against host defenses. Investigation of the distribution of modM alleles in a panel of M. catarrhalis strains, isolated from the nasopharynx of healthy children or middle ear effusions from patients with otitis media, revealed a statistically significant association of modM3 with otitis media isolates. The modulation of gene expression via the ModM phase-variable regulon (phasevarion), and the significant association of the modM3 allele with otitis media, suggests a key role for ModM phasevarions in the pathogenesis of this organism.