Bacterial Outer Membrane Vesicles. Mediators of virulence and antibiotic resistance.

Abstract: Outer membrane vesicles (OMV) have the size of nanoparticles and are released from the outer membrane of most Gram-negative bacteria. The OMV reflect the composition of the bacterial outer membrane, including lipids and virulence factors specific for the pathogen, and are involved in pathogenesis and bacterial survival. Moraxella catarrhalis and Haemophilus influenzae are Gram-negative bacterial species that cause respiratory tract infections such as acute otitis media and exacerbations in chronic obstructive pulmonary disease (COPD). The aim of this thesis was to investigate what role OMV derived from respiratory pathogens such as M. catarrhalis and H. influenzae plays in immune activation and evasion, as well as how the OMV interact with other pathogens of the same colonization niche. In our first study, we determined the protemoic composition of M. catarrhalis OMV, and found them to be composed of mainly outer membrane and periplasmic proteins. We also established that OMV bind to and activate respiratory epithelial cells and cause a pro-inflammatory response both in vitro and in vivo. Considering that more than 97% of all M. catarrhalis strains are positive for β-lactamase, we determined in our second study that OMV from this pathogen contain β-lactamase. We further showed that OMV rescued other amoxicillin susceptible co-pathogens such as non-typeable H. influenzae (NTHi) and Streptococcus pneumoniae from killing. Thereafter, we established in our third study that serum from healthy adults contain anti-Moraxella β-lactamase antibodies. We showed that these antibodies bind to and neutralize β-lactamase in OMV to a certain degree, whereas OMV also act as protective vesicles, shielding the enzyme in the extracellular space. Finally, in the last study we investigated the role of OMV in co-infections with other species dwelling in the respiratory tract. Previous studies have shown treatment failures of group A streptococci (GAS) in pharyngotonsillits even though the GAS are completely susceptible to β-lactams, and that NTHi and M. catarrhalis are often found associated with GAS during treatment failures. We found that OMV from β-lactamase positive NTHi and M. catarrhalis protect GAS from amoxicillin-induced killing, and we suggest OMV secretion to be a mechanism for bacteria in polymicrobial infections to interact and protect each other. In summary, we determined that OMV secreted from Gram-negative bacteria of the upper respiratory tract are mediators of virulence and antibiotic resistance, interacting with host cells as well as other bacterial species found in polymicrobial infections.