Moraxella catarrhalis NucM is an entry nuclease involved in extracellular DNA and RNA degradation, cell competence and biofilm scaffolding

Aimee Tan,Tsitsi D Mubaiwa,Kate L Seib,Luke V Blakeway,Wing-Sze Li,Makrina Totsika,Anthony D Verderosa

doi:10.1038/s41598-019-39374-0

Aimee Tan, Tsitsi D Mubaiwa + Show 5 more

Open Access

https://doi.org/10.1038/s41598-019-39374-0

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Abstract

Moraxella catarrhalis is a host-adapted bacterial pathogen that causes otitis media and exacerbations of chronic obstructive pulmonary disease. This study characterises the conserved M. catarrhalis extracellular nuclease, a member of the ββα metal finger family of nucleases, that we have named NucM. NucM shares conserved sequence motifs from the ββα nuclease family, including the DRGH catalytic core and Mg2+ co-ordination site, but otherwise shares little primary sequence identity with other family members, such as the Serratia Nuc and pneumococcal EndA nucleases. NucM is secreted from the cell and digests linear and circular nucleic acid. However, it appears that a proportion of NucM is also associated with the cell membrane and acts as an entry nuclease, facilitating transformation of M. catarrhalis cells. This is the first example of a ββα nuclease in a Gram negative bacteria that acts as an entry nuclease. In addition to its role in competence, NucM affects cell aggregation and biofilm formation by M. catarrhalis, with ΔnucM mutants having increased biofilm biomass. NucM is likely to increase the ability of cells to survive and persist in vivo, increasing the virulence of M. catarrhalis and potentially affecting the behaviour of other pathogens that co-colonise the otorhinolaryngological niche.

Highlights

Moraxella catarrhalis is a Gram-negative, human-restricted bacterial species that colonises the upper and lower respiratory tracts
outer membrane vesicle (OMV) showed only partial degradation of the plasmid, with more appearing as nicked and linear DNA than intact plasmid (Fig. 1A). This suggests that the majority of the nuclease is secreted into the growth medium, but that there may be a portion of the nuclease associated with the cell membrane
Extracellular nucleases are known to be involved in the virulence of a range of bacterial species, where they mediate phenotypes such as competence, biofilm formation and degradation of neutrophil extracellular traps

Summary

Introduction

Moraxella catarrhalis is a Gram-negative, human-restricted bacterial species that colonises the upper and lower respiratory tracts. Studies identified nucleases in B. subtilis and S. pneumoniae as essential components for natural competence, the uptake of DNA from the environment into the cell[12,19]. The effect of nuclease production on biofilm formation in bacterial species that produce their own extracellular nucleases has been examined in a range of species, including V. cholerae[14], H. influenzae[13], S. aureus[16] and N. gonorrhoeae[30]. In these cases, disruption of the nuclease gene has correlated with increased biofilm mass and accumulation of eDNA to levels exceeding the parental strain. We characterise the effect of NucM on M. catarrhalis growth, competence and biofilm formation

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