A new study led by researchers at Oxford Population Health has provided the most detailed analysis to date of the structure of Moraxella catarrhalis, a common bacterium that can cause respiratory infections in children and adults.
The team developed a new tool and coding system to classify and track M catarrhalis strains worldwide, confirming two genetically distinct lineages that have evolved along separate evolutionary paths. The study is published in Nature Communications.
The human nasopharynx (part of the upper respiratory tract) is a habitat for a diverse range of bacterial species, and these bacteria form a protective barrier for the respiratory system. Mostly these bacteria are not harmful, but sometimes they do cause disease.
M catarrhalis are one type of bacteria that reside within the nasopharyngeal microbiome of healthy individuals. They do not usually cause serious life-threatening infections, but are a frequent cause of acute ear infections in children and can exacerbate chronic obstructive pulmonary disease (COPD) in adults.
Compared to other bacterial pathogens, relatively little is known about the biology and genetics of M catarrhalis. Previous work over several decades identified two genetic lineages, seroresistant (resistant to the human immune response) and serosensitive. It was believed that seroresistant M catarrhalis may be more virulent but their genetic differences were not clearly resolved.
In this study, the authors present a new genotyping scheme that is a bit like defining a ‘barcode’ for the bacteria, which makes it easier for researchers worldwide to compare and track them in a consistent way. They used this new genotyping scheme to characterise nearly 2,000 M catarrhalis genomes from 12 countries, including M catarrhalis recovered from the nasopharynx of healthy South African children recruited to the Drakenstein Child Health Study.
The authors investigated over 1,300 genes in the M catarrhalis genome, including genes associated with virulence and antimicrobial resistance, which revealed that seroresistant and serosensitive M catarrhalis were indeed genetically distinct.
Genes linked to antimicrobial resistance and bacterial competition within the nasopharynx were more prevalent among seroresistant M catarrhalis, and there was lineage-specific variation in genes that encode surface proteins and are potential vaccine targets.
Dr Iman Yassine, first author of the paper, said ‘M catarrhalis is unlikely to be a major clinical threat but it is frequently recovered from healthy individuals, causes mild to moderate infections, and antimicrobial resistance is a concern. Our findings underscore the importance of continued genomic surveillance using precise genotyping tools like the one we present in this paper.’
All publicly available M catarrhalis genomes were investigated and while more work remains to be done, this new methodology will help to unravel the role of M catarrhalis in infection, resistance, and disease progression; it is a valuable tool for epidemiological monitoring and longitudinal surveillance.
The new core genome multi-locus sequence typing (cgMLST) scheme and Life Identification Number (LIN) code system have been implemented in the PubMLST database, ensuring that future genomic data on M catarrhalis can be easily compared and classified in a consistent, reproducible manner.