Genetic diversity and immune evasion of bacterial pathogens

Within the framework of this thesis the genetic diversity of three bacterial pathogens, Mycobacterium ulcerans, Neisseria meningitidis and Streptococcus agalactiae was investigated. The aim of these analyses was to contribute to the understanding of how genetic properties of the pathogens contribute to immune evasion. Implications of the findings for vaccine design are discussed.
Mycobacterium ulcerans
Buruli ulcer is a disease of skin and soft tissue caused by the bacterial pathogen Mycobacterium ulcerans. M. ulcerans has recently diverged from an M. marinum progenitor through the acquisition of a virulence plasmid, lateral gene transfer and reductive evolution. Isolates of M. ulcerans deriving from different regions of the globe can be associated with two distinct lineages, either the ancestral or the classical lineage. Here, we show that the two copies of the esxB-esxA gene cluster present in the genome of M. marinum are both deleted from the genome of M. ulcerans strains belonging to the highly virulent classical lineage. Members of the ancestral M. ulcerans lineage instead retained copies of the esxB-esxA gene cluster. Additionally, the hspX gene was present in the strains of the ancestral lineage and absent in the classical lineage. Our results indicate that M. ulcerans is adapting to an environment that is screened by immune recognition mechanisms by loss of highly immunogenic proteins.
Neisseria meningitidis
Certain hypervirulent lineages of Neisseria meningitidis, a commensal of the human nasopharynx, are a major cause of meningitis and septicaemia. Here we have investigated subcapsular antigens of serogroup A Neisseria meningitidis strains isolated in the course of longitudinal colonization and disease surveys in the African meningitis belt. In the course of clonal waves of colonization and disease we observed no sequence diversification of the outer membrane proteins PorA, PorB
and FetA. In contrast, high variability in the expression of Opa proteins was observed due to changing numbers of pentamer repeats within the open reading frames of the four opa genes opaA, opaB, opaD and opaJ. Furthermore, we found some exchange of alleles of the opa genes OpaA and OpaJ by horizontal gene transfer. Herd immunity may thus be a stronger driving force for diversification of Opa proteins than for other outer membrane proteins.
Streptococcus agalactiae
While Streptococcus agalactiae, the group B streptococcus (GBS), is traditionally considered a neonatal pathogen, it is also emerging as a significant cause of morbidity in adults. Here we have analysed the population structure of GBS isolates, collected from carriers and clinical cases in Kenya. Multi-locus sequence typing differentiated the 173 strains analyzed into 22 sequence types (STs), including 5 novel STs. A close correlation between STs and distinct capsular serotypes was found with the disease isolates being more diversified with respect to both STs and capsular serotypes than carrier isolates. The STs and capsular serotypes most prevalent in Kenya were also commonly found in many other regions of the world.
In this investigation, the highest genetic variablility was found in our GBS collection. In the N. meningitidis isolates collected during clonal waves of meningococcal colonization and disease, there was a striking lack of diversification, with the exception of the opa genes. The most conserved bacterial pathogen in this study was M. ulcerans, where no genetic variability could be found within a geographic region.
No vaccines exist to date against M. ulcerans or S. agalactiae, and an affordable universal vaccine against N. meningitidis is urgently needed. The technological advances in whole genome sequencing are likely to facilitate efforts towards finding suitable candidate antigens for subunit vaccines.