Streptococcus suis is Gram-positive bacterium and its natural habitat is the upper respiratory tract of pigs, and in particular the tonsils and nasal cavity. Although it is considered to be a normal member of the adult pig microbiome, it can cause serious diseases in pigs and humans. S. suis is in fact one of the most important swine pathogens world-wide, causing a wide variety of diseases in pigs including septicemia, arthritis, endocarditis, and meningitis that leads often to a rapid death within 1-2 days. Although most human infections are considered the consequence of occupational exposure, in the last years the number of human cases has increased and isolates with multi-resistance genes have been isolated. Human infection caused by S. suis are characterized by a similar symptomatology as in pigs. Despite the economic loss in the pork industry due to S. suis infection and its importance as emerging zoonotic agent, experimental studies of S. suis virulence and pathology have been hampered by the lack of efficient methods for genetic transformation and the lack of a simple, cost-effective model to investigate S. suis virulence.
In some streptococcal species, genetic transformation can be carried out very efficiently as these species can be experimentally induced to take-up and recombine homologous extracellular DNA. The discovery of natural competence in some streptococci and the potential of opening up new avenues for genetic analysis of S. suis, was the motivation for investigating natural competence in this important pathogen.
In Chapter 2 we showed that a peptide pheromone induces competence in S. suis. The induction was dependent on ComX, a sigma factor that controls the streptococcal late competence regulon; the SigX-inducing peptide (XIP); and ComR, a regulator of comX. XIP was identified as an N-terminally truncated variant of ComS. This has resulted in the development of a novel methodology that will enable diverse research groups to accelerate discovery of novel features of S. suis ecology and pathology, especially with respect to virulence.
In Chapter 3 we investigated the genetic regulation of competence in S. suis and we provided a hypothetical model of the S. suis transformasome. We verified the essential role of the S. suis major pilin, and CinA for efficient competence development, supporting the notion that our predicted multi-protein transformasome indeed appears to function as described for other streptococci. We have also characterised the differential metabolic states that enable competence, and the metabolic state associated with competence exit (Chapter 4).
In Chapter 5 we investigated for the first time the use a zebrafish larvae model to assess the relative virulence of S. suis strains in porcine infections. Because of its convenience and cost-effectiveness, this model may be used to assay virulence of environmental S. suis strains, in particularly those of clinical relevance to infection of pigs and humans. Furthermore, a large number of bacterial mutants and strains can be screened for their virulence and in vivo pathogenicity, opening up new avenues to investigate the so far undiscovered pathways mediating successful host infection by S. suis.
In Chapter 6 we applied these two innovative methods (the competence system and the zebrafish larval model) to characterize two different two-component systems (TCS) of S. suis. TCS are important players in the regulation of bacterial adaptation to changes in environmental conditions, including those encountered in the host during infection. In this study, we studied the role of the two TCS of S. suis 2 strain S10 in virulence and in the survival of the bacteria in the bloodstream and host tissue.
Chapter 7 summarizes and discusses the key results and the future prospective of the thesis research.
|Qualification||Doctor of Philosophy|
|Award date||14 Dec 2015|
|Place of Publication||Wageningen|
|Publication status||Published - 2015|
- streptococcus suis
- gene expression
- direct dna uptake
- virulence factors