Streptococcus suis (S. suis) and Haemophilus parasuis (H. parasuis) bacteria are spread between pigs through snout contact. While some members of these bacterial families are harmless (avirulent), others cause disease (i.e. virulent strains). When the bacteria are avirulent, they are part of the normal microbiota of pigs. Whereas the virulent strains cause severe infections in weaning piglets.
S. suis and H. parasuis isolates from different countries have different virulence levels. This is of note as there are 35 S. suis serotypes and 15 H. parasuis serotypes worldwide. Although there is a vaccine that targets certain strains of H. parasuis, none target S. suis. As a result, these infections are usually treated with antibiotics. The decrease in antibiotic use has caused a significant increase in mortality due to these pathogens. This is why laboratories in Canada and Spain are collaborating to identify new therapeutic or prophylactic strategies against these pathogens.
Drs. Marcelo Gottschalk (CRIPA, Canada) and Virginia Aragon (CReSA, Spain) find it interesting that infections with these bacteria follow a similar pattern. In fact, what causes the most harm to the piglets is not the damage caused by the bacteria, but rather the excessive inflammatory response mounted by the piglets own immune system once the bacteria enter the bloodstream. Since both S. suis and H. parasuis infect the porcine respiratory tract, these researchers wondered whether they cooperate to cause an infection. With the support of a grant from the IXe Comité mixte Québec-Catalogne 2017-2019, these two research groups characterized bacteria known to be either virulent or avirulent. They evaluated whether these strains of S. suis and H. parasuis cause infection alone or they are combined. Potential interactions between strains were determined by either simultaneously co-infecting with both strains or by first infecting with one strain and then adding the other.
Dr. Mariela Segura (CRIPA), another collaborator, investigated whether the anti-phagocytic S. suis capsule affects the outcome of H. parasuis infections. A capsule is a sugary coat and, in the case of S. suis, it protects the bacterium from being eaten by immune system (phagocytosis) cells. This was of interest because Dr. Segura had previously shown that the protective effect of the S. suis capsule can also benefit surrounding bacterial species. Therefore, the researchers wanted to know if
H. parasuis can take advantage of this protective effect.
Is adhesion affected?
Regardless of if the strain is virulent, neither S. suis or H. parasuis help each other stick to cells, nor do they interfere or inhibit each others adhesion to respiratory tract cells.
Is the inflammatory response affected?
However, the ability of the bacterium to cause infection and the porcine cell type (tracheal epithelial cells or alveolar macrophages), the bacteria encounter will influence the porcine inflammatory response. The inflammatory response was occasionally increased, other times a synergistic response was observed, whereas other times there was no change in inflammatory response. No relationship was observed between virulence levels and the type of inflammatory response, so the effect of co-infection has to be determined every time it is encountered.
Does the S. suis anti-phagocytosis effect extend to H. parasuis?
Phagocytosis of H. parasuis strains with low virulence potential remained unchanged. Importantly, the highly infectious and, therefore, dangerous strains of H. parasuis are not phagocytosed.
In conclusion, preliminary in vitro tests suggest that S. suis and H. parasuis do not interact with one another when they bind to cells of the respiratory tract and, therefore, it will not be possible to develop an anti-adhesion treatment that targets both bacteria. The finding that these bad bugs do not engage in a turf war and cohabitate within the respiratory tract of pigs is important, especially given that this living arrangement can exacerbate the porcine immune response. Live animal infections will be performed to confirm whether this is the case in vivo.
Sources : Annabelle Mathieu-Denoncourt, Corinne Letendre, Jean-Philippe Auger, Mariela Segura, Virginia Aragon, Sonia Lacouture and Marcelo Gottschalk. Limited Interactions between Streptococcus Suis and Haemophilus Parasuis in In Vitro Co-Infection Studies. Pathogens 2018, 7, 7.
Mathieu Houde, Marcelo Gottschalk, Fleur Gagnon, Marie-Rose Van Calsteren, Mariela Segura. Streptococcus suis capsular polysaccharide inhibits phagocytosis through destabilization of lipid microdomains and prevents lactosylceramide-dependent recognition. Infect. Immun. 2012, 80, 506–517.