Streptococcus suis

Streptococcus (S.) suis is a bacterium of the genus Streptococcus within the order Lactobacillales. It is a Gram-positive, facultative anaerobic bacterium. This means that S. suis usually grows under aerobic conditions but can also survive under anaerobic conditions due to a change in metabolism. It is catalase negative, immobile, forms pairs or chains and has a diameter of about 2 microns. In the fermentative metabolism lactate is mainly produced (Selbitz, Truyen, and Valentin-Weigand 2015).

The reservoir of S. suis are domestic pigs (Lowe et al. 2011) and wild boars (Baums et al. 2007;

Verónica Sánchez del Rey et al. 2014). Nevertheless, the bacterium was also found in other animals species such as rabbits, dogs or lambs (Muckle et al. 2010, 2014; V. Sánchez del Rey et al. 2013).

The cultivation of Streptococcus suis can be performed in special liquid media, e.g. Todd Hewitt Broth (THB) (Todd and Hewitt 1932) or on blood agar plates at 37 ° C, both aerobically and anaerobically. The bacterium shows white-greyish colonies on sheep blood agar with a diameter between 0.5 and 1 mm with an occurring α-hemolysis (Wewer 2009).

S. suis is a very heterogeneous bacterium with different serotypes and strains. There are currently 35 serotypes (ST) known that differ in their capsular antigenicity. However, for a few serotypes (ST 32 and ST 34), it has been suggested to assign them to a different bacterial species (Okura et al. 2016). In diseased pigs serotype 2 is the most prevalent worldwide followed by ST 9 and ST3 (Goyette-Desjardins et al. 2014).

Virulence factors help a pathogen to survive in the host organism (Cross 2008). S. suis is characterized by a variety of virulence factors (Fittipaldi et al. 2012), so it has e.g. a polysaccharide capsule to evade phagocytosis (Houde et al. 2012; M. Segura, Gottschalk, and Olivier 2004; Chabot-Roy et al. 2006). In addition it is discussed that the capsule helps the bacterium against entrapping by the so-called neutrophil extracellular traps (Zhao et al. 2015).

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Further it was shown that S. suis is able to generate biofilms, which make them more resistant to certain antibiotics (Grenier, Grignon, and Gottschalk 2009). Another virulence factor is the exotoxin suilysin (SLY), which can destroy host cells by forming pores in their cell membranes (Barnett et al. 2015; Tenenbaum et al. 2016). This was - in addition to other cells - shown for porcine brain microvascular endothelial cells (pBMEC) (Vanier et al. 2004) or for human brain microvascular endothelial cells (hBMEC), in which the mechanism of an alteration of the host’s actin cytoskeleton due to the influence of SLY was discovered (Lv et al. 2014).

The mechanisms of how S. suis exactly adapts in the body of the host are largely unknown (Jörg Willenborg and Goethe 2016). However, transcriptome analyses of S. suis under infectious conditions have been recently studied in vitro and demonstrated differences in the activities of the amino acid metabolism for example between the medium blood and the medium cerebrospinal fluid (Koczula et al. 2017).

S. suis is a commensal bacterium predominantly in the upper respiratory tract (tonsils and nasal cavities), but also in the genital and digestive tracts (Robertson and Blackmore 1989;

Gottschalk and Segura 2000). However, synergistic interaction with other agents may increase the risk of invasive infection (Meng et al. 2015) that can cause severe symptoms such as septicaemia, arthritis, endocarditis, pneumonia or meningitis or even a per acute dying (Sanford and Tilker 1982; Gottschalk et al. 2010; Arends et al. 1984).

Compared with the high spread of the germ in pig herds (prevalence about 100 %), severe symptoms in pigs are rarely observed (Goyette-Desjardins et al. 2014). Infected but healthy animals are of great relevance as they function as transmitters. Horizontal transmission is usually via nose-to-nose contact (Dekker et al. 2013), but vertical transmission between the sow and the piglet is also possible during phase of farrowing (Amass, SanMiguel, and Clark 1997). Clinical symptoms of infection are most prevalent in pigs at weaning age.

The bacterium shows also a zoonotic character: A S. suis infection in a human was first described in Denmark in 1968 (Arends and Zanen 1988). In contrast to pigs S. suis in humans usually causes most bacterial meningitis, nevertheless, other symptoms such as septicaemia, pneumonia, endocarditis or peritonitis may also occur (Thi Hoang Mai et al. 2008; Suankratay et al. 2004; Gottschalk et al. 2010). In addition, a late episode of the infection can be deafness (Thi Hoang Mai et al. 2008). S. suis ST 2 is most commonly found in human S. suis infections with 97% (Goyette-Desjardins et al. 2014). A major outbreak of the disease in China in the year

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2005 resulted in a mortality rate of approximately 20% (Gottschalk, Segura, and Xu 2007).

However, compared to pigs, the prevalence of S. suis in the human population is estimated to be only about 5% and carriers are believed to be in intensive contact with pigs (e.g., farmers, butchers, veterinarians) (Strangmann, Fröleke, and Kohse 2002; Goyette-Desjardins et al.

2014), although cases of human infection without previous contact to pigs have been reported (Kerdsin et al. 2016).

People can be infected by the distortion of raw pork (Fongcom et al. 2001) or by small skin wounds that allow the bacteria to enter the bloodstream (Wertheim et al. 2009).

The disruption of epithelial barriers is considered to be the first step of invasion of the bacteria into the bloodstream (Mariela Segura et al. 2016). After having successfully entered the blood the bacteria spread via the blood flow in the whole body and can reach the brain structures or the cerebrospinal fluid (CSF) after breaking through the brain barriers (K. S. Kim 2008;

Pulzova, Bhide, and Andrej 2009) and thus initiate the early phase of meningitis (Gottschalk and Segura 2000; Fittipaldi et al. 2012; Mariela Segura et al. 2016). Several studies have shown that especially the choroid plexus epithelium of the blood-cerebrospinal fluid barrier (BCSFB) is used as an entry into the CSF compartment of S. suis (Schwerk et al. 2015; Wewer et al.

2011; Schwerk et al. 2012).

To overcome the brain barriers there are three possibilities most mentioned for the bacteria:

transcellularly, paracellularly or attached to phagocytotic cells, which transmigrate through the barriers (so-called "Trojan Horse Theory") (K. S. Kim 2008; Pulzova, Bhide, and Andrej 2009; Dando et al. 2014). This theory has been broadened to postulate that S. suis attaches to non-disease-associated phagocytes that transmigrate normally with a small number into the CSF. By this mechanism they reach the CSF compartment where they start to replicate. This is called the "modified Trojan horse theory" (Gottschalk and Segura 2000).

At present it is still unclear why a disease leads to meningitis in some individuals, but leads to different symptoms or no disease in others (Higgins et al. 1990). In addition to the further investigation of the pathogen itself, intensified research on the pathogen-host interaction, regarding the immune system, should be underlined here.

In the following work S. suis Serotype 2, strain 10 is used for infection studies in vivo and in vitro. This strain was cultivated and published in 1999 at the DLO Institute for Animal Science

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and Health, Lelystad, The Netherlands (Smith et al. 1999). It was shown that S. suis Serotype 2, strain 10 is highly virulent in experimental infections of piglets (Vecht et al. 1989). Therefore, it was used by many research groups in experimental infections in vivo and in vitro (J. Seele et al. 2013; de Buhr et al. 2016, 2015; Seitz et al. 2012; Jana Seele et al. 2015).