Date Published: January 17, 2019
Publisher: Public Library of Science
Author(s): T. Louise Prüfer, Judith Rohde, Jutta Verspohl, Manfred Rohde, Astrid de Greeff, Jörg Willenborg, Peter Valentin-Weigand, Baochuan Lin.
Streptococcus suis is an economically important pathogen of pigs as well as a zoonotic cause of human disease. Serotyping is used for further characterization of isolates; some serotypes seem to be more virulent and more widely spread than others. This study characterizes a collection of German field isolates of Streptococcus suis from pigs dating from 1996 to 2016 with respect to capsular genes (cps) specific for individual serotypes and pathotype by multiplex PCR and relates results to the clinical background of these isolates. The most prominent finding was the reduction in prevalence of serotype-2/serotype-1/2 among invasive isolates during this sampling period, which might be attributed to widely implemented autogenous vaccination programs in swine against serotype 2 in Germany. In diseased pigs (systemically ill; respiratory disease) isolates of serotype-1/serotype-14, serotype-2/serotype-1/2, serotype 3 to 5 and 7 to 9 were most frequent while in carrier isolates a greater variety of cps types was found. Serotype-1/serotype-14 seemed to be preferentially located in joints, serotype 4 and serotype 3 in the central nervous system, respectively. The virulence associated extracellular protein factor was almost exclusively associated with invasive serotype-1/serotype-14 and serotype-2/serotype-1/2 isolates. In contrast, lung isolates of serotype-2/serotype-1/2 mainly harbored the gene for muramidase-released protein. Serotype 4 and serotype 9 isolates from clinically diseased pigs most frequently carried the muramidase-released protein gene and the suilysin gene. When examined by transmission electron microscopy all but one of the isolates which were non-typable by molecular and serological methods showed various amounts of capsular material indicating potentially new serotypes among these isolates. Given the variety of cps types/serotypes detected in pigs, not only veterinarians but also medical doctors should consider other serotypes than just serotype 2 when investigating potential human cases of Streptococcus suis infection.
Streptococcus (S.) suis is a facultative pathogen affecting humans, feral and domestic pigs. In Asia, where humans and pigs often live in close proximity, zoonotic epidemics with large numbers of human cases and even fatalities have been recorded in 1998 and 2005 . S. suis is the most common cause of adult meningitis in Vietnam . By contrast, in Western countries infections in humans usually occur only sporadically in those persons professionally occupied with keeping, handling or slaughtering pigs or processing their meat .
In all 711 isolates from collection A (1996–2004) and B (2015–2016) investigated in this study, including all non-cps-typable isolates, gdh and recN could be detected by the two PCR assays implemented. This is in accordance with our finding that none of the isolates was assigned to serotype 20, 22, 26, 32, 33 or 34 by PCR, which are no longer considered to belong to the species S. suis but rather to other Streptococcus species .
The capsule of S. suis is a major virulence factor for several serotypes during certain stages of the infection [12, 14–20] It is also an important target for initial epidemiological characterization of this pathogen by serotyping. Until recently, serotyping required availability of rabbit hyperimmune sera. Furthermore, it was hampered by some methodological challenges, such as co-agglutination (auto-, poly- or cross-agglutination) and subjectivity of interpretation of results.