A novel intranasal mouse model for mucosal colonization by Streptococcus suis serotype 2

Abstract

Streptococcus suis causes meningitis and various other diseases in pigs and humans. Healthy piglets carrying virulent Streptococcus suis strains on their mucosal surfaces are epidemiologically very important. The objective of this study was to establish an intranasal Streptococcus suis mouse model for invasion and colonization of the respiratory tract. CD1 mice were infected intranasally with a highly virulent Streptococcus suis serotype 2 strain under different conditions. Clinical, histological and bacteriological screenings revealed that invasion of host tissue occurred in the majority of mice only after predisposition with 12.5 ml 1 % acetic acid per nostril. Severe fibrinosuppurative or purulent necrotizing pneumonia associated with Streptococcus suis was a common manifestation. Furthermore, a novel model to study nasopharyngeal colonization was established by reducing the volume of 1 % acetic acid per nostril to 5 ml prior to Streptococcus suis application. This model mimics asymptomatic carriage in swine, as all mice carried Streptococcus suis on their respiratory mucosa at 7 days post-infection (p.i.) in moderate to high numbers without the development of pneumonia or any other invasive Streptococcus suis disease. This intranasal Streptococcus suis model was applied to investigate the function of suilysin (SLY) in colonization. Although an isogenic SLY mutant was isolated from the upper respiratory tract at a lower recovery rate than its wild-type parental strain at 14 days p.i., the differences were not significant and did not indicate severe attenuation in colonization. In conclusion, this work describes to the best of our knowledge the first intranasal mouse model to study colonization of the respiratory tract by a highly virulent Streptococcus suis pathotype.

Role of Capsule and Suilysin in Mucosal Infection of Complement-Deficient Mice with Streptococcus suis

“…S. suis wild-type (WT) strain 10, kindly provided by H. Smith (Lelystad, Netherlands), is a virulent serotype 2 strain that expresses suilysin, muramidase-released protein, extracellular factor, opacity factor of S. suis, and the immunoglobulin M-degrading enzyme of S. suis (IdeSsuis) (8,26,27). It has been used by different groups successfully for experimental intranasal infections of piglets (8,26) and recently by our group in intranasal infection of mice (25). The unencapsulated isogenic mutant 10cps⌬EF and the suilysin mutant 10⌬sly were generated in previous studies by insertion of spectinomycin and erythromycin resistance gene cassettes into genes involved in capsule biosynthesis (cpsE and cpsF) and the suilysin gene of strain 10, respectively (8,28).…”

“…For experimental S. suis infection and as source of murine blood, the following mice strains with a C57BL/6J background were used at an age of 5 weeks: C57BL/6J WT mice (Charles River WIGA, Sulzfeld, Germany), C3-deficient (C3 Ϫ/Ϫ ) mice (B6.129S4 -C3 tm1Crr/J ) (29), and C5aR-deficient (C5aR Ϫ/Ϫ ) mice (B6.129S4 -C5ar1 tm1Cge/J ) (30). Intranasal infections were conducted as described previously (25), except that a lower dose was used (see below). Briefly, mice were predisposed to infection through application of 5 l of 1% acetic acid (pH 4.0) in each nostril 1 h prior intranasal infection (conducted in anesthesia via inhalation of isoflurane (IsoFlo, Albrecht, Germany).…”

“…In the competitive infection experiments, the following numbers of mice per group were used in the first (second) trials: 5 (4) Clinical score. The health status of the mice was examined twice daily using a clinical score sheet (25), including weight development, clinical signs of general sickness (rough coat, rapid breathing, and dehydration), and clinical signs indicating meningitis (apathy, apraxia) or septicemia (swollen eyes, depression). A cumulative score of 3 to 4 indicated mild clinical signs, a score of 5 to 6 indicated moderate clinical signs, and a score Ͼ6 indicated severe clinical signs with specific regard to neural failure.…”

“…Necropsy and sampling of organs for bacteriological and histological investigations were conducted exactly as described previously (25). Fibrinosuppurative and purulent lesions were scored as originally described for piglets (26) and subsequently modified for mice (25).…”

“…A mucosal mouse model for S. suis meningitis has not until now been available. We recently described an intranasal colonization model for S. suis serotype 2 in C57BL/6J mice (25). Colonization of mucosal surfaces is regarded to be the first step in the pathogenesis of S. suis diseases in piglets.…”