Initial steps of the pathogenesis of the infection caused by Streptococcus suis: fighting against nonspecific defenses.

Interactions between a bacterial pathogen and its potentially susceptible host are initiated with the colonization step. During respiratory/oral infection, the pathogens must compete with the normal microflora, resist defense mechanisms of the local mucosal immunity, and finally reach, adhere, and breach the mucosal epithelial cell barrier in order to induce invasive disease. This is the case during infection by the swine and zoonotic pathogen Streptococcus suis, which is able to counteract mucosal barriers to induce severe meningitis and sepsis in swine and in humans. The initial steps of the pathogenesis of S. suis infection has been a neglected area of research, overshadowed by studies on the systemic and central nervous phases of the disease. In this Review article, we provide for the first time, an exclusive focus on S. suis colonization and the potential mechanisms involved in S. suis establishment at the mucosa, as well as the mechanisms regulating mucosal barrier breakdown. The role of mucosal immunity is also addressed. Finally, we demystify the extensive list of putative adhesins and virulence factors reported to be involved in the initial steps of pathogenesis by S. suis.

Focal bacterial meningitis following ascending bite wound infection leading to paraparesis in a captive California sea lion (Zalophus californianus).

Magnetic resonance imaging was performed on a 15-yr-old captive female California sea lion (Zalophus californianus) with a 2-wk history of progressive paraparesis and a 9-mo history of exudative skin lesion on the left thoracic wall. Magnetic resonance images showed a well-defined muscle infiltrating lesion ventrolateral to the seventh cervical to the third thoracic vertebra on the left side, which extended through the left intervertebral foramina C7 to T3 into the vertebral canal, causing spinal cord compression and displacement as well as inflammation of the spinal cord and nerves. This lesion surprisingly caused no forelimb deficits. Differential diagnoses included abscess formation or neoplasia. Pathologic examination revealed chronic focal purulent meningitis associated with widespread paraspinal fistulous inflammation originating from a chronic dermal ulcer. Mainly Escherichia coli var. haemolytica and Clostridium perfringens were identified as the underlying agents.

Host-pathogen Interaction at the Intestinal Mucosa Correlates With Zoonotic Potential of Streptococcus suis.

BACKGROUND: Streptococcus suis has emerged as an important cause of bacterial meningitis in adults. The ingestion of undercooked pork is a risk factor for human S. suis serotype 2 (SS2) infection. Here we provide experimental evidence indicating that the gastrointestinal tract is an entry site of SS2 infection. METHODS: We developed a noninvasive in vivo model to study oral SS2 infection in piglets. We compared in vitro interaction of S. suis with human and porcine intestinal epithelial cells (IEC). RESULTS: Two out of 15 piglets showed clinical symptoms compatible with S. suis infection 24-48 hours after ingestion of SS2. SS2 was detected in mesenteric lymph nodes of 40% of challenged piglets. SS2 strains isolated from patients showed significantly higher adhesion to human IEC compared to invasive strains isolated from pigs. In contrast, invasive SS9 strains showed significantly higher adhesion to porcine IEC. Translocation across human IEC, which occurred predominately via a paracellular route, was significantly associated with clonal complex 1, the predominant zoonotic genotype. Adhesion and translocation were dependent on capsular polysaccharide production. CONCLUSIONS: SS2 should be considered a food-borne pathogen. S. suis interaction with human and pig IEC correlates with S. suis serotype and genotype, which can explain the zoonotic potential of SS2.

Bacterial meningitis after sinus surgery in five adult horses.

OBJECTIVE: To report meningoencephalitis as a complication after paranasal sinus surgery in 5 horses. STUDY DESIGN: Case series. ANIMALS: Adult horses (n = 5). METHODS: Medical records (2005-2010) of 5 horses that developed neurologic signs after sinus surgery were reviewed to identify potential risk factors, cause(s), or common pathways for infection. RESULTS: Underlying diseases were primary (n = 1) and secondary sinusitis (4) because of apical dental infection (1), sinus cyst (2), or masses in the ethmoturbinate region (2). Horses were treated by conventional surgical approaches and aftercare including repeated sinus lavage. Four horses had undulating pyrexia postoperatively despite antimicrobial therapy. All horses developed neurologic signs, eventually unresponsive to treatment. Suppurative meningoencephalitis was diagnosed macro- and/or microscopically on necropsy in all horses. CONCLUSION: Meningitis is a rare but fatal complication after sinus surgery in horses.

Two rare clinical manifestations of coenurosis in sheep.

Two rare clinical manifestations of coenurosis in sheep are reported. (i) A case of partial seizure disorder in a ram of 11 months old. During seizure episodes the animal lay down in lateral recumbency displaying initially a stuporous condition and subsequently began to revolve its head from the base of the cervix. At the necropsy of the case, Coenurus cerebralis cyst (young bladder worm) was found dorsally inside the brainstem, in the site of the tectum mesencephaly. (ii) The second-reported manifestation was a bacterial meningoengephalitis that was witnessed in two lambs of 6-7 weeks old. The lambs displayed lateral recumbency with seizure activity. At necropsy, meningoencephalitis with congestion and abscesses were observed in both of them. Interestingly, C. cerebralis cysts were also found in both brains. Streptococcus dysgalactiae was isolated from the abscesses. Possibly, S. dysgalactiae translocation of the blood-brain barrier was facilitated by the migration of the immature stages of C. cerebralis to and through the brain.

Fatal meningitis in a calf caused by Mannheimia varigena.

Mannheimia varigena was identified as the etiologic agent of meningitis in a young Belgian White Blue heifer calf. Species identification of the bacterium was done by phenotyping and molecularly confirmed by tDNA-PCR. Standard bacteriological examination might fail to differentiate species belonging to the genus Mannheimia.

Trojan horse effect: phagocyte-mediated Streptococcus iniae infection of fish.

The salmonid macrophage-like cell line RTS-11 and purified trout pronephros phagocytes were used to analyze in vitro entry and survival of two Streptococcus iniae serotypes. Efficient invasion by S. iniae occurred in both cells, but only the type II strain persisted in pronephros phagocytes for at least 48 h. Ex vivo models of opsonin-dependent phagocytosis by pronephros phagocytes demonstrated increased phagocytosis efficacy. Analysis of phagocytes collected from diseased fish demonstrated that approximately 70% of the bacteria contained in the blood during the septic phase of the disease were located within phagocytes, suggesting an in vivo intracellular lifestyle. In addition to the augmented levels of bacteremia and enhanced survival within phagocytes, S. iniae type II induces considerable apoptosis of phagocytes. These variabilities in intramacrophage lifestyle might explain differences in the outcomes of infections caused by different serotypes. The generalized septic disease associated with serotype II strains is linked not only to the ability to enter and multiply within macrophages but also to the ability to cause considerable death of macrophages via apoptotic processes, leading to a highly virulent infection. We assume that the phenomenon of survival within phagocytes coupled to their apoptosis plays a crucial role in S. iniae infection. In addition, it may provide the pathogen an efficient mechanism of translocation into the central nervous system.

Role of suilysin in pathogenesis of Streptococcus suis capsular serotype 2.

Three suilysin (SLY) knockout mutant strains of Streptococcus suis serotype 2 were generated by allelic replacement from one North American and two European wild type strains. The mutants were characterized by Southern blot, Western blot and phenotyping. In vitro bactericidal testing showed that both wild type and SLY mutants were resistant to bactericidal factors in whole pig blood. To demonstrate the role of SLY during S. suis infection, four animal trials were carried out using young pigs. Either high dose (4 x 10(6)CFU/ml/pig) or low dose (0.5 x 10(6)CFU/ml/pig) live cell aerosol was applied to the pharynx. In one trial, a low challenge dose of North American strain SX332 and its isogenic sly(-) mutant strain (SX932) resulted in acute disease in 3/5 of pigs exposed to the wild type strain, while 5/5 of pigs exposed to the mutant strain survived the trial. In the repeat trial, 1/8 of pigs in wild type group and 6/8 of pigs in mutant group developed disease. The high dose trial with 332/932 pair showed that 4/8 pigs challenged with wild type and 5/8 of pigs challenged with mutant strain developed disease respectively. The third low dose trial, using European strain 31533 and its isogenic sly(-) mutant strain SX911, showed that 1/8 of pigs challenged with the wild type strain and 4/8 of pigs challenged with the corresponding mutant strain developed disease. All the diseased pigs showed fever, clinical signs and developed septicemia. S. suis was isolated from tissue samples such as brain, submandibular lymph node, lung, spleen, liver, heart or joint. Serum antibody titer against cell surface proteins changed little while the antibody titer against SLY increased only in the wild type group after challenge. sly gene was cloned and expressed in E. coli. The recombinant SLY (rSLY) protein showed 800 hemolysin units per microg protein. In vitro study showed that rSLY triggered TNFalpha production by human monocytes and IL-6 production by pig pulmonary alveolar macrophages and monocytes. Thus, the results of this study suggest that SLY does not seem to be a critical virulence factor for S. suis serotype 2 respiratory infection, but by stimulating cytokine release it may play a role in innate immunity.

Relatedness of Streptococcus suis isolates of various serotypes and clinical backgrounds as evaluated by macrorestriction analysis and expression of potential virulence traits.

We evaluated the genetic diversity of Streptococcus suis isolates of different serotypes by macrorestriction analysis and elucidated possible relationships between the genetic background, expression of potential virulence traits, and source of isolation. Virulence traits included expression of serotype-specific polysaccharides, muramidase-released protein (MRP), extracellular protein factor (EF), hemolysin activity, and adherence to epithelial cells. Macrorestriction analysis of streptococcal DNA digested with restriction enzymes SmaI and ApaI allowed differentiation of single isolates that could be assigned to four major clusters, named A1, A2, B1, and B2. Comparison of the genotypic and phenotypic features of the isolates with their source of isolation showed that (i) the S. suis population examined, which originated mainly from German pigs, exhibited a genetic diversity and phenotypic patterns comparable to those found for isolates from other European countries; (ii) certain phenotypic features, such as the presence of capsular antigens of serotypes 2, 1, and 9, expression of MRP and EF, and hemolysin activity (and in particular, combinations of these features), were strongly associated with the clinical background of meningitis and septicemia; and (iii) isolates from pigs with meningitis and septicemia showed a significantly higher degree of genetic homogeneity compared to that for isolates from pigs with pneumonia and healthy pigs. Since the former isolates are considered highly virulent, this supports the theory of a clonal relationship among highly virulent strains.

Interactions between Streptococcus suis serotype 2 and different epithelial cell lines.

Streptococcus suis is an important swine pathogen responsible for cases of sudden death, septicaemia, meningitis, endocarditis and pneumonia. It is also recognized as a zoonotic agent in people occupationally exposed to pigs or pig products. Knowledge on virulence factors of S. suis serotype 2 is limited and the pathogenesis of the infection is poorly understood. It has been suggested that the disease due to S. suis serotype 2 begins with colonization of the nasopharyngeal epithelium, followed by either spread within the respiratory tract or invasion of the bloodstream. The mechanisms involved in the access of bacteria from the bloodstream to the central nervous system are unknown. It is possible that epithelial cells of the choroid plexus also play an important role in the pathogenesis of the meningitis. Different interactions (adhesion, invasion and toxic effects) of S. suis serotype 2 with epithelial cell lines [LLC-PK1, PK(15), A549, HeLa and MDCK] were studied and compared to those of a human pathogen which also causes meningitis, group B Streptococcus (GBS). The results showed that S. suis serotype 2, in contrast to GBS, is able to adhere to but not to invade epithelial cells. The adhesin(s) involved seem(s) to be partially masked by the capsule and are a part of the cell wall. The haemolysin produced by S. suis serotype 2 is responsible for a toxic effect observed on epithelial cells. The results described give additional evidence that pathogenesis of the infection differs between S. suis and GBS. In particular, it is possible that suilysin-positive S. suis strains use adherence and cell injury, as opposed to direct cellular invasion, as part of a complicated multistep process which leads to bacteraemia and meningitis in pigs.

Endocarditis and meningitis caused by Streptococcus suis after cardiac surgery in a sheep.

A female Dorset-cross sheep developed labored respirations and was anorexic, weak, ataxic and febrile (42.0 degrees C) 15 days after implantation of a cardiac device. Clinical pathologic evaluation identified a mild leukocytosis (11,800 cells/microl), neutrophilia (8,969 cells/microl), and hypokalemia (2.9 mmol/L). Despite intensive therapy, the animal was euthanized 7 days later. Necropsy of the sheep revealed meningitis and vegetative endocarditis associated with the cardiac device. Blood cultures collected antemortem and tissue cultures collected at necropsy yielded Streptococcus suis. Histopathology confirmed the diagnosis of bacterial valvular endocarditis and meningoencephalitis. A variety of environmental and host factors are proposed as contributing to the infection, and the zoonotic potential of S. suis is discussed.

Granulomatous leptomeningitis in beagle dogs.

A granulomatous leptomeningitis was seen in seven laboratory Beagle dogs (four males, three females), 14 to 15 months old, that had been used as control or experimental subjects in a toxicologic study. The dogs were clinically normal during the experimental period. Microscopic lesions were characterized by typical noncaseating granulomas in leptomeninges and slight scattered perivascular cuffing in gray and white matter throughout the central nervous system, with no site of predilection. Although no microorganisms could be found by ordinary light microscopic examination, there was a positive reaction for Escherichia coli antigen in the cytoplasm of macrophages, which corresponded to periodic acid-Schiff-positive, calcium-negative, and iron-negative materials in the granulomas and cuffs. Electron microscopic examination revealed that these materials were large phagolysosomes indicative of abnormal lysosomal function. These findings indicate that at least some cases of canine granulomatous leptomeningitis could be very likely caused by E. coli and represent an entity distinct from classical canine granulomatous meningoencephalitis.