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PubMed1983

Synovial fluid inhibits killing of Staphylococcus aureus by neutrophils.

Serum in the extracellular environment promotes neutrophil bactericidal activity apart from its opsonizing properties. We examined the effect of non-inflammatory osteoarthritic synovial fluid on serum-mediated neutrophil killing of Staphylococcus aureus. This was done to evaluate the effect of synovial fluid on neutrophil bactericidal activity independent of opsonin concentration. With an initial inoculum of 5 X 10(6) CFU/ml, 1.47 +/- 0.14% bacteria survived after 120 min of incubation with 10% serum and neutrophils. In contrast, 4.07 +/- 0.33% bacteria survived after incubation in serum plus synovial fluid (P less than 0.001). This inhibitory effect was directly related to the concentration of synovial fluid in the incubation mixture. Increasing the concentration of synovial fluid resulted in an increased percent survival. Studies utilizing preopsonized bacteria and radiolabeled organisms demonstrated that synovial fluid did not interfere with opsonization or phagocytosis. Intracellular bactericidal activity was assayed separately from phagocytosis by utilizing a brief ingestion period followed by the removal of extracellular bacteria by either differential centrifugation or lysostaphin treatment. The reincubation of cells and associated bacteria with serum or serum plus synovial fluid revealed that synovial fluid significantly inhibited the promoting effect of serum on neutrophil bactericidal activity. After 60 min of incubation with 10% serum, 13.0 +/- 1.2% bacteria survived, whereas 21.5 +/- 2.3% survived after incubation in serum plus synovial fluid (P less than 0.005). Superoxide production was not affected by the presence of synovial fluid. These findings suggest that the inhibitory effect of synovial fluid is due to an interaction between synovial fluid and the serum factors that promote intracellular killing.

Blood Bactericidal Activity↗
PubMed1983

Opsonic activity of blister fluid from burn patients.

The combination of skin loss and immune depression after thermal injury predisposes burn patients to an increased risk of infection. Since the commonest site of infection in the burn patient is the burn wound itself, we elected to study the opsonic activity of locally produced blister fluid, from 18 thermally injured patients, for the two most common organisms colonizing the burn wound (Pseudomonas aeruginosa and Staphylococcus aureus). Blister fluid was as good an opsonin source for staphylococcus as normal serum. In contrast, the blister fluid did not support either the phagocytosis of the intracellular killing of P. aeruginosa. The poor opsonic activity of blister fluid for P. aeruginosa did not appear to be due to the presence of an inhibitory factor(s) since the addition of normal serum restored the opsonic activity of the blister fluid to normal. The concentrations of immunoglobulins and the complement components C3 and C4 in the blister fluid samples were less than half the level of those in normal serum. The opsonic activity of the blister fluid could not be restored to normal by the addition of either immunoglobulin or heat-inactivated serum (56 degrees C for 30 min). Thus, the opsonic factor(s) missing from the blister fluid was heat labile and thus probably represents complement components. That blister fluid had impaired opsonic activity for P. aeruginosa but not for S. aureus indicated that a local humoral defect may be responsible, at least in part, for the high incidence of gram-negative organisms, especially pseudomonads, colonizing the burn wound after thermal injury.

Adolescent↗
PubMed1984

Polyamino acid enhancement of bacterial phagocytosis by human polymorphonuclear leukocytes and peritoneal macrophages.

Cationic polyamino acids are known to enhance a variety of cell-cell interactions by virtue of their ability to alter electrostatic forces of cell surfaces. In this study, the effect of polyamino acids on phagocytosis of 3H-labeled bacteria by human polymorphonuclear leukocytes (PMNs) and peritoneal macrophages was investigated. Negatively charged and neutral polyamino acids did not influence phagocytosis of unopsonized Staphylococcus epidermidis, whereas protamine, poly-L-arginine, and poly-L-lysine stimulated phagocytosis in a dose-dependent manner. At 50 micrograms/ml, greater than 30% uptake by PMNs was seen with each of these cationic polyamino acids. Although cationic polyamino acids promoted PMN and peritoneal macrophage phagocytosis of unopsonized S. epidermidis, Staphylococcus aureus M (encapsulated) and M variant (unencapsulated), and Escherichia coli J5, little effect was seen with the parent E. coli O111:B4 or a serotype O222:H16 strain. Pretreatment of bacteria and phagocytes separately demonstrated that the phagocytosis-promoting property of polyamino acids is manifest predominantly on the bacteria. Bacteria pretreated with cationic polyamino acids also elicited a PMN chemiluminescent response, and PMN-associated bacteria were killed, as determined by a fluorochrome microassay. Thus, cationic polyamino acids promote the phagocytosis and killing of many but not all bacterial strains, and in this respect polyamino acids function as opsonins.

Escherichia coli↗
PubMed1984

Nonopsonic phagocytosis of strains of Pseudomonas aeruginosa from cystic fibrosis patients.

Pseudomonas aeruginosa is the predominant respiratory pathogen in patients with cystic fibrosis, but its mechanism of persisting in pulmonary secretions is poorly understood. We observed that three nonmucoid cystic fibrosis P. aeruginosa strains were phagocytized and one strain resisted phagocytosis by human polymorphonuclear leukocytes in the absence of serum. Phagocytosis was assessed by luminol-enhanced chemiluminescence, inspection of stained smears, bactericidal assay, reduction of nitroblue tetrazolium dye, and electron microscopy. Phagocytosis, determined by visual inspection, occurred at 35 degrees C but not at 4 degrees C. Nonopsonic phagocytosis was inhibited most efficiently by D-mannose, mannose-containing saccharides, and D-fructose. Opsonin-dependent phagocytosis of P. aeruginosa and of zymosan was not markedly inhibited by mannose, suggesting different leukocyte receptors for nonopsonic and opsonic phagocytosis.

Adult↗
PubMed1987

Oxidative response of human neutrophils, monocytes, and alveolar macrophages induced by unopsonized surface-adherent Staphylococcus aureus.

In contrast to results with bacterial suspensions, phagocytosis of unopsonized bacteria readily occurs when bacteria are adhered to glass or plastic surfaces. However, in contrast to neutrophils, alveolar macrophages produced much less DNA denaturation as measured by acridine orange metachromasia of phagocytized Staphylococcus aureus. We have studied the phagocytosis of unopsonized surface-adherent S. aureus and the subsequent production of reactive oxygen species by peripheral blood neutrophils, monocytes, and alveolar macrophages. Phagocyte-free systems were then used to show the relationship of the reactive oxygen species produced by neutrophils and alveolar macrophages and the denaturation of unopsonized S. aureus DNA with acridine orange. Peripheral blood neutrophils, monocytes, and alveolar macrophages from normal human volunteers were added to vials with adherent S. aureus without opsonin. Bacterial uptake and luminol- and lucigenin-dependent chemiluminescence were measured. Neutrophils developed much greater luminol-dependent chemiluminescence than monocytes or alveolar macrophages. Compared with neutrophils and monocytes, alveolar macrophages developed significantly greater concentrations of superoxide, as measured by lucigenin-dependent chemiluminescence and ferricytochrome c reduction. These findings suggested that products of the myeloperoxidase-hydrogen peroxide-halide pathway were generated when peripheral blood neutrophils were stimulated and that alveolar macrophages primarily produced superoxide. When these reactive oxygen species were generated in phagocyte-free systems containing S. aureus, products of the myeloperoxidase-hydrogen peroxide-halide pathway produced denaturation of S. aureus DNA, whereas superoxide did not. Thus, differences in reactive oxygen species produced during phagocytosis may be related to the different capacities of neutrophils and alveolar macrophages to denature unopsonized adherent S. aureus DNA.

Acridine Orange↗
PubMed1990

Opsonic activity of cerebrospinal fluid in experimental cryptococcal meningitis.

The role of antibody in protection against infection with Cryptococcus neoformans is undefined. In this paper we describe the development of opsonic activity in the cerebrospinal fluid (CSF) of rabbits in response to cryptococcal meningitis. The opsonin appeared to be immunoglobulin G (IgG); the activity was heat stable, copurified with the IgG fraction during protein A separation, and could be absorbed by encapsulated cryptococci. Immunosuppression with cyclosporine could be administered to prevent or allow in vivo deposition of IgG on the polysaccharide capsule of yeast in the CSF. Both early and late cyclosporine regimens resulted in prolonged, severe meningeal infections corresponding to the complete absence of in vitro opsonic activity in the CSF. While the production of opsonic antibody is part of the successful host response against C. neoformans in the central nervous system of rabbits, the presence of specific immunoglobulin by itself is insufficient for complete protection.

Animals↗
PubMed1991

Phagocytosis of virulent Porphyromonas gingivalis by human polymorphonuclear leukocytes requires specific immunoglobulin G.

No studies to date clearly define the interactions between Porphyromonas gingivalis and human peripheral blood polymorphonuclear leukocytes (PMN), nor has a protective role for antibody to P. gingivalis been defined. Using a fluorochrome phagocytosis microassay, we investigated PMN phagocytosis and killing of P. gingivalis as a function of P. gingivalis-specific antibody. Sera from a nonimmune rabbit and a healthy human subject were not opsonic for virulent P. gingivalis A7436, W83, and HG405; phagocytosis of these strains (but not 33277) required opsonization with hyperimmune antiserum (RaPg). Diluting RaPg with a constant complement source decreased proportionally the number of P. gingivalis A7436 cells phagocytosed per phagocytic PMN. Enriching for the immunoglobulin G fraction of RAPg A7436 enriched for opsonic activity toward A7436. An opsonic evaluation of 18 serum samples from adult periodontitis patients revealed that only 3 adult periodontitis sera of 17 with elevated immunoglobulin G to P. gingivalis A7436 were opsonic for A7436 and, moreover, that the serum sample with the highest enzyme-linked immunosorbent assay titer was most opsonic (patient 1). However, the opsonic activity of serum from patient 1 was qualitatively and not just quantitatively different from that of the nonopsonic human sera (but was less effective opsonin than RaPg). Strain variability was observed in resistance of P. gingivalis to phagocytosis, and opsonization was strain specific for some, but not all, strains tested. An evaluation of killing of A7436 revealed that serum killing and extracellular killing of P. gingivalis were less effective alone when compared with intracellular PMN killing alone.

Adult↗
PubMed1994

Direct activity of human T lymphocytes and natural killer cells against Cryptococcus neoformans.

Lymphocytes constitute a critical component of host defenses against cryptococcosis. Previously, we demonstrated that human lymphocytes cultured with interleukin-2 formed conjugates with, and directly inhibited the growth of, Cryptococcus neoformans. Here, we explore the anticryptococcal activity of freshly isolated, highly purified populations of human peripheral blood lymphocytes. Lymphocytes were incubated with encapsulated C. neoformans for 24 h, after which the lymphocytes were lysed, dilutions and spread plates were made, and CFU were counted. Fungistasis was determined by comparing growth in wells with and without lymphocytes. Nylon wool-nonadherent peripheral blood mononuclear cells (NWNA PBMC) were highly fungistatic, even if either T cells or natural killer (NK) cells were depleted by panning. A mixed population of T cells and NK cells, obtained by rosetting NWNA PBMC with sheep erythrocytes, completely inhibited cryptococcal growth, whereas the nonrosetting cells had little fungistatic activity. CD4+, CD8+, and CD16/56+ lymphocytes, isolated by positive immunoselection, had potent growth-inhibitory activity. In contrast, purified B cells had no activity. Fungistasis was seen even in the absence of opsonins. Antifungal activity was markedly diminished when surface receptors on NWNA PBMC were cleaved by treatment with trypsin or bromelain. Supernatants from stimulated lymphocytes or concentrated lymphocyte sonicates were not active. Lymphocyte-mediated fungistasis was seen with two different strains of C. neoformans. CD4+, CD8+, and CD16/56+ lymphocytes formed conjugates with C. neoformans, as observed under Nomarski differential interference contrast microscopy and videomicroscopy. These data demonstrate that freshly isolated peripheral blood T cells and NK cells have the capacity to bind and directly inhibit the growth of C. neoformans.

Antigens, Fungal↗
PubMed1995

Binding of host collectins to the pathogenic yeast Cryptococcus neoformans: human surfactant protein D acts as an agglutinin for acapsular yeast cells.

Cryptococcus neoformans is an opportunistic pathogen in AIDS patients causing disseminated disease and lethal meningitis after inhalation of acapsular or sparsely encapsulated yeast cells. In this study we have investigated whether a recently described family of primitive opsonins, termed collectins, contribute to innate resistance against C. neoformans. The pulmonary surfactant proteins SP-A and SP-D as well as the serum collectins mannose-binding protein and CL-43 bound in a calcium-dependent manner to acapsular C. neoformans in vitro. Binding was concentration dependent and abolished by competition with defined mono- and oligosaccharides. In contrast, no binding of the collectins was observed with the encapsulated form of the yeast. Furthermore, binding of purified collectin SP-D, but not SP-A, mannose-binding protein, or CL-43, led to a concentration-dependent agglutination of acapsular C. neoformans. These data indicate that collectins recognize carbohydrate structures in the cell wall of an initial infectious form of C. neoformans and may play a role in early antifungal defenses in the lung.

Agglutinins↗
PubMed1996

Burkholderia pseudomallei activates complement and is ingested but not killed by polymorphonuclear leukocytes.

The mechanism by which Burkholderia pseudomallei is resistant to lysis by human serum is unknown but may include interference with complement activation, effective opsonization, or complement-mediated lysis. We investigated the interaction of B. pseudomallei with complement in the presence and absence of specific antibody to determine potential mechanisms of serum resistance. We demonstrated rapid activation and consumption of complement by B. pseudomallei which, in the absence of specific antibody, occurred predominantly via the alternative pathway. Complement activation was associated with deposition of the opsonically active C3b and iC3b fragments on the bacterial surface. C5b-9, detected on the bacterial surface after opsonic periods of 1 to 60 min, was susceptible to elution by 1 M NaCl, indicating that resistance to complement-mediated lysis may result from deposition of the membrane attack complex in a nonmicrobicidal location. To define the role of opsonins, we investigated the ability of polymorphonuclear leukocytes (PMNL) to phagocytose B. pseudomallei. Phagocytosis of bacteria by PMNL, and the observed oxidative response, was significantly increased by opsonization of organisms with complement and/or specific antibody. Despite opsonophagocytosis by PMNL and the production of an oxidative response, no significant bacterial killing was observed.

Burkholderia pseudomallei↗
PubMed1997

Enhancement of neutrophil-mediated killing of Plasmodium falciparum asexual blood forms by fatty acids: importance of fatty acid structure.

Effects of fatty acids on human neutrophil-mediated killing of Plasmodium falciparum asexual blood forms were investigated by using a quantitative radiometric assay. The results showed that the antiparasitic activity of neutrophils can be greatly increased (>threefold) by short-term treatment with fatty acids with 20 to 24 carbon atoms and at least three double bonds. In particular, the n-3 polyenoic fatty acids, eicosapentaenoic and docosahexaenoic acids, and the n-6 fatty acid, arachidonic acid, significantly enhanced neutrophil antiparasitic activity. This effect was >1.5-fold higher than that induced by an optical concentration of the known agonist cytokine tumor necrosis factor alpha (TNF-alpha). At suboptimal concentrations, the combination of arachidonic acid and TNF-alpha caused a synergistic increase in neutrophil-mediated parasite killing. The fatty acid-induced effect was independent of the availability of serum opsonins but dependent on the structure of the fatty acids. The length of the carbon chain, degree of unsaturation, and availability of a free carboxyl group were important determinants of fatty acid activity. The fatty acids which increased neutrophil-mediated killing primed the enhanced superoxide radical generation of neutrophils in response to P. falciparum as detected by chemiluminescence. Scavengers of oxygen radicals significantly reduced the fatty acid-enhanced parasite killing, but cyclooxygenase and lipoxygenase inhibitors had no effect. These findings have identified a new class of immunoenhancers that could be exploited to increase resistance against Plasmodium species.

Adjuvants, Immunologic↗
PubMed1999

Relationship between cell surface carbohydrates and intrastrain variation on opsonophagocytosis of Streptococcus pneumoniae.

Streptococcus pneumoniae undergoes spontaneous phase variation between a transparent and an opaque colony phenotype, the latter being more virulent in a murine model of sepsis. Opaque pneumococci have previously been shown to express lower amounts of C polysaccharide (cell wall teichoic acid) and in this study were shown to have a higher content of capsular polysaccharide by immunoelectron microscopy. This report then examined the relationship between expression of these two cell surface carbohydrate structures and their relative contribution to the increased virulence of opaque variants. Comparison of genetically related strains showed that the differential content of capsular polysaccharide did not affect the amount of teichoic acid as measured by a capture enzyme-linked immunosorbent assay (ELISA). In contrast, when the teichoic acid structure was altered by replacing choline in the growth medium with structural analogs, the quantity of capsular polysaccharide as measured by a capture ELISA was decreased, demonstrating a linkage in the expression of the two surface carbohydrate structures. A standardized assay was used to assess the relative contribution of cell surface carbohydrates to opsonophagocytosis. The opaque variants required 1.2- to 30-fold more immune human serum to achieve 50% opsonophagocytic killing than did related transparent variants (types 6B and 9V). The opsonophagocytic titer was proportional to the quantity of capsular polysaccharide rather than teichoic acid. The major factor in binding of the opsonin, C-reactive protein (CRP), was also the amount of capsular polysaccharide rather than the teichoic acid ligand. Only for the transparent variant (type 6B), which bound more CRP, was there enhanced opsonophagocytic killing in the presence of this serum protein. Increased expression of capsular polysaccharide, therefore, appeared to be the major factor in the decreased opsonophagocytic killing of opaque pneumococci.

Animals↗
PubMed2000

Effects of opsonization and gamma interferon on growth of Brucella melitensis 16M in mouse peritoneal macrophages in vitro.

Entry of opsonized pathogens into phagocytes may benefit or, paradoxically, harm the host. Opsonization may trigger antimicrobial mechanisms such as reactive oxygen or nitric oxide (NO) production but may also provide a safe haven for intracellular replication. Brucellae are natural intramacrophage pathogens of rodents, ruminants, dogs, marine mammals, and humans. We evaluated the role of opsonins in Brucella-macrophage interactions by challenging cultured murine peritoneal macrophages with Brucella melitensis 16M treated with complement- and/or antibody-rich serum. Mouse serum rich in antibody against Brucella lipopolysaccharide (LPS) (aLPS) and human complement-rich serum (HCS) each enhanced the macrophage uptake of brucellae. Combinations of suboptimal levels of aLPS (0. 01%) and HCS (2%) synergistically enhanced uptake. The intracellular fate of ingested bacteria was evaluated with an optimal concentration of gentamicin (2 microg/ml) to control extracellular growth but not kill intracellular bacteria. Bacteria opsonized with aLPS and/or HCS grew equally well inside macrophages in the absence of gamma interferon (IFN-gamma). Macrophage activation with IFN-gamma inhibited replication of both opsonized and nonopsonized brucellae but was less effective in inhibiting replication of nonopsonized bacteria. IFN-gamma treatment of macrophages with opsonized or nonopsonized bacteria enhanced NO production, which was blocked by N(G)-monomethyl L-arginine (MMLA), an NO synthesis inhibitor. MMLA also partially blocked IFN-gamma-mediated bacterial growth inhibition. These studies suggest that primary murine macrophages have limited ability to control infection with B. melitensis, even when activated by IFN-gamma in the presence of highly opsonic concentrations of antibody and complement. Additional cellular immune responses, e.g., those mediated by cytotoxic T cells, may play more important roles in the control of murine brucellosis.

Animals↗
PubMed2001

Role of complement in Mycobacterium avium pathogenesis: in vivo and in vitro analyses of the host response to infection in the absence of complement component C3.

We investigated the importance of the host complement system in the pathogenesis of disease mediated by the intramacrophage pathogen Mycobacterium avium. Mycobacteria opsonized with complement are efficiently ingested by macrophages through various complement receptors. Furthermore, unlike other bacteria, mycobacteria can activate both the alternative and classical complement pathways in the absence of specific antibodies. Therefore, to examine the role of complement in the mycobacterial infection process in vivo, mice deficient in complement component C3 were infected with M. avium. Surprisingly, C3-deficient mice infected intravenously with M. avium displayed no difference in bacterial burden or granulomatous response compared to wild-type control mice. C3-sufficient mice and C3-deficient mice were equally susceptible to infection by M. avium regardless of the genotype at the bcg locus, a locus known to confer susceptibility to infection with intracellular pathogens. In vitro studies using mouse bone marrow-derived macrophages resulted in significant M. avium invasion of macrophages in the absence of C3; however, the kinetics of infection were delayed compared to complement-mediated invasion. The data indicate that complement does not play an essential role in mediating M. avium infections in the mouse and suggest either that other invasion mechanisms can compensate for the absence of complement-mediated entry or that complement is not a major mycobacterial opsonin in vivo.

Animals↗
PubMed2001

Mannan-binding lectin enhances susceptibility to visceral leishmaniasis.

Levels of the serum opsonin mannan-binding lectin (MBL) were directly correlated with the probability of developing visceral leishmaniasis. Monocytes infected with MBL-opsonized Leishmania chagasi promastigotes secreted higher levels of tumor necrosis factor alpha and interleukin-6 than cells infected with nonopsonized parasites. Our findings indicate that MBL can modulate the clinical outcome of infection with L. chagasi and the function of infected macrophages.

Adolescent↗
PubMed2002

Intracellular survival of Leishmania major in neutrophil granulocytes after uptake in the absence of heat-labile serum factors.

The role of polymorphonuclear neutrophil granulocytes (PMN) in defense against the intracellular parasite Leishmania is poorly understood. In the present study, the interaction of human PMN with Leishmania major promastigotes was investigated in vitro. In the presence of fresh human serum, about 50% of PMN phagocytosed the parasites within 10 min and the parasite uptake led to PMN activation, resulting in the killing of most ingested parasites. Heat inactivation of the serum markedly reduced the rate of early parasite phagocytosis, suggesting a role of complement components in the early uptake of Leishmania. However, over 50% of PMN were able to ingest parasites in the presence of heat-inactivated serum if the coincubation was extended to 3 h. After 3 h, 10% of the PMN were found to internalize Leishmania even under serum-free conditions. These findings indicate that PMN possess mechanisms for both opsonin/complement-dependent and -independent uptake of Leishmania. Both pathways of uptake could be partially blocked by anti-CR3 antibody. Mannan-binding lectin was found not to be involved in this process. When phagocytosed in the absence of opsonin, the majority of Leishmania parasites survived intracellularly in PMN for at least 1 day. These data suggest a dual role of PMN in the early response to L. major infection. On the one hand, PMN can rapidly eliminate the intracellular parasites, and on the other hand, Leishmania can survive intracellularly in PMN. These data, together with the finding that intact parasites were seen in PMN isolated from the skin of infected mice, suggest that PMN can serve as host cells for the intracellular survival of Leishmania within the first hours or days after infection.

Animals↗
PubMed2002

Opsonization modulates Rac-1 activation during cell entry by Leishmania amazonensis.

Lesions caused by Leishmania amazonensis normally heal, but relapses occur due to parasite persistence in host tissues. It has been proposed that infection of fibroblasts plays an important role in this process by providing the parasites with a safe haven in which to replicate. However, most previous studies have focused on the entry of Leishmania into macrophages, a process mediated by serum opsonins. To gain insight into a possible role of nonopsonic entry in the intracellular persistence of amastigotes, we examined the invasion of Chinese hamster ovary (CHO) cells. Amastigotes entered CHO cells by a cytochalasin D, genistein, wortmannin, and 2,3-butanedione monoxime-sensitive pathway and replicated within phagolysosomes. However, unlike most phagocytic processes described to date, amastigote internalization in CHO cells involved activation of the GTPases Rho and Cdc42 but not Rac-1. When uptake was mediated by fibronectin or when amastigotes were opsonized with immunoglobulin G and internalized by Fc receptor-expressing CHO cells, Rac-1 activation was restored and found to be required for parasite internalization. Given the essential role of Rac in assembly of the respiratory burst oxidase, invasion through this nonopsonic, Rac-1-independent pathway may play a central role in the intracellular survival of Leishmania in immune hosts.

Animals↗
PubMed2003

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.

Animals↗