Pathogenesis of bacterial infections: some determinants of virulence in Gram negative bacteria

Antigenic homology of endotoxin with a coliform mastitis vaccine strain, Escherichia coli O111:B4 (J5)

This study examined recognition of heterologous Gram-negative endotoxin by antibodies recognizing common lipopolysaccharide core antigens. Gram-negative endotoxins from 11 heterologous bacterial strains were tested for recognition by antibodies against common lipopolysaccharide core antigens. Serum was harvested from a calf immunized with the Rc mutant, Escherichia coli O111:B4 (J5), and affinity purified against endotoxin derived from an Ra mutant, Salmonella typhimurium, producing an antibody reagent recognizing homologous Gram-negative core antigens present in the Rc mutant vaccinal antigen. This reagent demonstrated reactivity against 11 chemically purified Gram-negative endotoxins. Included were endotoxins derived from 3 smooth E. coli species, 2 Salmonella spp., Shigella flexneri, Klebsiella pneumoniae, Pseudomonas aeruginosa, Serratia marcescens, and lipid A. Endotoxin derived from K. pneumoniae had significantly higher ELISA reactivity with core antigen specific antibodies than did endotoxin derived from either E. coli O111:B4 (J5) or P. aeruginosa. These results suggest immunization with R mutant bacterins may have utility in the prevention of Gram-negative mastitis even when whole bacteria react poorly with antibodies recognizing common core antigens.

Endotoxemia in horses. A review of cellular and humoral mediators involved in its pathogenesis

Endotoxemia remains the leading cause of death in horses, being intimately involved in the pathogenesis of gastrointestinal disorders that cause colic and neonatal foal septicemia. Endotoxins, normally present within the bowel, gain access to the blood across damaged intestinal mucosa, or endotoxemia occurs when gram negative organisms proliferate in tissues. Endotoxins are removed from the circulation by the mononuclear phagocyte system, and the response of mononuclear phagocytes to these lipopolysaccharides (LPS) play an important role in determining the severity of clinical disease. Macrophages become highly activated for enhanced secretory, phagocytic and cidal functions by LPS. Macrophage-derived cytokines are responsible for many of the pathophysiologic consequences of endotoxemia. The arachidonic acid metabolites, prostacyclin and thromboxane A2 likely mediate early hemodynamic dysfunction and the leukotrienes may potentiate tissue ischemia during endotoxemia. Interleukin 1 (IL-1) induces fever and is responsible for the inflammatory cascade, which constitutes the acute phase response. Tumor necrosis factor (TNF), an important proximal mediator of the effects of LPS, acts to initiate events and formation of other molecules that affect shock and tissue injury. Systemic administration of TNF produces most of the physiologic derangements that are associated with endotoxemia and antibodies that are directed against TNF significantly reduce LPS-induced mortality in experimental animals. In response to endotoxins, mononuclear phagocytes express thromboplastin-like procoagulant activity (PCA), which initiates microvascular thrombosis. Both IL-1 and TNF induce PCA expression, creating a positive feedback loop for LPS-induced coagulopathy. A macrophage-derived platelet activating factor contributes to coagulation dysfunction and further stimulates arachidonic acid metabolism. The ultimate consequences of endotoxemia are multiple system organ failure and death. The numerous feedback loops and intertwining cascades of mediators during endotoxemia defy simplistic methods of treatment. The optimal therapy likely involves methods to alter the generation of inflammatory mediators by mononuclear phagocytes.

Antigenic homology among gram-negative organisms isolated from cattle with clinical mastitis

This study examined the degree of serologic homology among mastitis pathogens. Antibodies were raised against the Rc mutant, Escherichia coli O111:B4 (strain J5) and affinity purified against lipopolysaccharide derived from the Ra mutant, Salmonella typhimurium TV119. These antibodies reacted with a battery of unrelated Gram-negative bacteria in whole cell ELISA. Bacteria with strong cross-reactions included a heterologous, smooth E. coli, Salmonella dublin, S. typhimurium, Salmonella newport, and Pseudomonas aeruginosa. Recognition of Klebsiella pneumoniae and Bordetella bronchisepticum was observed, but reactions were weaker than with the other isolates. The reduced recognition of these isolates probably reflects a masking effect of the bacterial capsule and variations in lipopolysaccharide structure. The polyclonal antibody did not recognize a Gram-positive isolate, Staphylococcus aureus. These immunoglobulins were then tested using whole cell ELISA against a panel of bacteria recovered from the mammary glands of cattle with clinical mastitis. Marked reactivity was noted against a variety of Gram-negative pathogens. Gram-positive isolates had lower recognition by Gram-negative core antigen specific immunoglobulin. The results suggest immunization with rough mutant bacteria may have broad application in the prevention of coliform mastitis.

Resistance to host immune defense mechanisms afforded by capsular material of Pasteurella haemolytica, serotype 1

Selected serum-mediated host immune defense mechanisms against Pasteurella haemolytica were studied using encapsulated and decapsulated organisms. When the capsular material was removed from P. haemolytica, it became more susceptible to serum agglutination, complement-mediated serum killing, and phagocytosis by polymorphonuclear leukocytes. When encapsulated organisms were used, phagocytosis was enhanced by antibodies to capsular material produced by vaccination of calves with any of three P. haemolytica vaccines. The serum bactericidal activity, however, was not facilitated by increased levels of anticapsular antibody in vaccinated cattle. By contrast, when decapsulated organisms were used, vaccination enhanced both the bactericidal and opsonizing capacities of sera from the calves. These studies indicate that capsular material should be considered a principal virulence factor for P. haemolytica.

Cross-reactive affinity purification of immunoglobulin recognizing common gram-negative bacterial core antigens

A procedure isolating immunoglobulins specific for common gram-negative bacterial core antigens is described. A polyclonal reagent was purified by ammonium sulfate precipitation, dialysis, and column affinity chromatography. The initial vaccinal antigen was an Ra mutant Escherichia coli O111:B4 (strain J5). The capture antigen was lipopolysaccharide derived from an Ra mutant, Salmonella typhimurium TV119 covalently-linked to an agarose matrix. Column eluants were characterized in terms of total protein concentration, IgG concentration, and EIA titer recognizing E. coli (J5). Low protein, low IgG, high EIA reading fractions were isolated, demonstrating the utility of the described technique to purify broad spectrum cross-reactive immunoglobulin reagents.

Immunity targeting common core antigens of gram-negative bacteria

Antibodies against common gram-negative core antigens can provide protection against environmental, commensual, and contagious bacteria that cause life-threatening gram-negative sepsis/endotoxemia. Cross-protective immunity may be effective against many common livestock diseases, including neonatal coliform septicemia, coliform mastitis, septic metritis, and severe enteritis. The greatest potential benefit of cross-protective immunity may be realized in the reduction of ill-thrift and mortality of neonates. The use of active and passive immunization against core antigens common to gram-negative bacteria has been successful in human beings, laboratory animals, and livestock. Protection has been observed following challenge with either heterologous endotoxin or unrelated, live, virulent gram-negative bacteria.

Humoral response in neonatal calves following immunization with Escherichia coli (strain J5): the effects of adjuvant, age and colostral passive interference

Serologic responses in 61 calves 3 to 34 days of age following immunization with bacterins containing a heat-killed rough mutant, Escherichia coli 0111:B4 (strain J5) were determined by an enzyme-linked immunosorbent assay specific for the IgG isotype. Administration of either heat-killed bacteria or oil-based adjuvants alone failed to enhance serologic recognition of common core antigens when comparing to nonvaccinate controls. Increased titers were uniquely and specifically limited to calves receiving the antigen in an oil emulsion. In a second experiment, age and initial, passively acquired titer recognizing the vaccinal antigen were not found to have any effect on the magnitude of the humoral response of 57 calves following immunization.

Plasma endotoxin concentrations in experimental and clinical equine subjects

Endotoxin (LPS) was quantitated in experimental subjects and in horses with naturally occurring gastrointestinal strangulation obstruction and/or septicaemic diseases to establish the fate of LPS and the clinical usefulness of the Limulus amoebocyte lysate (LAL) assay. The assay was validated for sensitivity (10 pg/ml), recovery (90 to 106 per cent), intra-assay precision (CV = 5.5 per cent) inter-assay precision (CV = 11 per cent), and stability of diluted, heat treated, frozen samples (at least 90 days). Plasma concentrations of LPS after sublethal (3 micrograms/kg) jugular or portal vein bolus injections of LPS rose to 4000 pg/ml and 1500 pg/ml respectively followed by a rapid phase of clearance. Peak plasma concentrations of LPS, associated with slow portal infusion, were lower than peak values associated with bolus injections, remained elevated during the infusion (2 h), but rapidly decreased after infusion was stopped. Thirty seven horses with 38 episodes of naturally occurring gastrointestinal or septicaemic disease were assayed for LPS. Eight episodes involving gastrointestinal disease and eight involving septicaemic disease were positive for LPS. It is concluded that the LAL assay is sensitive and reliable for detecting LPS in equine plasma and it may have clinical value for establishing the severity of endotoxaemia or for distinguishing between septic and non-septic conditions. Problems of rapid clearance of LPS from plasma, low concentrations, the possibility of sample contamination, and the time and method of sample procurement remain to be addressed.

Therapy of suspected septicemia in neonatal foals using plasma-containing antibodies to core lipopolysaccharide (LPS)

Equine antiserum to core lipopolysaccharide (LPS) was evaluated in a double-blind prospective study for therapeutic benefit in suspected septicemia in neonatal foals. Forty foals younger than 7 days of age were included in the study by satisfaction of clinical and laboratory criteria, suggestive of gram-negative septicemia. Twenty-two foals were treated with core LPS antiserum (plasma produced from horses which were hyperimmunized with rough gram-negative mutant bacterin) and 18 foals received "nonimmune" plasma (from horses prior to immunization against core LPS). All foals received antimicrobials, fluids, and other supportive care measures, depending on clinical signs and according to accepted current practice. The clinical and laboratory data of each foal were monitored and recorded daily for 14 days after plasma treatment or until death. The overall survival rate of these 40 foals with septicemia was 52.5%. The most prevalent diagnoses in addition to septicemia were enteritis and pneumonia. Of 30 positive bacterial cultures, 93% were due to gram-negative organisms. There was no statistically significant increase in survival rate in the 22 foals given core LPS antiserum (P greater than 0.05).