Some bacteria have an outer capsule that serves as a virulence factor, preventing phagocytosis by the host's immune system and enhancing the organisms' ability to cause disease. This capsule is composed of polysaccharides, and completely surrounds the bacterium.
Bacterial Virulence Factors are synthesized by certain bacteria and increases their ability to infect or damage human tissues. The polysaccharide capsule is considered a virulence factor because it inhibits phagocytosis by host Macrophages and Neutrophils, allowing bacterial evasion of the host immune response. However, the immune system, with the aid of the spleen, can develop antibodies to capsular antigens. These antibodies then coat and opsonize encapsulated bacteria, allowing for their phagocytosis. Vaccines can also be generated using capsular sugars and form the basis of conjugate vaccines.
The spleen plays an important role in sequestering bacteria which are not well opsonized and is key in the production of opsonizing antibodies. This organ has an abundance of lymphoid tissue, including splenic macrophages that attack foreign organisms. These phagocytic immune cells work with capsular antibodies and opsonins to phagocytose and remove encapsulated organisms from the bloodstream. In asplenic patients, the absence of these macrophages severely diminishes the patient's ability to fight off encapsulated bacteria. Asplenia can be anatomic, usually due to splenectomy, or functional (loss of splenic function). Various conditions may lead to asplenia, such as trauma, sickle cell anemia, hemolytic anemias, Thalassemias, benign hematologic conditions, benign lymphoid disorders, and some malignant cancers such as non-Hodgkin’s lymphoma.
Streptococcus agalactiae, commonly referred to as Group B Streptococci, is a gram-positive cocci that can colonize the vagina of women and cause serious disease in infants that acquire the organism during passage through the birth canal.
Streptococcus pneumoniae is an encapsulated, gram- positive, lancet-shaped diplococci that is a major cause of pneumonia, meningitis and otitis media.
Haemophilus influenzae is an encapsulated, gram-negative coccobacilli that can cause several diseases, including meningitis, pneumonia, otitis media and epiglottitis.
Neisseria meningitidis, often called meningococcus, is an encapsulated gram-negative diplococci that can cause meningitis and life-threatening sepsis.
Escherichia coli, commonly abbreviated E. coli, is an encapsulated gram-negative bacilli often found as normal flora in the intestines. Most E. coli strains are harmless, but pathogenic strains can cause disease; including food poisoning, neonatal pneumonia and meningitis, septic shock and UTIs.
Salmonella typhi, Salmonella enterica, and other Salmonella species are encapsulated gram negative bacilli. Salmonella typhi is the causative agent for typhoid fever, while Salmonella enterica causes salmonellosis. Some other clinical conditions Salmonella spp. causes include gastroenteritis, osteomyelitis in sickle cell patients, and reactive arthritis.
Klebsiella is an encapsulated gram-negative, oxidase- negative bacilli with a prominent polysaccharide capsule. Infections can lead to a wide range of diseases including pneumonia and nosocomial urinary tract infections.
Pseudomonas aeruginosa is an encapsulated gram negative, aerobic bacilli that has been increasingly recognized as an opportunistic human pathogen that can cause infections in burn patients, osteomyelitis, endocarditis, pneumonia, and urinary tract infections.
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