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Biology LibreTexts Neisseria meningitidis

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    Neisseria gonorrhoeae diplococci

    Electron micrograph of Neisseria. ("Neisseria gonorrhoeae diplococci" by Microbe World is licensed under CC BY-NC-SA 2.0)


    • Commonly referred to as the meningococcus
    • Neisseria meningitidis is a Gram-negative diplococcus, typically flattened where the cocci meet.
    • Oxidase positive
    • Aerobic
    • There are 13 serogroups of meningococci. Serogroups B and C commonly cause meningitis and meningococcemia in developed countries; serogroups Y and W135 typically cause pneumonia.


    • Humans are the only natural host.


    • Transmitted person-to-person by aerosolized respiratory tract secretions.

    Clinical Disease

    • One of the three major causes of bacterial meningitis (the others being Strep. pneumoniae and Haemophilus influenzae)
    • There are between 2000 and 3000 cases of meningococcal meningitis per year in the U.S. A total of 2725 cases were reported to CDC in 1998.
    • N. meningitidis infects the nasopharynx of humans causing a usually mild or subclinical upper respiratory infection. However in about 15% of these individuals, the organism invades the blood and disseminates, causing septicemia (specifically meningococcemia) and from the there may cross the blood-brain barrier causing meningitis  A petechial skin rash (purple splotches) (Figure \(\PageIndex{1}\)), caused by endotoxin in the blood, appears in about 75 percent of the septic cases and fatality rates for meningococcal septicemia (meningococcemia) are as high as 30 percent as a result of the shock cascade (endotoxic shock). A fulminating form of the disease, called Waterhouse-Frederichsen syndrome, can be fatal within several hours due to massive intravascular coagulation and resulting shock, probably a result of massive endotoxin release. N. meningitidis is especially dangerous in young children.
    • Typical symptoms are headache, meningeal signs, and fever.
    • Mortality is close to 100% if untreated; less than 10% with prompt and appropriate antibiotic therapy.
    • Effective vaccines targeting the polysaccharide capsules of major pathogenic strains (serogroups) are available.

    Figure \(\PageIndex{1}\): Petechiae (1-2 mm) and purpura (3-10 mm) are caused by minor bleeding of capillaries (2010; 

    James Heilman, MDCC BY-SA 3.0 )

    Primary Virulence Factors

    • Polysaccharide capsule prevents phagocytosis
    • Fimbriae allow adherence to tissues
    • Endotoxin.  N. meningitidis displays an unusual behavior called "blebbing" (Figure \(\PageIndex{2}\)) in which bits of the outer membrane (containing LPS/endotoxin) are released from the cell while it is alive (in most cases LPS is only released when a cell lyses).  This results in the extreme inflammation and endotoxic shock associated with N. meningitidis.



    Figure \(\PageIndex{2}\): Neisseria meningtidis blebbing. Transmission electron micrograph 45,000X.  Large arrows point to cell well blebs. (1973; The Rockefeller University Press under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 Unported license, as described at

    From Meningococcal Infections, by Thomas A Hoffman, MD, Professor, Department of Internal Medicine, Division of Infectious Diseases, Jackson Memorial Hospital, University of Miami. Neisseria meningitidis is shared under a CC BY license and was authored, remixed, and/or curated by LibreTexts.

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