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MICROBIOLOGY
Pathogenic Gram-Negative
Cocci (Neisseria)
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Gram-Negative Bacteria
Constitute the largest group of human pathogens
Due in part to the presence of lipid A in the bacterial
cell wall
• Triggers fever, vasodilatation, inflammation, shock,
and disseminated intravascular coagulation (blood
clots within blood vessels)
Almost every Gram-negative bacterium that can
breach the skin or mucous membranes, grow at 37C,
and evade the immune system can cause disease and
death in humans
Neisseria
Only genus of Gram-negative cocci that regularly
causes diseases in humans
Nonmotile, aerobic bacteria often arranged as
diplococci
Distinguished from many other Gram-negative
pathogens by being oxidase positive
2 species are pathogenic to humans
The meningococcus, N. meningitidis
The gonococcus, N. gonorrhoeae
Non-pathogenic Neisseria
Non-capsulated
Can grow on ordinary media
Can grow at room temperature
Do not require extra CO2
Have different fermentation reactions
Cultural characters
They grow on enriched media (chocolate blood
agar)
They are exacting in their nutritional
requirements, Neisseria gonorrhea being more
exacting than Neisseria meningitidis)
The selective medium is Thayer Martin medium
(Chocolate blood agar + VCN)
They require extra CO2 for growth especially
upon primary isolation.
Pathogenicity
Polysaccharide capsules protect the bacteria from
the lytic enzymes within phagocytes
Fimbriae enable them to attach to human cells
Lipooligosaccharide: composed of lipid A
(endotoxin) and sugar molecules
IgA protease produced by the bacteria cleaves
secretory IgA in mucous
Can easily change its surface antigens leading to
evasion of the immune response and multiple
infections especially in Neisseria gonorrhoea
Neisseria meningitidis
Humans are the only natural carrier of N.
meningitidis
Can be a member of the normal microbiota of the
upper respiratory tract
Causes life-threatening disease when the bacteria
invade the blood or cerebrospinal fluid
Most common cause of meningitis in individuals
under 20
Respiratory droplets transmit the bacteria among
people living in close contact, especially students
living in dormitories, soldiers, prisoners &household
contacts
Neisseria meningitidis
More than 13 known antigenic types
Types A, B, C, Y & W135 are more commonly
associated with human disease
Polysaccharide capsules resist lytic enzymes
inside phagocytes
Neisseria meningitidis
Meningococcal meningitis can result in death as
early as 6 hours after initial symptoms
Initial symptoms include fever, sore throat,
headache, stiff neck, vomiting and convulsions
Meningococcal septicemia, blood poisoning, can
also be life threatening
Can produce blood coagulation and the formation of
minute hemorrhagic lesions
Diagnosis
Presence of Gram-negative diplococci in
phagocytes of the central nervous system
Treatment, and Prevention
Treatment
Penicillin, administered intravenously, is the drug of
choice
Prevention
Eradication is unlikely due to the
presence of asymptomatic carriers
Vaccination
Chemoprophylaxis for close contacts eg, rifampicin,
sulfonamides, tetracycline
Neisseria gonorrhoeae
Causes gonorrhea, a sexually transmitted disease
Gonococci adhere to epithelial cells of the
mucous membranes lining the genital, urinary, and
digestive tracts of humans spreading to deeper tissue as
they multiply
As few as 100 pairs of cells are enough to cause
disease
Gonorrhea in men
Usually symptomatic producing inflammation that
causes painful urination and purulent discharge
Can cause scarring and infertility if prostate and
epididymis are involved
Neisseria gonorrhoeae
Gonorrhea in women
Often asymptomatic (50%)
Can infect the cervix and other parts of the uterus,
including the Fallopian tubes
Can result in pelvic inflammatory disease (PID)
• Can result in ectopic pregnancy or sterility
Gonococcal infection of children can occur during
childbirth producing inflammation of the cornea
(ophthalmmia neonatorum) and sometimes blindness.
Infection of the respiratory tracts can also occur
Pathogenesis
Diagnosis and Treatment
Diagnosis
Gonorrhea in men can be identified by the presence
of Gram-negative diplococci in pus from an inflamed
penis, they appear characteristically as gram-negative
diplococci intra and extracellular)
Asymptomatic cases can be identified with
commercially available genetic probes
Treatment
Complicated due to resistant gonococcal strains
Broad-spectrum antimicrobial drugs are often used
(ceftriaxone, ciprofloxacin)
Prevention
Most effective prevention is sexual abstinence
No vaccine is available
Chemical prophylaxis is ineffective
Routine administration of antimicrobial agents to
the eyes of newborns successfully prevents ophthalmic
disease