Neisseria meningitides: Properties of the Bacteria

Neisseria meningitides

N. meningitidis causes a spectrum of diseases ranging frommeningococcemia (which is rapidly fatal) to a transient bac-teremia (which is relatively benign). It is also the second most common cause of community-acquired meningitis in adults.

Properties of the Bacteria

◗ Morphology

N. meningitidis shows following features:

·           N. meningitidis are Gram-negative, spherical, or oval cocciarranged in pairs with the adjacent sides flattened. The cocci are generally intracellular in PMN in smears from pus cells and other specimens.

·           They measure 0.6–0.8 mm in diameter.

·           Freshly isolated bacteria are usually capsulated.

·           They are nonmotile and nonsporing.

◗ Culture

Meningococci are strict aerobes. They grow optimally at a tem-perature between 36°C and 39°C and optimum pH of 7.4–7.6. Their growth is enhanced by incubation in a moist atmosphere in the presence of 5% CO2.

Meningococci are fastidious bacteria with complex nutri-tional requirements. They do not grow on ordinary media, but grow well on the medium enriched with blood or serum, such as blood agar, chocolate agar, and Mueller–Hinton agar. The blood or serum promotes growth of bacteria by neutralizing inhibitory substances found in the media rather than by pro-viding additional requirements.

1.        Blood agar: On blood agar,N. meningitidisproduces small,round (1–2 mm in diameter), convex, gray, and translucent nonpigmented colonies with entire edges after 24 hours of incubation. At 48 hours, the colonies become larger with an opaque raised center and crenated with transparent margin. It does not produce any hemolysis on blood agar. Strains of meningococci with large polysaccharide capsule appear as mucoid colonies. Meningococci produce large colonies on chocolate agar.

2.        Selective media: Thayer Martin medium with antibiot-ics (vancomycin, colistin, nystatin, and trimethoprim) and New York City medium are the selective media commonly used for the isolation of the bacteria from clinical speci-mens containing mixed bacterial flora.

◗ Biochemical reactions

N. meningitidis shows following reactions:

·           N. meningitidis is oxidase and catalase positive. These twotests are important biochemical markers for preliminary identification of this organism. Alcaligenes spp., Aeromonas spp., Vibrio spp., Campylobacter spp., and Pseudomonas spp. are the other bacteria that are oxidase positive.

·           Oxidase test: The test can be performed in two ways: In thefirst method, 1% solution of oxidase reagent (tetramethyl paraphenylene-diamine-dihydrocholoride) is poured on the culture media; the Neisseria colonies turn deep purple. In the second method, a few colonies of Neisseria are rubbed with a glass rod on a strip of filter paper moistened with oxidase reagent. A deep purple color develops immediately.

·           N. meningitidis ferments glucose and maltose with acid butno gas. It does not ferment sucrose or lactose. Fermentation tests are required for final identification of Neisseria species. It does not produce hydrogen sulphide and does not reduce nitrates.

◗ Other properties

Susceptibility to physical and chemical agents: N. menin-gitidis are highly delicate organisms. They are highly sensitive toheat, desiccation, and disinfectants.