bacteriology

Culture Chocolate agar is used for isolation of pathogenic species from sterile sites Thayer-Martin or Modified Thayer-Martin (selective) agar used for isolation from contaminated specimens with normal flora. Non pathogenic species grow on ordinary medium (Nutrient agar). After 48hrs. on enriched media, gonococci and meningococci form convex, glistening, transparent or opaque colonies, non pigmented and non hemolytic. The colonies are 1-5mm in diameter.

Growth characteristics Neisseria grow best under aerobic condition. Meningococci and gonococci required complex organic substances for their growth Neisseria produce oxidase enzyme. Most Neisseria utilize carbohydrates producing acid but no gas. Sugar utilization used in the identification of species. The organisms are rapidly killed by drying, moist heat, sun light, and disinfectants.

Neisseria meningitidis Antigenic structure At least 13 serogroups of meningococci identified by immunologic specificity of capsular polysaccharides. The important serogroups associated with disease in human are A,B,C,Y, and W-135. The outer membrane proteins of meningococci have been divided into classes on the basis of molecular weight. These proteins help to form pores in the cell walls of this bacteria and about 20 serotypes are present. The Opa (class5) protein is comparable to Opa of the gonococci. Meningococci are piliated (hair like appendages), serve as adhesion factor for the oropharyngeal mucosa. Meningococci lipolysaccharide (LPS) is responsible for many toxic effect found in meningococcal disease.

Pathogenesis and clinical findings Humans are the only natural hosts for whom meningococci are pathogenic. The nasopharynx is the portal of entry. The organisms attach to the epithelial cells with the aid of pili. Fulminant meningococcemia (water house-Friderichsen syndrome) is more severe, with high fever, hemorrhagic rash and disseminated intravascular coagulation, which lead to thrombosis of small blood vessels. There may be interstitial myocarditis and arthritis. Meningitis is the common complication of meningococcemia. It begins suddenly, with intense headache, vomiting, stiff neck, and progresses to coma within a few hours.

Diagnostic laboratory testsSpecimens: Smears: Gram – stained smears of sediment of centrifuged spinal fluid or other specimens show typical Neisseriae within polymorphonuclear leucocytes or extracellular. Culture: incubated at 37°C in an atmosphere of 5%CO2. Modified Thayer - Martin medium with antibiotics (Vancomycin, colistin, amphotericin) used for contaminated specimens as nasopharyngeal swab.Colonies of Neisseriae on solid media identified by Gram stain and oxidase test. Further identification by carbohydrate fermentation and agglutination with type specific or polyvalent serum.

Treatment Penicillin G is the drug of choice for treating meningococcal disease. Chloramphenicol or third-generation cephalosporin such as cefotaxime or ceftriaxone used in persons allergic to penicillin. Epidemiology, prevention and control N. meningitides is highly contagious transmitted by respiratory secretions. Five to 30% of the normal population harber meningococci in nasopharynx during interepidemic periods During epidemics the carrier rate rise to 70-80%. The carrier state is an important source of infection more than the clinical cases. Penicillin not eradicate the carrier state. Rifampin or minocycline can eradicate the carrier state and serve as chemoprophylaxis. Specific polysaccharides of group A,C,Y and W-135 can stimulate antibody response and protect susceptible persons against infection. Such vaccine used in selected populations as in the military and in civilian epidemics.