(a)
What are the common causes of pyogenic meningitis?
(b)
Describe the laboratory diagnosis of pyogenic meningitis including
collection and transport of specimens. Also briefly comment on the relative
usefulness of microscopy, culture, serology, and rapid tests in identifying the
microorganisms that you listed.
Suggested
Answer:
(a)
Pyogenic meningitis is an inflammatory response to bacterial infection
involving the pia and arachnoid membranes over the brain and spinal cord.
(b)
In cases of suspected meningococcal disease, specimens of blood, cerebrospinal fluid, and nasopharyngeal secretions should be collected before administration of any antimicrobial agents and examined for the presence of N meningitidis. Success in isolation is reduced by prior therapy; however, the microscopic diagnosis is not significantly affected. The cerebrospinal fluid should be concentrated by centrifugation and a portion of the sediment cultured on chocolate or blood agar. The plates should be incubated in a candle jar or CO2 incubator. The presence of oxidase-positive colonies and Gram-negative diplococci provides a presumptive identification of N meningitidis. Production of acid from glucose and maltose but not sucrose, lactose, or fructose may be used for confirmation (Table 14-1). The serologic group may be determined by a slide agglutination test, using first polyvalent and then monovalent antisera.
Nasopharyngeal specimens must be obtained from the posterior nasopharyngeal wall behind the soft palate and then should be inoculated onto a selective medium such as Thayer-Martin medium and processed as above.
Blood specimens are inoculated in 10- to 15-ml aliquots onto each of three blood bottles to give a final concentration of 10% (vol/vol). Evacuated bottles should be vented. Some strains of N meningitidis are inhibited by the sodium polyanetholsulfonate contained in blood medium. Toxicity may be overcome by the addition of gelatin. Sodium amylosulfate is not toxic for the meningococcus. Blood cultures are subcultured blindly onto chocolate or blood agar for confirmation.
Gram-stained smears of cerebrospinal fluid may be diagnostic; however, finding neisseriae in these smears is often more difficult than finding the strains that cause pneumococcal meningitis. Quellung tests may be of value.