Microbiology Cheat Sheet
Meningitis: Csf to Organism
Meningitis CSF to organism for NEET-PG: pneumococcus, meningococcus, Listeria, CSF lactate, neonatal empirical therapy, TB cobweb clot, and cryptococcal CrAg.
MedNext Academy | 3 min read
Meningitis: Csf to Organism
Meningitis CSF to organism for NEET-PG: pneumococcus, meningococcus, Listeria, CSF lactate, neonatal empirical therapy, TB cobweb clot, and cryptococcal CrAg.
This topic covers the etiopathogenesis, CSF analysis, age-based aetiology, and microbial identification of bacterial, tuberculous, fungal, and viral meningitis.
High-yield lines
- Streptococcus pneumoniae is the most common cause of bacterial meningitis in adults and carries the highest case-fatality rate among the classic triad.
- Neisseria meningitidis is the only common meningitis organism that causes epidemics, produces a petechial rash, and requires chemoprophylaxis for close contacts.
- Bacterial meningitis CSF shows neutrophil predominance, high protein, and very low glucose with a CSF-to-serum ratio below 0.4.
- CSF lactate above 3.5 millimoles per litre distinguishes bacterial from viral meningitis even after prior antibiotics, unlike Gram stain and culture.
- The polysaccharide capsule is the single most important virulence factor shared by all major meningitis-causing bacteria.
- Neonatal meningitis is caused by Group B Streptococcus, E. coli K1, and Listeria monocytogenes, requiring ampicillin plus cefotaxime.
- Ceftriaxone is avoided in neonates because it displaces bilirubin and risks kernicterus, so cefotaxime is used instead.
- Third-generation cephalosporins do not cover Listeria, so ampicillin must be added for neonates, the elderly, pregnant women, and immunocompromised patients.
- Gram-positive lancet-shaped diplococci indicate S. pneumoniae, confirmed by optochin sensitivity and bile solubility.
- Gram-negative kidney-shaped intracellular diplococci indicate N. meningitidis, which ferments glucose and maltose and is oxidase-positive.
- Listeria shows Gram-positive short rods with tumbling motility at 25 degrees, beta-haemolysis, and a positive CAMP test with a rectangular zone.
- Tuberculous meningitis shows lymphocytic pleocytosis, low glucose, very high protein with a cobweb clot, and elevated CSF adenosine deaminase.
- Cryptococcal meningitis is diagnosed by India ink showing a capsular halo and by the cryptococcal antigen lateral flow assay with over 95 percent sensitivity.
- Enteroviruses are the most common cause of aseptic viral meningitis, diagnosed by CSF RT-PCR, and the illness is self-limiting.
Mapped competency codes
- MI5.1
- MI5.3
Continue into the full chapter
This summary maps to MI5-central-nervous-system-infections.
Frequently Asked Questions
Which CSF parameter best distinguishes bacterial from viral meningitis after antibiotics?
CSF lactate above 3.5 millimoles per litre, which retains sensitivity even after prior antibiotic therapy unlike Gram stain and culture.
Why must ampicillin be added to empirical meningitis therapy in certain groups?
Third-generation cephalosporins do not cover Listeria monocytogenes, so ampicillin is added for neonates, elderly, pregnant, and immunocompromised patients.
What is the best diagnostic test for cryptococcal meningitis?
The cryptococcal antigen (CrAg) lateral flow assay, with over 95 percent sensitivity, which is superior to India ink.
Why is ceftriaxone avoided in neonatal meningitis?
It displaces bilirubin from albumin and risks kernicterus, so cefotaxime is used instead in neonates.
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