Bacterial Meningitis

System: CNS Infections · Parent MOC: MOC - Diseases by System

One-Sentence Definition

Acute infection of the meninges by bacteria, a medical emergency characterized by fever, nuchal rigidity, and altered mentation, diagnosed primarily from CSF.

Clinical Presentation

• Classic triad (incomplete in many): fever, neck stiffness, altered mental status
• Headache, photophobia, nausea; petechial rash → think meningococcus
• Neonates/elderly: may be subtle (poor feeding, lethargy, hypothermia)

Pathogens

Context	Organism	Notes
All ages, common	Streptococcus pneumoniae	Lancet diplococci; Capsule
Adolescents / outbreaks	Neisseria meningitidis	Petechiae / purpura
Neonates	Streptococcus agalactiae, E. coli K1, Listeria monocytogenes	Age-specific empiric coverage
Unvaccinated children	Haemophilus influenzae type b	Rare where Hib vaccine used
Post-op / trauma / shunt	Staphylococcus aureus, GNRs	Device → Device-Associated Infections

Pathogenesis

• Nasopharyngeal colonization → bacteremia → meningeal invasion (encapsulated organisms resist phagocytosis)
• Inflammation ↑ ICP, cranial neuropathies, sequelae

Diagnosis

Step	Test	Note
Immediate	Blood cultures	Before antibiotics if no delay
Key	LP → CSF cell count, protein, glucose	Bacterial: ↑WBC (PMN), ↑protein, ↓glucose
Rapid	CSF Gram Stain	Guides empiric narrowing
ID	Culture + multiplex PCR	PCR helps if pretreated
Adjunct	Imaging before LP if mass-effect risk	Don’t delay abx for CT if indicated

Treatment Principles

• Empiric IV bactericidal therapy immediately after cultures (age/risk-based: pneumococcus, meningococcus, Listeria when relevant)
• Dexamethasone timing per guidelines (pneumococcal benefit)
• Definitive: narrow to AST/ID
• Droplet precautions until meningococcus excluded/treated window met

Prevention

• Vaccines: pneumococcal, meningococcal, Hib
• Chemoprophylaxis for close contacts of meningococcal disease

Differential / Pitfalls

• Viral meningitis, encephalitis, subarachnoid hemorrhage, abscess with meningeal signs
• Pretreated CSF may be culture-negative → rely on PCR/Gram

Learning Aids

Diagrams

• Figure - Diagnostic Workflow
• Figure - Gram Envelope Comparison

Clinical Example

Example

Case: 62M, fever, confusion; CSF Gram shows lancet-shaped Gram-positive diplococci.
Question: Most likely organism and priority actions?
Answer: Streptococcus pneumoniae. Start empiric meningitis regimen + dexamethasone per protocol; blood cultures; isolate ID/AST; assess hearing/neuro sequelae risk.

Videos

Video	Why watch
Khan Academy — Gram staining	Read the CSF smear morphology

Learning Media Hub

Related

• CNS Infections · Bloodstream and Sepsis
• Streptococcus pneumoniae · Capsule
• MOC - Diagnostic & Lab Methods · MOC - Antimicrobials

Active Recall

1. Typical bacterial CSF pattern (cells/protein/glucose)?
2. Why is Capsule central to meningitis pathogens?
3. When is CT before LP considered?