Bacterial meningitis is one of those “don’t-miss” diagnoses on Step 1: it’s common, deadly if untreated, and the exam loves testing age-based etiologies, CSF patterns, and next best step when you suspect it. If you can quickly recognize the presentation, interpret lumbar puncture (LP) results, and start the right empiric antibiotics (plus dexamethasone when indicated), you’ll pick up a ton of easy points.
What It Is (Definition) — and Why It Matters
Bacterial meningitis is an acute pyogenic infection of the leptomeninges (pia + arachnoid) and subarachnoid space, leading to a neutrophil-predominant inflammatory response in the cerebrospinal fluid (CSF).
High-stakes takeaway: Treatment is time-sensitive. Many questions reward starting empiric antibiotics before confirmatory tests if suspicion is high.
Pathophysiology (Step 1-Friendly Mechanisms)
Most bacterial meningitis follows this sequence:
- Colonization of nasopharynx (common with S. pneumoniae, N. meningitidis, H. influenzae)
- Invasion into bloodstream (bacteremia)
- Crossing the blood–brain barrier (often via choroid plexus/endothelium)
- Inflammation in subarachnoid space
- Bacterial components stimulate cytokines (TNF-α, IL-1)
- Increased BBB permeability → vasogenic edema
- Impaired CSF resorption → hydrocephalus
- Cerebral vasculitis/thrombosis → ischemia
- ↑ intracranial pressure (ICP) → herniation risk
Why CSF Glucose Drops
Bacteria and recruited neutrophils consume glucose, and inflammation impairs glucose transport across the BBB → low CSF glucose.
Etiology: High-Yield Bugs by Age/Risk Factor
This is a classic Step 1 table—learn it cold.
| Patient group | Most likely organisms | High-yield associations |
|---|---|---|
| Neonates (<1 month) | GBS (Strep agalactiae), E. coli (K1), Listeria monocytogenes | Listeria = cold deli meats, unpasteurized dairy; can cause neonatal sepsis/meningitis |
| Infants/children | S. pneumoniae, N. meningitidis, (less now: H. influenzae type b) | Hib decreased with vaccination; still relevant in unvaccinated |
| Adolescents/young adults | N. meningitidis | Crowded living (dorms, military); petechial rash; Waterhouse-Friderichsen |
| Adults | S. pneumoniae most common | Post-otitis/sinusitis, alcoholism, asplenia |
| Elderly | S. pneumoniae, Listeria, gram-neg rods | Listeria risk increases with age |
| Asplenia / sickle cell | Encapsulated: S. pneumoniae, H. influenzae, N. meningitidis | Remember: “SHiN” encapsulated organisms |
| Basilar skull fracture / CSF leak | S. pneumoniae, H. influenzae | Rhinorrhea/otorrhea; recurrent meningitis |
| Neurosurgery, shunts | S. epidermidis, S. aureus, gram-neg rods | Biofilms; device-associated infections |
| Immunocompromised (esp. cell-mediated) | Listeria, gram-neg rods | Think transplant, steroids, pregnancy |
First Aid cross-reference: Microbiology (encapsulated bacteria; meningitis etiologies), Neurology (meningitis vs encephalitis), Immunology (asplenia risks).
Clinical Presentation: What to Recognize Fast
Classic symptoms (the “triad”)
- Fever
- Neck stiffness (nuchal rigidity)
- Altered mental status
Only a minority have all three, so don’t rely on the full triad.
Other high-yield findings
- Severe headache
- Photophobia
- Nausea/vomiting
- Seizures (especially severe cases)
- Signs of raised ICP (papilledema, focal deficits)
- Meningococcemia signs: petechiae/purpura, hypotension/shock
Physical exam maneuvers (testable but imperfect)
- Kernig sign: pain/resistance with knee extension when hip flexed
- Brudzinski sign: involuntary hip/knee flexion with neck flexion
Diagnosis: The Step 1 Algorithm (and “CT before LP”)
Core principle
If bacterial meningitis is strongly suspected:
Draw blood cultures → start empiric antibiotics (± dexamethasone) immediately.
LP is crucial, but do not delay therapy in high-suspicion scenarios.
When do you need a head CT before LP?
CT first if risk of herniation due to mass effect/increased ICP. Classic indications include:
- Focal neurologic deficits
- New-onset seizure
- Papilledema
- Altered mental status (esp. severe)
- Immunocompromised state (often tested as a reason to image first)
High-yield exam move: If CT is required, you still give antibiotics first after blood cultures—don’t wait.
CSF Findings: Pattern Recognition Table (Must-Know)
Typical bacterial meningitis CSF:
| CSF parameter | Bacterial meningitis | Why |
|---|---|---|
| Opening pressure | ↑ | inflammation/edema, impaired resorption |
| WBC count | ↑↑ (neutrophils/PMNs) | acute pyogenic response |
| Protein | ↑ | BBB disruption |
| Glucose | ↓ | consumption + impaired transport |
| Gram stain/culture | Often positive | depends on organism and pretreatment |
Memory anchor: Bacterial = PMNs, Protein up, Pressure up, Glucose down (“PPP down G”).
Quick compare: viral vs bacterial (Step 1 favorite)
- Viral: lymphocytes, normal glucose, mild protein increase
- Bacterial: neutrophils, low glucose, high protein
Treatment: Empiric Regimens You’re Expected to Know
Treat immediately—this is where Step questions often live.
Start with: blood cultures → dexamethasone (selected cases) → antibiotics
Dexamethasone
- Often given before or with first antibiotic dose to reduce inflammatory damage (especially in suspected pneumococcal meningitis).
- Most testable benefit: ↓ hearing loss with Hib (classically) and improved outcomes in some pneumococcal cases.
Empiric antibiotics by age/risk
| Group | Empiric therapy (classic) | Coverage logic |
|---|---|---|
| Neonate (<1 month) | Ampicillin + cefotaxime or ampicillin + gentamicin | Ampicillin covers Listeria + GBS; add gram-negative coverage |
| Age 1 month–50 years | Ceftriaxone (or cefotaxime) + vancomycin | Covers S. pneumoniae (incl resistant via vanc) + N. meningitidis |
| >50 years or immunocompromised | Ceftriaxone + vancomycin + ampicillin | Add ampicillin for Listeria |
Why vancomycin? Rising resistance in S. pneumoniae → need empiric coverage until sensitivities return.
Targeted therapy (once organism identified)
- N. meningitidis: ceftriaxone/cefotaxime (or penicillin G if sensitive)
- S. pneumoniae: ceftriaxone + vancomycin initially; tailor based on susceptibilities
- Listeria: ampicillin (often plus gentamicin)
- Hib: ceftriaxone/cefotaxime
Special High-Yield Associations (Exam Gold)
1) Neisseria meningitidis
- Gram-negative diplococci, oxidase positive
- Thayer-Martin medium, ferments maltose + glucose
- Can cause meningococcemia → petechiae/purpura, DIC
- Waterhouse–Friderichsen syndrome: adrenal hemorrhage → shock
- Prevention: vaccine (capsular polysaccharide conjugate) + rifampin/ciprofloxacin prophylaxis for close contacts
First Aid cross-reference: Micro (Neisseria), Immunology (complement deficiency—see below).
2) Complement deficiency and Neisseria
- C5–C9 deficiency → impaired MAC formation → recurrent Neisseria infections
This is a classic linkage on Step 1.
3) Streptococcus pneumoniae
- Gram-positive lancet-shaped diplococci
- α-hemolytic, optochin sensitive, bile soluble
- Major cause in adults, post-URI/otitis/sinusitis, asplenia, alcoholism
4) Haemophilus influenzae type b (Hib)
- Gram-negative coccobacillus
- Risk in unvaccinated children
- Needs factor V (NAD+) and factor X (heme) (chocolate agar)
- Capsule = PRP (polyribosylribitol phosphate)
- Vaccine is conjugate (PRP linked to protein carrier)
5) Listeria monocytogenes
- Gram-positive rod, tumbling motility, intracellular
- From unpasteurized dairy, deli meats; severe in neonates, elderly, pregnant, immunocompromised
- Treated with ampicillin
“Meningitis vs Encephalitis” (Common Trap)
- Meningitis: fever, headache, neck stiffness; consciousness may be affected but less prominent early
- Encephalitis: more altered mental status, seizures, focal neuro findings
Step 1 often pairs encephalitis with HSV-1 (temporal lobe) — different CSF profile (usually lymphocytes, RBCs possible).
High-Yield Question Stems to Anticipate
- College student with fever, headache, petechial rash → think meningococcus; give ceftriaxone, prophylax contacts.
- Older adult with meningitis symptoms → don’t forget ampicillin for Listeria.
- Suspected meningitis + papilledema/focal deficit → blood cultures + antibiotics, then CT before LP.
- CSF: high PMNs + low glucose + high protein → bacterial.
- Recurrent Neisseria infections → C5–C9 deficiency.
Rapid Review (One-Minute Checklist)
- Top adult causes: S. pneumoniae, N. meningitidis
- Neonate causes: GBS, E. coli (K1), Listeria
- CSF bacterial pattern: ↑ opening pressure, PMNs, ↑ protein, ↓ glucose
- Empiric therapy (most tested):
- 1 mo–50 yrs: ceftriaxone + vancomycin
-
50 or immunocompromised: ceftriaxone + vanc + ampicillin
- CT before LP if focal deficits, papilledema, seizure, severe AMS, immunocompromised—but don’t delay antibiotics