STEPTRACKERMICROBIOLOGY + NEUROLOGY | INFECTIONS BY SYSTEM | HIGH-YIELD REVIEW

Meningitis & CNS Infections

Age picks the organism, the CSF picks the category (bacterial, viral, fungal/TB), and the speed of empiric therapy decides the outcome.

FOCUS ON
By age: 0–1 month → GBS, E coli, Listeria (ampicillin + cefotaxime/gentamicin); 1 month–adult → S pneumoniae, N meningitidis (Hib if unvaccinated), enteroviruses (ceftriaxone + vancomycin); > 50 or immunocompromised → pneumococcus, meningococcus, Listeria (add ampicillin). CSF: bacterial — ↑↑ neutrophils, ↑ protein, ↓ glucose, ↑ pressure; viral — lymphocytes, normal glucose; fungal/TB — lymphocytes, ↑↑ protein, ↓ glucose. Give dexamethasone with the first antibiotic dose (pneumococcus). Don't delay antibiotics for CT/LP. Encephalitis: HSV-1 (temporal), arboviruses; brain abscess: strep, anaerobes, S aureus.
GROUP · SYNDROME
ORGANISMS
CLUE
EMPIRIC THERAPY
Neonates (0–1 month)
Group B strep, E coli (K1), Listeria
Fever or hypothermia, poor feeding, bulging fontanelle; maternal GBS status
Ampicillin + cefotaxime (or gentamicin)
Children and adults
S pneumoniae, N meningitidis, enterovirus; Hib if unvaccinated
Fever, headache, neck stiffness, altered mental status; petechiae (meningococcus)
Ceftriaxone + vancomycin + dexamethasone
Older adults, immunocompromised
S pneumoniae, Listeria, N meningitidis, gram-negative rods
Alcohol, pregnancy, transplant, steroids
Add ampicillin for Listeria
After neurosurgery, shunt, trauma
S aureus, coagulase-negative staph, Pseudomonas, gram-negatives
Device present
Vancomycin + cefepime/meropenem
Recurrent meningitis
Pneumococcus (CSF leak), Neisseria (C5–C9 deficiency)
Clear rhinorrhea after skull fracture
Fix leak · complement workup
Viral (aseptic) meningitis
Enteroviruses (summer–fall), HSV-2, HIV, mumps, arboviruses, LCMV
Well-appearing; lymphocytic CSF, normal glucose
Supportive; acyclovir if HSV
Fungal meningitis
Cryptococcus (AIDS), Coccidioides (Southwest)
Subacute; very high opening pressure
Amphotericin + flucytosine
Tuberculous meningitis
M tuberculosis
Subacute basilar meningitis, cranial nerve palsies, hydrocephalus
RIPE + dexamethasone
Encephalitis
HSV-1 (most common sporadic), VZV, enteroviruses, West Nile, rabies, autoimmune (anti-NMDA)
Fever + altered behavior, seizures, focal signs
IV acyclovir immediately
Brain abscess
Viridans/S anginosus, anaerobes (sinus, dental); S aureus (bacteremia); Nocardia, Toxo (immunocompromised)
Headache, fever, focal deficit; ring-enhancing lesion
Drainage + ceftriaxone + metronidazole
Spinal epidural abscess
S aureus
Fever, back pain, then cord compression (IV drug use, diabetes)
MRI, surgery, vancomycin

Mechanism — why bacterial CSF glucose falls

Bacteria + neutrophils in CSFConsume glucose
→
Inflamed choroid/BBBImpaired glucose transport
→
CSF glucose ↓< 40% of serum

Protein rises because the blood-brain barrier leaks; pressure rises from edema and blocked CSF absorption.

CSF patterns

TYPE
CELLS
PROTEIN
GLUCOSE
Bacterial
↑↑ neutrophils
↑
↓
Viral
↑ lymphocytes
Normal / mild ↑
Normal
Fungal / TB
↑ lymphocytes
↑↑
↓
HSV encephalitis
Lymphocytes + RBCs
↑
Normal

How it's tested

COMMON CONFUSIONPartially treated bacterial meningitis can look lymphocytic with near-normal glucose — history of recent antibiotics matters.
THE TRAPIf a CT must come first (papilledema, focal deficit, immunocompromise, seizure), draw blood cultures and give antibiotics + dexamethasone before the scan.
YOUR APPROACHAge and host → likely organisms → CSF pattern → empiric regimen now → narrow by Gram stain and culture → prophylaxis for contacts if meningococcus or Hib.

If you see… think…

Neonate with meningitis → GBS, E coli, Listeria
Pregnant woman with meningitis after soft cheese → Listeria (ampicillin)
Gram ⁺ lancet diplococci in CSF of a 65-year-old → S pneumoniae
Petechial rash with meningitis in a dorm → N meningitidis
Lymphocytic CSF with normal glucose in August → enterovirus
Recurrent lymphocytic meningitis with genital lesions → HSV-2 (Mollaret)
AIDS, CD4 40, opening pressure 35 cm H₂O → Cryptococcus
Basilar meningitis with CN VI palsy and low glucose → TB meningitis
Adjunct given with the first dose for adult bacterial meningitis → dexamethasone
Brain abscess after dental infection → viridans strep + anaerobes