Bacterial Meningitis

• Nasopharyngeal colonization and bacteremia: Pathogenic bacteria (particularly encapsulated organisms) adhere to nasopharyngeal epithelium via specialized surface structures including pili, adhesins, and other ligands that interact with host epithelial receptors. The bacteria evade mucosal immunity through polysaccharide capsules that resist opsonization and complement deposition. Following mucosal penetration, organisms enter the bloodstream, where they must resist killing by circulating antibodies, complement, and phagocytes. The polysaccharide capsule is the critical virulence determinant—unencapsulated mutants are avirulent. Complement-mediated opsonization is ineffective against polysaccharide capsules, explaining why patients with complement deficiencies (C3, C5-C9, properdin, factor D) have dramatically increased susceptibility to meningococcal disease.

• Blood-brain barrier (BBB) invasion: The BBB normally excludes most pathogens through tight junctions between endothelial cells, minimal pinocytosis, and absence of lymphatic drainage. Bacterial meningitis pathogens cross the BBB through multiple mechanisms: (1) paracellular transit via increased vascular permeability induced by bacterial toxins and inflammatory mediators, (2) transcytosis through endothelial cells via receptor-mediated pathways (meningococci express PorA and PorB that interact with endothelial cells; pneumococci express various adhesins), and (3) bacterial lipopolysaccharide (LPS) and peptidoglycans directly increase endothelial permeability through TLR4 signaling. Once bacteria establish CNS infection, the immune privilege of the CNS becomes a liability—the CNS lacks lymph nodes and has limited innate immune responses, allowing unrestricted bacterial replication.

• Meningeal inflammation and inflammatory cascade: Following bacterial invasion of the subarachnoid space, organisms are phagocytosed by resident microglia and infiltrating macrophages/monocytes. Bacterial cell wall components (LPS, peptidoglycans, teichoic acids) are potent activators of pattern recognition receptors including TLR2, TLR4, and NOD-like receptors, triggering massive release of pro-inflammatory cytokines: TNF-α, IL-1β, IL-6, and IL-8. These cytokines mediate local vasculitis, increased BBB permeability, vasogenic edema, and neutrophil infiltration into the CNS. Complement activation (primarily via classical pathway) generates C5a, amplifying chemotaxis and leukocyte recruitment. The inflammatory response, while attempting to control bacterial growth, paradoxically causes much of the pathology through several mechanisms: (a) increased intracranial pressure from cerebral edema and impaired CSF reabsorption, (b) vasculitis causing cerebral vasospasm, infarction, and hemorrhage, (c) direct neurotoxicity from cytokines and free radicals, and (d) subdural empyema formation in some cases.

• Cytotoxic and vasogenic edema: Bacterial lipopolysaccharide and cell wall products stimulate endothelial cells and astrocytes to release aquaporin-4 water channels and increase vascular permeability, leading to vasogenic edema (fluid accumulation in extracellular space). Simultaneously, impaired cerebral metabolism from hypoxia and acidosis causes cytotoxic edema (intracellular fluid accumulation). Elevated intracranial pressure (ICP) results from the combined effect of cerebral edema, increased CSF production from meningeal inflammation, and impaired CSF reabsorption. Elevated ICP reduces cerebral perfusion pressure and can precipitate herniation and sudden death if uncorrected.

• Abscess formation and loculation: In some cases, particularly with pneumococcal meningitis, the inflammatory response becomes localized, forming subdural empyema or loculated ventriculitis. Bacterial enzymes (pneumolysin, hyaluronidase) and host inflammatory response create focal areas of pus collection. Subdural empyema carries particularly poor prognosis as it impairs CSF circulation and requires urgent neurosurgical drainage.

• Vascular complications: Bacterial meningitis causes vasculitis of cortical vessels and meningeal blood vessels through direct bacterial invasion and inflammatory mediator release. Vasospasm can cause focal or multifocal cerebral infarctions, particularly in the distribution of middle and anterior cerebral arteries. Intracranial hemorrhage can occur from vascular rupture or septic thrombophlebitis. These vascular complications explain neurological sequelae including focal deficits, seizures, and cognitive impairment in survivors.

• Bacterial virulence factors: Different organisms express specific virulence factors determining disease severity. Streptococcus pneumoniae produces pneumolysin (a cholesterol-dependent cytolysin causing direct cellular damage and inflammatory activation) and is highly invasive due to polysaccharide capsule and various adhesins. Neisseria meningitidis produces lipooligosaccharide (LOS) instead of LPS, which is even more potent in inducing cytokine release. Listeria monocytogenes produces internalins allowing crossing of the BBB and placental barrier, and can cause ventriculitis and brain abscess formation due to poor CNS immune response in susceptible populations.

• Streptococcus pneumoniae: The most common cause of bacterial meningitis in adults in developed countries, responsible for 40-50% of community-acquired cases. Transmission occurs via respiratory droplets from colonized nasopharynx. Risk factors include asplenia, CSF leaks, basilar skull fractures (allowing direct inoculation), complement deficiencies, and immunoglobulin deficiencies. Penicillin-resistant strains are increasingly prevalent globally, necessitating empiric high-dose third-generation cephalosporin therapy. Pneumococcal vaccines (PCV13, PPSV23) reduce incidence but do not eliminate risk.

• Neisseria meningitidis (meningococcus): Accounts for 20-40% of meningitis cases and is the most common cause in adolescents and young adults in developed countries. Meningococci can cause rapidly progressive fulminant disease with mortality rates reaching 50% if untreated. Transmission is via respiratory droplets from asymptomatic carriers or infected individuals. Serogroups B, C, Y, and W135 cause most invasive disease; serotypes A and C predominate in sub-Saharan Africa and other areas without universal vaccination. Risk factors include functional or anatomical asplenia, complement deficiencies (especially C5-C9, properdin), college dormitory residence, military service, and close household contacts. Vaccination (meningococcal conjugate vaccines) is recommended but does not provide lifetime immunity.

• Listeria monocytogenes: Causes 5-10% of bacterial meningitis in the United States and is the most common cause in neonates (< 3 months), pregnant women, and elderly individuals (>50 years). Risk factors include immunosuppression (particularly cell-mediated immunity defects such as HIV/AIDS, organ transplantation, chronic corticosteroid use), extremes of age, pregnancy, and renal failure. Transmission occurs via consumption of contaminated foods (unpasteurized dairy, delicatessen meats, soft cheeses). Listeria is intrinsically resistant to cephalosporins, necessitating addition of ampicillin or penicillin G to empiric regimens in high-risk patients. Bacteremia may precede meningitis by several days.

• Gram-negative bacilli (particularly Escherichia coli with K1 capsule, Klebsiella, Pseudomonas aeruginosa, Acinetobacter): Predominate in hospital-acquired meningitis, nosocomial infections, or meningitis complicating neurosurgery, ventriculoperitoneal shunt placement, or penetrating head trauma. E. coli K1 is the most common cause of neonatal meningitis (accounting for ~40% of cases). Risk factors include recent neurosurgery, CSF shunts, immunosuppression, and healthcare-associated infection. These organisms are resistant to penicillins and cephalosporins unless specifically targeted (e.g., ceftazidime for Pseudomonas).

• Haemophilus influenzae (non-typeable strains): Formerly a leading cause of meningitis in unvaccinated children; now rare in countries with universal Hib conjugate vaccine. Risk factors include vaccine non-compliance, asplenia, and agammaglobulinemia. Typeable H. influenzae (type b) meningitis is now uncommon in vaccinated populations.

• Group B Streptococcus (GBS): The most common cause of neonatal meningitis in the first 3 months of life. Transmitted vertically during delivery from colonized maternal vaginal flora. Presents with subacute course compared to other neonatal pathogens, with fever, irritability, and feeding difficulties. Can cause late-onset infection (7-90 days) from vertical transmission or environmental acquisition.

• Risk factors for meningitis overall: Asplenia (functional or anatomical), terminal complement deficiencies (C5-C9, properdin, factor D), CSF leaks or anatomical defects (basilar skull fractures, cochlear implants), recent neurosurgery or shunt placement, penetrating head trauma, immunosuppression (HIV/AIDS, organ transplant, chronic corticosteroids), diabetes mellitus, renal failure, hepatic cirrhosis, alcoholism, and close household contact with meningitis cases.

• Fever: Present in >90% of cases and typically develops acutely (over hours), distinguishing bacterial meningitis from viral meningitis which may have more gradual onset. Temperature elevation reflects pyrogenic cytokine release (TNF-α, IL-1β) in response to bacterial antigens. Absence of fever should not exclude the diagnosis, particularly in immunocompromised patients, neonates, or elderly individuals who may mount blunted fever responses.

• Headache: Occurs in 80-90% of conscious patients and is typically severe, diffuse, and frontal or temporal in distribution. Headache results from meningeal inflammation, increased intracranial pressure, and vasculitis. Photophobia is present in 40-50% of cases due to meningeal irritation affecting the ophthalmic division of the trigeminal nerve. Neck stiffness (nuchal rigidity) is a sign of meningeal irritation, present in 70% of cases, though may be absent in young children and immunocompromised patients. Positive Kernig sign (inability to extend knee while hip is flexed to 90 degrees, eliciting pain) and Brudzinski sign (spontaneous hip and knee flexion when neck is flexed forward) are classic signs of meningeal irritation but have variable sensitivity (50-60%) and specificity.

• Altered mental status and encephalopathy: Occurs in 75-80% of cases and ranges from subtle confusion and disorientation to profound obtundation and coma. Altered mental status results from multiple mechanisms including increased intracranial pressure, cytotoxic and vasogenic edema, vasculitis, seizures, and direct neuronal toxicity from inflammatory mediators. Severity of altered mental status at presentation correlates with poor prognosis. Delirium with behavioral changes may occur before obvious meningeal signs develop.

• Seizures: Occur in 20-40% of patients during acute illness and result from cortical irritation, focal infarction, hemorrhage, elevated intracranial pressure, or metabolic derangements. Seizures may be the presenting symptom and complicate management by obscuring neurological examination and increasing metabolic demands in the setting of compromised cerebral perfusion. Status epilepticus carries particularly poor prognosis.

• Petechial/purpuric rash: Present in 50-80% of meningococcal meningitis cases and is the most diagnostically helpful sign when present. The rash typically appears within 6-24 hours after symptom onset and is most evident on extremities, lower abdomen, and legs. Rash results from meningococcal invasion of small blood vessels causing vasculitis and thrombosis. A non-blanching petechial or purpuric rash is meningococcemia until proven otherwise. Absence of rash does not exclude meningococcal disease. Rash is rare in pneumococcal meningitis (<10%) and other bacterial causes.

• Focal neurological deficits: Present in 10-20% of cases at admission and result from vasculitis, infarction, hemorrhage, subdural empyema, or abscess formation. Deficits include hemiparesis, cranial nerve palsies (particularly CN II, III, IV, VI, VII, VIII from increased intracranial pressure and basilar involvement), and visual field abnormalities.

• Neonatal presentation (< 3 months): Differs significantly from older children and adults. Classic meningeal signs are absent; instead, nonspecific signs predominate including irritability, poor feeding, lethargy, fever (or hypothermia in sick infants), hypotonia, bulging anterior fontanelle, and seizures. Septic appearance with hypotension and shock may occur rapidly.

• Elderly or immunocompromised patients: May present with minimal fever, absent meningeal signs, and subtle alteration in mental status easily attributed to other causes. The threshold for lumbar puncture (LP) should be very low in these populations. Listeria meningitis in elderly and immunocompromised hosts may present subacutely (over 3-7 days) compared to acute presentation in immunocompetent hosts.

• Fulminant presentation: Meningococcal meningitis can progress with shocking rapidity, progressing from initial malaise and fever to altered mental status, shock, and death within 12-24 hours if untreated. Meningococcemia with sepsis may present without obvious meningitis features.

• Clinical suspicion: The diagnosis should be suspected in any patient with fever and altered mental status, fever with headache and neck stiffness, or unexplained fever in an at-risk population (neonate, immunocompromised host, asplenic patient, recent neurosurgery). The combination of fever and petechial/purpuric rash mandates immediate empiric antibiotic therapy before any diagnostic testing given the high likelihood of meningococcal meningitis.

• Lumbar puncture (LP) and cerebrospinal fluid (CSF) analysis: The definitive diagnostic procedure and should be performed urgently in all suspected cases. However, LP should be delayed and neuroimaging obtained first if: (1) papilledema or focal neurological deficits suggesting increased intracranial pressure or space-occupying lesion, (2) recent neurosurgery or shunt placement, (3) immunocompromised state with opportunistic CNS infections, or (4) hemodynamic instability requiring stabilization. In such cases, empiric antibiotics should be given before LP.
• CSF appearance: Bacterial meningitis typically produces turbid or cloudy CSF (in contrast to clear CSF

Immediate steps (first 60 minutes)

• Do not delay antibiotics: obtain blood cultures and give empiric IV antibiotics immediately; if neuroimaging is indicated before lumbar puncture, antibiotics and steroids precede the CT scan (IDSA 2004 bacterial meningitis guideline). Support airway, treat shock with isotonic crystalloid and vasopressors, and place the patient on droplet precautions until 24 hours of effective therapy if meningococcus is possible.
• Adjunctive dexamethasone: 0.15 mg/kg IV every 6 hours for 4 days, given before or with the first antibiotic dose. Corticosteroids blunt the TNF-α/IL-1β surge released by bacteriolysis, reducing mortality and sensorineural hearing loss in pneumococcal meningitis (IDSA). Benefit is lost if given after antibiotics; it is not recommended in neonates.

Empiric antimicrobial therapy (age/host-based, per IDSA)

• 1 month–50 years: third-generation cephalosporin (ceftriaxone) plus vancomycin for penicillin/cephalosporin-resistant pneumococcus. Vancomycin is dosed to a 24-hour AUC/MIC of 400–600 (2020 IDSA/ASHP consensus), not to a trough goal.
• >50 years, pregnant, alcohol use disorder, or impaired cell-mediated immunity: add ampicillin for Listeria — cephalosporins have no listerial activity.
• Neonates: ampicillin plus cefotaxime or gentamicin; avoid ceftriaxone (bilirubin displacement, calcium precipitation).
• Post-neurosurgical, penetrating trauma, or CSF shunt: vancomycin plus an antipseudomonal beta-lactam (cefepime or meropenem), per the IDSA 2017 healthcare-associated ventriculitis/meningitis guideline.
• Severe beta-lactam allergy: meropenem, or aztreonam/moxifloxacin with TMP-SMX for Listeria. Cephalosporin cross-reactivity in penicillin allergy is roughly 1–3% and side-chain driven, so ceftriaxone is usually acceptable after non-anaphylactic reactions.

Definitive and procedural management

• Narrow by culture and susceptibility: penicillin G or ceftriaxone for susceptible pneumococcus/meningococcus; ampicillin plus gentamicin for Listeria. Duration is shortest for meningococcus and longest for Listeria and Gram-negative bacilli.
• Neurosurgery: hardware/shunt removal with external ventricular drainage for device infection; drainage of subdural empyema or abscess; EVD or CSF diversion for obstructive hydrocephalus.
• Chemoprophylaxis of close contacts of meningococcal disease with rifampin, ciprofloxacin, or ceftriaxone (CDC/ACIP); rifampin for unvaccinated child contacts of H. influenzae type b.

Neurologic emergencies

• Cerebral herniation: cerebral edema and impaired CSF reabsorption raise ICP beyond compensatory limits. Signaled by declining consciousness, unilateral fixed dilated pupil, posturing, or Cushing triad (hypertension, bradycardia, irregular respirations). Immediate airway control, head elevation, hyperosmolar therapy, and neurosurgical consultation; a post-LP deterioration is the feared scenario.
• Status epilepticus: cortical irritation, infarction, or hyponatremia lowers seizure threshold; ongoing or subtle convulsive activity with failure to awaken warrants EEG and benzodiazepine plus antiseizure loading.
• Waterhouse–Friderichsen syndrome: meningococcal endotoxin-driven DIC with bilateral adrenal hemorrhage. Refractory hypotension, expanding purpura, and hypoglycemia/hyperkalemia signal it; give stress-dose hydrocortisone with vasopressors.
• Septic shock and DIC: cytokine-mediated vasodilation and consumptive coagulopathy; falling platelets, prolonged INR, low fibrinogen, oozing from puncture sites.

Subacute and delayed complications

• Sensorineural hearing loss: inflammation spreading through the cochlear aqueduct damages the organ of Corti — the most common sequela, especially pneumococcal. Formal audiologic testing before discharge is standard (AAP).
• Hydrocephalus: purulent exudate obstructs arachnoid granulations (communicating) or aqueduct (obstructive); enlarging ventricles on imaging, persistent lethargy, or bulging fontanelle. May require EVD or shunt.
• Cerebral infarction and venous sinus thrombosis: septic vasculitis/thrombophlebitis producing new focal deficits or persistent seizures.
• Subdural effusion vs. empyema: sterile effusion is common in infant *H. influenzae*/pneumococcal disease and resolves; empyema (fever, focal deficits, mass effect) requires surgical drainage.
• SIADH and hyponatremia: inflammation-driven ADH release; low serum sodium with concentrated urine — aggravates seizures and edema.
• Cognitive impairment, cranial nerve palsies, and epilepsy in survivors.

Treatment-related

• Vancomycin nephrotoxicity: rising creatinine, particularly with high AUC exposure or concomitant piperacillin-tazobactam.
• Dexamethasone effects: hyperglycemia, GI bleeding, and reduced CSF penetration of vancomycin — watch for clinical failure in resistant pneumococcus.
• Ceftriaxone in neonates: biliary sludging and bilirubin displacement/kernicterus; Clostridioides difficile colitis and rifampin-induced orange secretions with CYP450 induction (oral contraceptive failure).

• The single best next step is almost always "give antibiotics now": when the stem includes an indication for CT before LP (papilledema, focal deficit, seizure, immunosuppression, altered consciousness), the correct sequence is blood cultures → dexamethasone + empiric antibiotics → CT → LP. Choosing "LP first" or "CT first, then antibiotics" is the classic trap.
• Dexamethasone must precede or accompany the first antibiotic dose: its benefit depends on suppressing the cytokine burst from bacteriolysis, and it is most clearly beneficial in pneumococcal meningitis (IDSA). Given after antibiotics, it is useless.
• **Ceftriaxone does not cover *Listeria*: any patient over 50, pregnant, neonatal, or with impaired cell-mediated immunity gets ampicillin** added. This is the most frequently tested empiric-regimen question.
• **Recurrent Neisseria infection = terminal complement deficiency (C5–C9, properdin, factor D): screen with a CH50**. Recurrent pneumococcal meningitis instead suggests a CSF leak, asplenia, or humoral immunodeficiency.
• Non-blanching petechial/purpuric rash with fever is meningococcemia until proven otherwise; Waterhouse–Friderichsen syndrome (adrenal hemorrhage, refractory shock) is the eponym examiners want.
• Bacterial CSF buzzword pattern: neutrophil-predominant pleocytosis, markedly elevated protein, and low glucose (low CSF:serum glucose ratio) — the low glucose separates bacterial from viral meningitis, and a lymphocyte-predominant profile with very low glucose should raise TB or fungal disease.
• Vancomycin is dosed to a 24-hour AUC/MIC of 400–600, not to a 15–20 mcg/mL trough; that trough target was retired in the 2020 IDSA/ASHP consensus.
• Don't forget public health: droplet precautions for 24 hours plus chemoprophylaxis (rifampin, ciprofloxacin, or ceftriaxone) for household, childcare, and intimate contacts of meningococcal cases, and audiologic testing before discharge in survivors.