Acute bacterial meningitis

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Bacterial meningitis is infection of leptomeninges and CSF, most often caused in UK adults by S pneumoniae and N meningitidis; Listeria risk rises with older age, pregnancy and immune suppression. Organisms enter from blood or contiguous ear, sinus, skull-base or device infection and multiply within an immune-poor CSF compartment.

Inflammatory mediators disrupt the blood-brain barrier, causing oedema, vasculitis, infarction, raised intracranial pressure and neuronal injury. Antibiotics kill bacteria but can intensify inflammation transiently, which is why dexamethasone is timed before or with the first dose. Diagnostic sampling and treatment must run in parallel rather than sequentially.

Key points

The classic fever, neck stiffness and altered-consciousness triad is insensitive; severe headache or cognitive change with fever warrants urgent evaluation.
Take blood cultures and EDTA blood for meningococcal and pneumococcal PCR before antibiotics only when this causes no delay.
Give ceftriaxone 2 g intravenously every 12 hours in adults with suspected bacterial meningitis.
Give dexamethasone 10 mg intravenously every 6 hours for 4 days, starting with or immediately before the first antibiotic and reviewing when the organism is known.
Add amoxicillin 2 g intravenously every 4 hours for people with Listeria risk under NICE and local renal guidance.
Lumbar puncture is the key diagnostic test when safe; do not perform it during shock, uncontrolled seizure or clinical evidence of mass effect.
Use CT before lumbar puncture only for specified neurological risks, not because meningitis is suspected in every patient.
Notify relevant disease, provide meningococcal contact prevention and arrange hearing and neurocognitive follow-up after survival.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Encapsulated respiratory bacteria

Pneumococcus and meningococcus enter blood after nasopharyngeal colonisation and cross meningeal barriers, with capsule protecting against innate clearance.

Listeria and contiguous sources

L monocytogenes enters through gastrointestinal exposure in susceptible hosts, while ear, sinus, skull fracture and devices permit direct spread.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
CSF bacterial multiplication
Low local antibody and complement allow rapid organism growth before recruited neutrophils enter the subarachnoid space.
2
Barrier and vascular injury
Cytokines increase permeability, oedema and intracranial pressure while vasculitis and thrombosis cause focal cerebral infarction and disability.
3
Inflammatory lysis response
Antibiotic-mediated killing releases bacterial components, amplifying inflammation that appropriately timed early intravenous dexamethasone seeks to moderate.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Meningeal syndromeRed flag

Fever, severe headache, neck stiffness, photophobia, vomiting and altered cognition creates a high-probability syndrome even when one feature is absent.

Meningococcal sepsisRed flag

Purpura, mottling, severe limb pain and shock suggests concurrent bloodstream disease requiring critical care.

Raised intracranial pressureRed flag

Falling consciousness, focal deficit, papilloedema, abnormal pupils or repeated seizure indicates mass effect and changes lumbar-puncture safety.

Listeria contextRed flag

Older age, pregnancy, malignancy, transplant or major immune suppression increases L monocytogenes probability despite absence of a distinctive rash.

Red flags requiring action

Altered consciousness, shock, focal neurology, papilloedema or uncontrolled seizure changes lumbar-puncture safety and requires senior escalation.
A rapidly spreading non-blanching rash or severe limb pain raises meningococcal septicaemia and needs immediate sepsis care.
Older age, pregnancy or immune suppression increases Listeria risk and requires amoxicillin in addition to ceftriaxone.
Antibiotics already given do not remove the need for lumbar puncture once safe because PCR and CSF pattern remain valuable.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Blood cultures and blood PCRFirst step
Why
Identify bacteria without requiring lumbar puncture safety.
Interpretation and limitations
Obtain immediately before antibiotics when possible; negative culture after treatment does not exclude disease and molecular tests may remain diagnostic.
02
Lumbar puncture
Why
Measure opening pressure and obtain CSF cell count, glucose, protein, Gram stain, culture and PCR.
Interpretation and limitations
Neutrophils, low CSF-to-blood glucose and high protein support bacterial disease, but early or partially treated profiles overlap viral and TB meningitis.
03
CT brain before selected lumbar puncture
Why
Identify mass effect when clinical neurological features create herniation risk.
Interpretation and limitations
Use for focal deficit, markedly reduced consciousness, papilloedema or specified seizures; routine CT delays antibiotics and can falsely reassure.
04
Paired glucose and organ baseline
Why
Interpret CSF glucose and guide antimicrobial safety.
Interpretation and limitations
Take blood glucose with CSF and check FBC, renal, liver, coagulation and lactate; shock and coagulopathy may preclude immediate puncture.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Viral meningitis

Lymphocytic CSF and preserved glucose often support viral disease, but early bacterial cases and prior antibiotics can overlap.

Encephalitis

Prominent behavioural change, focal seizure or temporal imaging suggests brain parenchymal infection and requires immediate aciclovir alongside meningitis care.

Subarachnoid haemorrhage

Thunderclap headache and blood in CSF can mimic meningism; fever and inflammatory CSF require careful parallel evaluation.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01ASSESSTreat before diagnostic delayFirst stepBacterial meningitis is clinically suspected in an adult.

1Use ABCDE, glucose and early senior review; take blood cultures and PCR and start therapy immediately when sampling would delay.
2Give dexamethasone with or immediately before ceftriaxone and add amoxicillin when Listeria risk exists.
3Treat seizure, shock, hypoglycaemia, hypoxia and raised intracranial pressure with critical-care and neurological support.
4Notify suspected meningococcal disease and use appropriate droplet precautions until the public-health and treatment criteria are met.

02TREATObtain safe microbiological evidenceThe patient is stable enough for lumbar puncture or testing can continue after initial treatment.

1Assess lumbar-puncture contraindications clinically and perform puncture promptly without routine CT when safe.
2Record opening pressure and send sufficient labelled CSF for microscopy, culture, bacterial PCR and exposure-directed viral or TB testing.
3If puncture was initially unsafe, reassess after stabilisation; prior ceftriaxone does not make later CSF worthless.
4Investigate ear, sinus, skull-base, endocardial or device sources when organism or course suggests contiguous or persistent seeding.

03REVIEWNarrow and prevent sequelaeOrganism, susceptibility and clinical trajectory become available.

1Narrow antimicrobials and set duration from organism, susceptibility, CSF clearance and focal complications with infection specialists.
2AlternativeStop or continue dexamethasone according to organism and timing rather than completing automatically after an alternative diagnosis.
3Repeat imaging or CSF only for poor response, unusual organism, hydrocephalus, abscess, device or diagnostic uncertainty.
4Arrange hearing assessment, neurological and cognitive review, rehabilitation, notification and meningococcal contact actions before handover.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Provides rapid CSF-active cover for pneumococcus, meningococcus and susceptible H influenzae.

Ceftriaxone empirical meningitis treatment

Give ceftriaxone 2 g intravenously every 12 hours in adults with suspected bacterial meningitis.

Check immediate cephalosporin allergy and local resistance, obtain cultures when safe and never delay for CT or lumbar puncture.

Reduces inflammatory neurological and hearing injury, particularly in pneumococcal meningitis.

Dexamethasone adjunct

Give dexamethasone 10 mg intravenously every 6 hours for 4 days, starting with or immediately before the first antibiotic.

Review glucose, gastrointestinal bleeding and organism; late initiation has less evidence and continuation depends on the confirmed cause.

Adds essential L monocytogenes activity missing from third-generation cephalosporins.

Amoxicillin for possible Listeria

Add amoxicillin 2 g intravenously every 4 hours when older age, pregnancy or immune suppression creates Listeria risk.

Adjust for renal impairment, define allergy urgently with microbiology and stop when Listeria is no longer credible.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Hearing loss

Cochlear and eighth-nerve inflammation produces permanent hearing impairment requiring prompt formal audiology assessment and multidisciplinary rehabilitation.

Cerebral infarction and seizure

Vasculitis, venous thrombosis, oedema and cortical injury cause focal deficits, epilepsy and substantial long-term cognitive disability.

Hydrocephalus and pressure

Impaired CSF flow raises intracranial pressure, threatens fatal herniation and may require urgent specialist neurosurgical drainage.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat Glasgow Coma Scale, pupils, focal neurology, seizure activity and cardiorespiratory physiology frequently during the acute phase.
Track sodium, glucose, renal function, fluid balance and signs of SIADH, cerebral salt loss or drug toxicity.
Review blood and CSF culture and PCR daily and document antimicrobial narrowing, dexamethasone decision and planned stop date.
Arrange hearing, cognition, mobility, seizure and psychological follow-up and explain recurrence warning symptoms before discharge.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

CT does not exclude pressure

A normal scan cannot prove lumbar puncture safety when clinical mass effect or shock remains, and CT should not postpone antibiotics.

Partial treatment changes CSF

Antibiotics reduce culture yield and can alter cells and glucose, but PCR and overall pattern still contribute after treatment.

Listeria lacks cephalosporin response

A standard ceftriaxone regimen leaves L monocytogenes untreated, making risk recognition and amoxicillin addition essential.

Survival is not full recovery

Hearing loss, executive dysfunction, fatigue and emotional change can be missed unless follow-up asks specifically and includes rehabilitation.

11Common pitfallsFrequent interpretation and management errors.

01
Waiting for all three classic features before treating bacterial meningitis.
02
Performing routine CT before antibiotics and lumbar puncture in every suspected case.
03
Forgetting amoxicillin in a person with Listeria risk because ceftriaxone seems broad.
04
Discharging without hearing, cognitive, neurological and public-health follow-up.