Bacterial meningitis organisms by age and risk group
The cerebrospinal fluid tells you the meningitis is bacterial, but not which bacterium. Age and risk group predict the organism, and the Gram stain confirms it.
Predict the most likely bacterial meningitis organism from age and risk group, match it to its Gram stain, and recognise the bacterial CSF pattern.
The short version
- Newborns get group B streptococcus, E coli and Listeria.
- Children and adults get pneumococcus and meningococcus; Listeria is added after fifty or with weak cell-mediated immunity.
- After neurosurgery or with a shunt, think staphylococci and gram-negative rods, including Pseudomonas.
- Bacterial CSF has mostly neutrophils, high protein and low glucose; in children and adults the CSF to blood glucose ratio is typically at or below 0.4.
- Viral CSF is the contrast: lymphocytes, normal glucose and only a modest rise in protein.
Same CSF, different bacteria
A three-week-old baby and an eighty-year-old man are both feverish and drowsy. Both have a lumbar puncture, and both samples are cloudy, full of neutrophils, with very little sugar.[1]
The fluids look alike, yet the bacteria differ. For bacterial meningitis organisms by age, the rule is simple: the CSF says "bacterial", and age and risk group say which organism.
Bacteria usually reach the meninges from the blood. In children and adults they come from the nose and throat. In newborns they come from the mother or the gut.[1]
Five organisms and their Gram stains
Five organisms cover most questions, and the Gram stain tells them apart.[8] If you need a refresher on reading the stain, revisit the cell wall lesson.
| Organism | Gram stain |
|---|---|
| Group B streptococcus | Gram-positive cocci in chains |
| Pneumococcus | Gram-positive diplococci |
| Meningococcus | Gram-negative diplococci |
| E coli | Gram-negative rods |
| Listeria | Gram-positive rods |
Who gets which: newborns, children, adolescents and adults
Walk through life. Newborns and young infants, up to about three months, get group B streptococcus, E coli and Listeria.[3] Group B streptococcus comes from the mother's genital tract, and E coli from the gut. Listeria comes from contaminated food, passed on by the mother.[1]
Older infants and children get pneumococcus and meningococcus. Haemophilus influenzae type b used to be a major cause, but it has fallen with vaccination.[2][3]
Adolescents and young adults get meningococcus, with a second peak of disease around that age, for example in students living in dormitories.[4] Meningococcal disease can also cause a purpuric rash.[1]
Adults: pneumococcus and meningococcus again lead the list.[1][8]
| Age group | Likely organisms |
|---|---|
| Newborns, up to about 3 months | Group B streptococcus, E coli, Listeria |
| Older infants and children | Pneumococcus, meningococcus (Hib has fallen with vaccination) |
| Adolescents and young adults | Meningococcus (second peak; purpuric rash possible) |
| Adults | Pneumococcus, meningococcus |
The extra group: Listeria, neurosurgery and shunts
Listeria joins the list in people over fifty and in anyone with weak cell-mediated immunity. Pregnancy is also a Listeria risk, but there it usually causes bacteraemia that threatens the fetus rather than meningitis.[5][8] Ageing and immunosuppressive drugs weaken the T cell and macrophage defence that clears Listeria.[5]
After neurosurgery, or with a shunt, think staphylococci and gram-negative rods, including Pseudomonas.[7][8]
Bacterial meningitis CSF findings
In bacterial meningitis, the cell count is high and mostly neutrophils, often over a thousand per cubic millimetre. The protein is high, and the glucose is low.[1][6]
In children and adults, bacterial meningitis typically gives a CSF to blood glucose ratio at or below 0.4. Newborns use a different cut-off, so this one is for children and adults.[8][9]
Newborns use a different cut-off.
- Blood glucose 90, CSF glucose 30
- 30 / 90 = 1/3 ≈ 0.33
- 0.33 is below 0.4, so this suggests bacterial
Viral meningitis is the contrast: lymphocytes, normal glucose, and only a modest rise in protein.[1][6]
| Feature | Bacterial | Viral |
|---|---|---|
| Cells | High, mostly neutrophils (often over 1000/mm³) | Mostly lymphocytes |
| Protein | High | Modest rise |
| Glucose | Low | Normal |
Listeria is the one that can show a mixed picture of neutrophils and lymphocytes.[5]
Three things to keep
- Newborns get group B streptococcus, E coli and Listeria.
- Children and adults get pneumococcus and meningococcus, and Listeria is added after fifty or with weak cell-mediated immunity.
- Bacterial CSF has neutrophils, high protein and low glucose, with a ratio typically at or below 0.4 in children and adults.
The next lesson moves to the febrile infant and how to assess risk, building on the newborn organisms covered here.
Practice question
Original · USMLE styleA 71-year-old woman who takes prednisone and methotrexate for rheumatoid arthritis has had fever, headache and new confusion for 1 day. Lumbar puncture shows a CSF white blood cell count of 1850/mm³ (74% neutrophils), a protein concentration of 160 mg/dL and a glucose concentration of 24 mg/dL; the serum glucose concentration is 110 mg/dL. Gram stain of the CSF shows gram-positive rods.
Common questions
What causes bacterial meningitis in newborns?
Newborns and young infants, up to about three months, get group B streptococcus, E coli and Listeria. Group B streptococcus comes from the mother's genital tract, E coli from the gut, and Listeria from contaminated food passed on by the mother.
Which organisms cause bacterial meningitis in older children and adults?
Pneumococcus and meningococcus lead the list in older infants, children and adults. Meningococcus has a second peak in adolescents and young adults, for example in students living in dormitories.
When does Listeria cause meningitis?
Listeria joins the list in people over fifty and in anyone with weak cell-mediated immunity. Pregnancy is also a Listeria risk, but there it usually causes bacteraemia that threatens the fetus rather than meningitis. Ageing and immunosuppressive drugs weaken the T cell and macrophage defence that clears it.
What does bacterial meningitis look like in the CSF?
The cell count is high and mostly neutrophils, often over a thousand per cubic millimetre, with high protein and low glucose. In children and adults the CSF to blood glucose ratio is typically at or below 0.4.
How does viral meningitis CSF differ from bacterial?
Viral meningitis shows lymphocytes, normal glucose and only a modest rise in protein.
Transcript
Read the full narration, by chapter631 words
0:00The hook: same CSF, different bacteria
A three-week-old baby and an 80-year-old man are both feverish and drowsy. Both have a lumbar puncture, and both samples are cloudy, full of neutrophils, with very little sugar. The fluids look alike, yet the bacteria differ. Age and risk group tell you which. Today: predict the likely meningitis organism from age and risk group, and read the cerebrospinal fluid. Bacteria usually reach the meninges from the
0:26How bacteria reach the meninges
blood. In children and adults they come from the nose and throat. In newborns they come from the mother or the gut.
0:37Five organisms and their Gram stains
Recall the Gram stain from the cell wall lesson. Five organisms cover most questions, and the Gram stain tells them apart. Group B streptococcus: gram-positive cocci in chains. Pneumococcus: gram-positive diplococci. Meningococcus: gram-negative diplococci. E coli: gram-negative rods. Listeria: gram-positive rods. Now walk through life.
1:03Newborns and young infants
Newborns and young infants, up to about three months. The causes are group B streptococcus, E coli, and Listeria. Group B streptococcus comes from the mother's genital tract, and E coli from the gut. Listeria comes from contaminated food, passed on by the mother.
1:24Older infants and children
Older infants and children: pneumococcus and meningococcus. Haemophilus influenzae type b used to be a major cause, but it has fallen with vaccination.
1:35Adolescents, young adults and adults
Adolescents and young adults: meningococcus, with a second peak of disease around that age, for example in students living in dormitories. Meningococcal disease can also cause a purpuric rash. Adults: pneumococcus and meningococcus again lead the list.
1:53The extra group: Listeria, and neurosurgery or shunts
Now the extra group. Listeria joins the list in people over 50 and in anyone with weak cell-mediated immunity, including in pregnancy. Ageing and immunosuppressive drugs weaken the T cell and macrophage defence that clears Listeria. After neurosurgery, or with a shunt, think staphylococci and gram-negative rods, including Pseudomonas.
2:18The fluid itself: bacterial CSF
Now the fluid itself. In bacterial meningitis, the cell count is high and mostly neutrophils, often over a 1000 per cubic millimetre. The protein is high, and the glucose is low. In children and adults, bacterial meningitis typically gives a CSF to blood glucose ratio at or below 0.4. Newborns use a different cut-off, so this one is for children and adults. Try it.
2:46The CSF to blood glucose ratio, worked example
Blood glucose 90, CSF glucose 30. Thirty over 90 is a third, which is below 0.4, so this is bacterial. Viral meningitis is the contrast:
2:59Viral CSF and the Listeria mixed picture
lymphocytes, normal glucose, and only a modest rise in protein. Listeria is the one that can show a mixed picture of neutrophils and lymphocytes. Your turn.
3:12Practice question
A 71-year-old woman who takes prednisone and methotrexate for rheumatoid arthritis has fever, headache, and new confusion for 1 day. A lumbar puncture shows white cells of 1850 per cubic millimetre, 74% neutrophils, protein of 160 milligrams per decilitre, and glucose of 24 milligrams per decilitre, with a serum glucose of 110. The CSF Gram stain shows gram-positive rods. Which organism is the most likely cause? A, Escherichia coli. B, Listeria monocytogenes. C, Neisseria meningitidis. D, Streptococcus agalactiae. E, Streptococcus pneumoniae.
4:03Pause, then the wrong options first
Pause here and decide. A is out: E coli is a gram-negative rod, so the Gram stain rules it out. C is out: meningococcus is a gram-negative diplococcus, and it peaks in infants and young adults. D is out: group B streptococcus is a gram-positive coccus in chains, and a newborn pathogen. E is out: pneumococcus is common at her age, but it is a gram-positive diplococcus, not a rod. So the answer is B. Age over 50, immunosuppression, and gram-positive rods all point to Listeria.
4:44Recap and next lesson
Three things to keep. One: newborns get group B streptococcus, E coli, and Listeria. Two: children and adults get pneumococcus and meningococcus, and Listeria is added after 50 or with weak cell-mediated immunity. Three: bacterial CSF has neutrophils, high protein, and low glucose, a ratio at or below 0.4 in children and adults. Next, the febrile infant, and how to assess risk.
Sources
Numbers in the text, e.g. [1], point here- CSF leukocytes in intracranial infections: pathogen table and CSF profiles. Cell Mol Neurobiol 2026. Europe PMC PMC12932793. europepmc.org/article/PMC/PMC12932793
- Pediatric bacterial meningitis in southern China: analysis of 838 cases. Front Cell Infect Microbiol 2025. Europe PMC PMC11835870. europepmc.org/article/PMC/PMC11835870
- Demographic and pathogen characteristics of incident bacterial meningitis in infants in South Africa. PLoS One 2024. Europe PMC PMC11423971. europepmc.org/article/PMC/PMC11423971
- Burden of invasive meningococcal disease: evolving epidemiology. Infect Dis Ther 2026. Europe PMC PMC13658686. europepmc.org/article/PMC/PMC13658686
- Invasive listeriosis in an otherwise immunocompetent elderly patient. Cureus 2026. Europe PMC PMC13391224. europepmc.org/article/PMC/PMC13391224
- Epidemiology, pathogen distribution, and comparative CSF profiles of viral and bacterial meningitis in South Lebanon. BMC Public Health 2026. Europe PMC PMC13729349. europepmc.org/article/PMC/PMC13729349
- Ventriculitis: a severe complication of central nervous system infections. Open Forum Infect Dis 2021. Europe PMC PMC8176394. europepmc.org/article/PMC/PMC8176394
- Meningitis; Streptococcus agalactiae; Streptococcus pneumoniae; Neisseria meningitidis; Escherichia coli; Listeria monocytogenes. Wikipedia (background). en.wikipedia.org/wiki/Meningitis
- Early mortality risk stratification in childhood bacterial meningitis using CSF glucose and protein levels. PLoS One 2026. Europe PMC PMC13249170. europepmc.org/article/PMC/PMC13249170
- Nomogram to predict probable meningitis in very preterm or very low birth weight infants with early-onset sepsis. BMC Pregnancy Childbirth 2026. Europe PMC PMC13445912. europepmc.org/article/PMC/PMC13445912
The worked glucose example on screen, blood 90 and CSF 30, uses invented teaching values. Newborns use a different CSF to blood glucose cut-off, which this lesson does not give as a number.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): newborn, infant, adolescent, young man, elderly man, sagittal CSF plate, healthy and inflamed meninges. Icons: Health Icons (healthicons.org), CC0: pregnancy, pills. Original diagrams drawn in code: the Gram-stain sketches, the age ladder, the incidence sketch, the CSF tubes and bars, the glucose ratio line, the T cell and macrophage sketch, the question card and all handwriting. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-MICRO-008.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
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- EP 02Acid-fast stain and the mycobacterial cell wall5:01
- EP 03Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids5:46
- EP 04Bacterial toxins that raise cAMP: cholera, pertussis, anthrax5:54
- EP 05Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile5:22
- EP 06Staph vs strep: catalase, coagulase and haemolysis5:23
- EP 07Encapsulated organisms and the spleen: asplenia and conjugate vaccines5:08
- EP 08Bacterial genetics: how bacteria swap genes5:13
- EP 09Beta-lactamase and antibiotic resistance: three ways bacteria win5:13
- EP 10DNA vs RNA virus replication: which enzyme each genome needs5:48
- EP 11HIV life cycle and drug targets, step by step5:39
- EP 12Hepatitis B serology: reading the panel5:17
- EP 13Antifungal drugs explained: mechanisms of action5:26
- EP 14Bacterial meningitis organisms by age and risk group5:19