1. Infections of the Central Nervous System
MLAB 2434 – Microbiology
Keri Brophy-Martinez

2. General Concepts
Infections of the CNS are of critical concern and positive laboratory findings are “critical values”
Infections may be caused by bacteria, fungi, viruses or parasites

3. Anatomy of CNS Brain, Spinal Cord and Cranial Nerves Meninges
Dura mater
Pia arachnoid
Pia mater
Subarachnoid space between pia arachnoid and pia mater; CSF

4. CSF
Unique body fluid produced by filtration and secretion from specialized capillary tufts of the four ventricles of the brain
Circulates around brain and spinal cord under pressure
Serves as cushion for brain and spinal cord

5. CSF Characteristics CSF is a clear and colorless sterile fluid
Adults
Protein mg/dL
Glucose mg/dL
WBC 0-5 mm3
Newborns
Higher levels of protein & glucose
Infections indicated by increased cell counts and alterations in protein and glucose

6. CNS Infections Portals of entry for bacteria
Respiratory (most common route of community-acquired infections)
Auditory
Bloodstream
Neural routes
Contiguous sites

7. Types of CNS Infections
Meningitis
Also called “leptomeningitis” (pia mater & arachnoid)
Inflammation around blood vessels within the subarachnoid space
Classifications
Acute
Chronic
Aseptic

8. Bacterial CNS Infections
Acute Bacterial Meningitis
Peaks in winter & early spring
Symptoms include photophobia, headache, nausea, vomiting, and stiff neck or Brundzinski sign
In infants, only symptoms may be irritability, poor feeding, and restlessness

9. Bacterial CNS Infections (cont’d)
Acute Bacterial Meningitis (cont’d)
Infants
Group B Strep
L. monocytogenes
H. influenzae
S. pneumoniae
E. coli
Children
N. meningitidis

10. Bacterial CNS Infections (cont’d)
Acute Bacterial Meningitis (cont’d)
Adolescents
N. meningitidis
S. pneumoniae
Adults
Elderly
Gram-negative bacilli

11. Bacterial CNS Infections (cont’d)
Acute Bacterial Meningitis (cont’d)
Lab findings
Leukocytosis with left shift and toxic changes, such as toxic granulation & Dohle bodies
Increased CSF protein
Decreased CSF glucose
Increased intracranial pressure

12. Types of CNS Infections (cont’d)
“Aseptic Meningitis”
CSF shows predominance of lymphocytes with no organisms on direct examination
Commonly caused by viruses
Enterovirus
Arbovirus
Mumps virus
Herpes Simplex Virus

13. Types of CNS Infections (cont’d)
Encephalitis/ Meningoencephalitis
Diffuse inflammation of the cerebral cortex
Observe mental changes or neurologic signs
Common in viral infections

14. Types of CNS Infections (cont’d)
Brain Abscesses
Areas of tissue destruction containing organisms and inflammatory cells
Occur as spread from other body sites or trauma to skull; infecting organism depends on predisposing conditions
Non-traumatic abscesses mostly are aerobic and anaerobic streptococci, Staphylococcus aureus, anaerobic GNR

15. Bacterial CNS Infections (cont’d)
Mycobacterial Infection
Most commonly M. tuberculosis
Enters by respiratory route and spread via bloodstream
CSF cells are mostly lymphocytes and monocytes
Mycobacteria are few and CSF should be centrifuged to concentrate before culturing/gram staining

16. Bacterial CNS Infections (cont’d)
Spirochetal Infections
Lyme Disease
Neurosyphilis
Viral Infections
Enterovirus
Arboviruses
Herpes Virus
Parasitic Infections
Toxoplasma gondii
Naegleria fowleri
Fungal Infections
Crytococcus neoformans
Candida species

17. Laboratory Diagnosis of CNS Infections
CSF Collection
Lumbar puncture in lower back
3 to 4 tubes collected
1st = chemistries
2nd = microbiology
3rd = cell counts
4th = special procedures (if collected)
Process ASAP

18. Laboratory Diagnosis of CNS Infections (cont’d)
CSF Analysis
Acute Bacterial Meningitis
CSF turbid or cloudy
WBC increased with predominance of neutrophils
Increased CSF protein
Decreased CSF glucose
Centrifugation concentrates organisms
Should be plated on at least BAP, CA and broth; MAC if Gram-negative bacilli are expected
Bacterial Antigen Testing
Current literature indicates routine bacterial antigen testing is of limited value

19. Laboratory Diagnosis of CNS Infections (cont’d)
CSF Analysis
Viral Meningitis
Number of lymphocytes increased
Diagnosis based on PCR or EIA
Fungal Meningitis
Gram stain or India ink
Culture held for 4-6 weeks

20. Laboratory Diagnosis of CNS Infections (cont’d)
CSF Analysis
Tuberculous meningitis
Increase in lymphocytes
Increase in protein
Decrease in glucose
AFB stain & culture
PCR can be used; however, false positives common

21. Characteristic Findings in Meningitis
Bacterial
Fungal
Tuberculous
Viral
Syphilitic
Parasitic
Organisms seen in CSF
See notes
None
Cell count
(leukocytes/mL)
,000
Neutrophils predominate
Normal- 500
Lymphocytes
predominate
50-500
Normal-200
Lymphocytes/
Eosinophils
Protein (mg/dL)
Normal- 250
Normal- 150
Frequently normal
Usually increased
Glucose (mg/dL)
<30
Normal- decreased
<45
Normal

22. References
Kiser, K. M., Payne, W. C., & Taff, T. A. (2011). Clinical Laboratory Microbiology: A Practical Approach . Upper Saddle River, NJ: Pearson Education.
Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook of Diagnostic Microbiology (4th ed.). Maryland Heights, MO: Saunders.