Infections of the
Central Nervous
System
MLAB 2434 – Microbiology
Keri Brophy-Martinez
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
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
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

CSF Characteristics

CSF is a clear and colorless sterile fluid

Adults
• Protein 15-45 mg/dL
• Glucose 40-80 mg/dL
• WBC 0-5 mm3

Newborns
• Higher levels of protein & glucose

Infections indicated by increased cell
counts and alterations in protein and
glucose
CNS Infections

Portals of entry for bacteria
Respiratory (most common route of
community-acquired infections)
 Auditory
 Bloodstream
 Neural routes
 Contiguous sites

Types of CNS Infections

Meningitis



Also called “leptomeningitis” (pia mater &
arachnoid)
Inflammation around blood vessels within
the subarachnoid space
Classifications
• Acute
• Chronic
• Aseptic
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

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
• S. pneumoniae
• H. influenzae
Bacterial CNS Infections
(cont’d)

Acute Bacterial Meningitis (cont’d)



Adolescents
• N. meningitidis
• S. pneumoniae
Adults
• N. meningitidis
• S. pneumoniae
Elderly
• Gram-negative bacilli
• N. meningitidis
• S. pneumoniae
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
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
Types of CNS Infections
(cont’d)

Encephalitis/ Meningoencephalitis



Diffuse inflammation of the cerebral
cortex
Observe mental changes or neurologic
signs
Common in viral infections
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
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
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
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
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
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
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
Characteristic Findings in Meningitis
Bacterial
Fungal
Tuberculous
Viral
Syphilitic
Parasitic
Organisms seen in
CSF
See notes
See notes
See notes
None
None
See notes
Cell count
(leukocytes/mL)
100-100,000
Neutrophils
predominate
Normal- 500
Lymphocytes
predominate
50-500
Lymphocytes
predominate
Normal-200
Lymphocytes
predominate
100-750
Lymphocytes
predominate
Normal-200
Lymphocytes/
Eosinophils
predominate
Protein (mg/dL)
100-500
Normal- 250
Normal- 150
Frequently
normal
50- 250
Usually increased
Glucose (mg/dL)
<30
Normaldecreased
<45
Normal
Normal
Normal- decreased
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.