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Learning Objectives: Microbiology of CNS Infections I: Meningitis

1. Review the unique anatomy of the brain, spinal cord, and blood-barrier and under- standhow infectious organisms can attack these structures.

BBB excludes macromolecules, microorganisms, immune system cells, and antibodies. It

contains cerebral capillaries, choroids plexus, and arachnoid cells which all use tight junctions.The BBB deters infectious invasion, but hampers clearance once infection starts.

There is no Brain-CSF barrier. Infection in ventricles will spread. But brain cells are so

close together its hard for anything to move between them (immune cells or microbes).

Some infections can spread by axoplasmic transport (viruses and bacterial toxins, likerabies and tetanus).

Some neural cells have special receptors used by viruses.

Direct transfer with craniotomy/skull fracture/sinus infection/ shunt/congenital ectodermaldefect for Staph aureus and Enterobacteria.

2. Understand how bacterial, viral, and fungal infections of the blood can lead to menin- gitis.

Septicemia of the blood happens, then large numbers of organisms develop, and some areselected to enter the CNS (rare variants) and these then proliferate because it is an immune

privileged site. Meningitis is inflammation of pia-arachnoid meninges. It can be within the cellsor in the subarachnoid space.

3. Know the major bacteria, viruses, and fungi that cause meningitis.

Bacteria:InfantsGroup B Strep (Streptococcus agalactiae), E. coli, Listeria

Children (pre-vaccine era)Hemophilus influenzae

AdultsNeisseria meningitides, Streptococcus pneumoniae (others if skull penetration)

Other bugsother strep, staphylococci, mycobacteria (TB)Viruses: Enteroviruses, Mumps, Lymphocytic chorio-meningitis, other, unknown

FungiCryptococcus Neoformans, Coccidioides immitis, other

4. Understand the utility of sampling cerebrospinal fluid (CSF) to detect organismsthat cause meningitis.

BacteriaNormal or high pressure, PMNs, high protein, low sugar, and cultures can be

done.ViralNormal pressure, mononuclear cells, normal or small elevation in protein, normal

sugar, some special tests exist.

Subacute Funga/MycobacterialNormal pressure, mononuclear cells, high protein, normalor slightly low sugar, some cultures or stains.

Brain Abscesshigh pressure, some PMNs, slightly high protein, normal sugar, no special

testsEncephalitishigh pressure, mononuclear cells, slightly high protein, normal sugar, some

special tests.

5. Know properties of the bacteria infecting the central nervous system, tests used todetect them, and that specific antibiotics that cross the blood-brain barrier must be

used to treat infections of the CNS

Bugs maintain bacteremia, use capsid polysaccaraides to avoid phagocytosis, or evadeclearance by growing in red blood cells.

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Symptoms are headache, fever, nuchal rigidity, Brudzinski sign (reflex flexion of legs

when neck is flexed), Kernig sign (limited extension of leg when neck flexed), obtundation,

seizures in kids. Later on pus can collect at base of brain and cause cranial nerve palsies and CSFobstruction or hydrocephalus. Vasculitis, infarction, and multifocal deficits will follow.

Meningococci cause a purpuric rash (petechiae) (neisseria).

Penicillin G, Cefotaxime, Ceftriaxone, Ampicillin, Ceftazidime, and VancomycinCephalosporin (cefotaxime or ceftriaxone) is appropriate for children over 3 months old.

Penetrates CNS, covers N. Meningitidis, S. Pneumoniae, H. Influenzae.

Add ampicillin in young infants < 3 months old.

6. Know the important fungal infections of the nervous system, fungi that cause them,

and currently recommended antibiotic treatments

Cryptococcus neoformans in immunocompromised people. Coccidioides immitissubacute. Use various fungicidals depending on the fungus and susceptibility.

7. Know important viruses that infect the nervous system and their effects on the CNS.

Mumps causes parotitis. Enterovirus causes rash. Only symptomatic treatment. Rule out

other causes.