Microbiology and Immunology

Meningitis, bacterial and viral WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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MMeningitis, bacterial and viralENINGITIS, BACTERIAL AND VIRAL

Meningitis is a potentially fatal inflammationof the meninges,

the thin, membranous covering of the brain and the spinal

cord. Meningitis is most commonly caused by infection (by

bacteria, viruses, or fungi), although it can also be caused by

bleeding into the meninges, cancer, or diseases of the immune

system.

The meninges are three separate membranes, layered

together, which serve to encase the brain and spinal cord. The

dura is the toughest, outermost layer, and is closely attached to

the inside of the skull. The middle layer, the arachnoid, is

important in the normal flow of the cerebrospinal fluid (CSF),

a lubricating fluid that bathes both the brain and the spinal

cord. The innermost layer, the pia, helps direct brain blood

vessels into the brain. The space between the arachnoid and

the pia contains CSF, which serves to help insulate the brain

from trauma. Through this space course many blood vessels.

CSF, produced within specialized chambers deep inside the

brain, flows over the surface of the brain and spinal cord. This

fluid serves to cushion these relatively delicate structures, as

well as supplying important nutrients for brain cells. CSF is

reabsorbed by blood vessels that are located within the

meninges.

The cells lining the brain’s capillaries (tiny blood ves-

sels) are specifically designed to prevent many substances

from passing into brain tissue. This is commonly referred to as

the blood-brain barrier. The blood-brain barrier prevents vari-

ous toxins (substances which could be poisonous to brain tis-

sue), as well as many agents of infection, from crossing from

the blood stream into the brain tissue. While this barrier obvi-

ously is an important protective feature for the brain, it also

serves to complicate therapy in the case of an infection, by

making it difficult for medications to pass out of the blood and

into the brain tissue where the infection resides.

The most common infectious causes of meningitis vary

according to an individual host’s age, habits and living envi-

ronment, and health status. In newborns, the most common

agents of meningitis are those that are contracted from the

newborn’s mother, including Group B Streptococci(becoming

an increasingly common infecting organism in the newborn

period), Escherichia coli,and Listeria monocytogenes.Older

children are more frequently infected by Haemophilus

influenzae, Neisseria meningitidis,and Streptococcus pneu-

moniae,while adults are infected by S. pneumoniaeand N.

meningitidis. N. meningitidisis the only organism that can

cause epidemicsof meningitis. These have occurred in partic-

ular when a child in a crowded day-care situation, a college

student in a dormitory, or a military recruit in a crowded train-

ing camp has fallen ill with N. meningitidismeningitis.

Viral causes of meningitis include the herpessimplex

viruses, mumpsand measlesviruses (against which most

children are protected due to mass immunizationprograms),

the virus that causes chicken pox, the rabiesvirus, and a num-

ber of viruses that are acquired through the bite of infected

mosquitoes. Patients with AIDS(Acquired Immune Deficiency

Syndrome) are more susceptible to certain infectious causes of

meningitis, including by certain fungal agents, as well as by

the agent that causes tuberculosis. Patients who have had their

spleens removed, or whose spleens are no longer functional

(as in the case of patients with sickle cell disease) are more

susceptible to certain infections, including those caused by N.

meningitidis and S. pneumoniae.

The majority of meningitis infections are acquired by

blood-borne spread. An individual may have another type of

infection (of the lungs, throat, or tissues of the heart) caused

by an organism that can also cause meningitis. The organism

multiplies, finds its way into the blood stream, and is delivered

in sufficient quantities to invade past the blood-brain barrier.

Direct spread occurs when an already resident infec-

tious agent spreads from infected tissue next to or very near

the meninges, for example from an ear or sinus infection.

Patients who suffer from skull fractures provide openings to

the sinuses, nasal passages, and middle ears. Organisms that

frequently live in the human respiratory system can then pass

through these openings to reach the meninges and cause infec-

tion. Similarly, patients who undergo surgical procedures or

who have had foreign bodies surgically placed within their

skulls (such as tubes to drain abnormal amounts of accumu-

lated CSF) have an increased risk of the organisms causing

meningitis being introduced to the meninges.

The most classic symptoms of meningitis (particularly

of bacterial meningitis) include fever, headache, vomiting,

photophobia (sensitivity to light), irritability, lethargy (severe

fatigue), and stiff neck. The disease progresses with seizures,

confusion, and eventually coma.

Damage due to meningitis occurs from a variety of phe-

nomena. The action of infectious agents on the brain tissue is

one direct cause of damage. Other types of damage may be

due to mechanical effects of swelling of brain tissue, and com-

pression against the bony surface of the skull. Swelling of the

meninges may interfere with the normal absorption of CSF by

blood vessels, causing accumulation of CSF and damage due

to resulting pressure on the brain. Interference with the brain’s

carefully regulated chemical environment may cause damag-

ing amounts of normally present substances (carbon dioxide,

potassium) to accumulate. Inflammation may cause the blood-

brain barrier to become less effective at preventing the passage

of toxic substances into brain tissue.

Antibiotic medications (forms of penicillins and

cephalosporins, for example) are the most important element

of treatment against bacterial agents of meningitis. Because of

the effectiveness of the blood-brain barrier in preventing pas-

sage of substances into the brain, medications must be deliv-

ered directly into the patient’s veins (intravenous or IV) at

very high doses. Antiviral medications (acyclovir) may be

helpful in the case of viral meningitis, and antifungal medica-

tions are available as well.

Other treatment for meningitis involves decreasing

inflammation (with steroid preparations) and paying careful

attention to the balance of fluids, glucose, sodium, potassium,

oxygen, and carbon dioxide in the patient’s system. Patients

who develop seizures will require medications to halt the

seizures and prevent their return.

A series of immunizations against Haemophilus influen-

zae,started at two months of age, has greatly reduced the inci-

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