Microbiology and Immunology

Pneumonia, bacterial and viral WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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the sporozoite enters the bloodstream and makes its way to the

liver. After multiplying in liver cells, the protozoan can pene-

trate red blood cells, which is a hallmark of the disease

malaria. Multiplication occurs in a red blood cell, which ulti-

mately bursts, releasing new forms of the protozoa that can

infect neighbouring red blood cells. Such cycles lead to mas-

sive destruction of red blood cells.

Malaria can produce a myriad of symptoms, including

high fever, generalized aches, tender spleen and liver, jaun-

dice, and, in severe cases, convulsions, failure of the kidneys,

shock, and collapse of the circulatory system. The fever tends

to be cyclical, reflecting the cyclical pattern of protozoan

release from red blood cells followed by a period of protozoan

multiplication inside other red blood cells. These cycles can

vary from 48 hours with Plasmodium vivaxto about 72 hours

with Plasmodium malariae.

Resistance of the protozoa, particularly Plasmodium

falciparum, to the drugs such as chloroquinine and

pyrimethamine that have previously been an effective control

was first reported in 1961. Since that time, the occurrence of

resistance has increased. A major factor in the development of

the resistance is the adaptivity of the protozoan. The genome

of the Plasmodiumis very complex, and genetic alteration to

new environmental pressures occurs quickly.

See alsoParasites; Zoonoses

PLASTID•seePLASMIDS

PPneumonia, bacterial and viralNEUMONIA, BACTERIAL AND VIRAL

Pneumonia is an infection of the lung, and can be caused by

nearly any class of organism known to cause human infec-

tions, including bacteria, viruses, fungi, and parasites. In the

United States, pneumonia is the sixth most common disease

leading to death, and the most common fatal infection

acquired by already hospitalized patients. In developing

countries, pneumonia ties with diarrhea as the most common

cause of death.

The main function of the respiratory system is to pro-

vide oxygen, the most important energy source for the body’s

cells. Inspired air travels down the respiratory tree to the alve-

oli, where the oxygen moves out of the alveoli and is sent into

circulation throughout the body as part of the red blood cells.

The oxygen in the inspired air is exchanged within the alveoli

for the body’s waste product, carbon dioxide, which leaves the

alveoli during expiration.

The normal, healthy human lung is sterile, meaning that

there are no normally resident bacteria or viruses (unlike the

upper respiratory system and parts of the gastrointestinal system,

where bacteria dwell even in a healthy state). There are multiple

safeguards along the path of the respiratory system that are

designed to keep invading organisms from leading to infection.

The first line of defense includes the hair in the nostrils,

which serves as a filter for larger particles. The epiglottis is a

trap door of sorts, designed to prevent food and other swal-

lowed substances from entering the larynx and then trachea.

Sneezing and coughing, both provoked by the presence of irri-

tants within the respiratory system, help to clear such irritants

from the respiratory tract.

Mucous, produced throughout the respiratory system,

also serves to trap dust and infectious organisms. Tiny hair-

like projections (cilia) from cells lining the respiratory tract

beat constantly, moving debris, trapped by mucus, upwards

and out of the respiratory tract. This mechanism of protection

is referred to as the mucociliary escalator.

Cells lining the respiratory tract produce several types

of immune substances which protect against various organ-

isms. Other cells (called macrophages) along the respiratory

tract actually ingest and kill invading organisms.

The organisms that cause pneumonia, then, are usually

carefully kept from entering the lungs by virtue of these host

defenses. However, when an individual encounters a large

number of organisms at once, either by inhaling contaminated

air droplets, or by aspiration of organisms inhabiting the upper

airways, the usual defenses may be overwhelmed and infec-

tion may occur.

In addition to exposure to sufficient quantities of

causative organisms, certain conditions may predispose an

individual to pneumonia. Certainly, the lack of normal

anatomical structure could result in an increased risk of pneu-

monia. For example, there are certain inherited defects of cilia

which result in less effective protection. Cigarette smoke,

inhaled directly by a smoker or second-hand by an innocent

bystander, interferes significantly with ciliary function, as well

as inhibiting macrophage function.

Stroke, seizures, alcohol, and various drugs interfere

with the function of the epiglottis, leading to a leaky seal on

the trap door, with possible contaminationby swallowed sub-

stances and/or regurgitated stomach contents. Alcohol and

drugs also interfere with the normal cough reflex, further

decreasing the chance of clearing unwanted debris from the

respiratory tract.

Viruses may interfere with ciliary function, allowing

themselves or other microorganism invaders, such as bacteria,

access to the lower respiratory tract. One of the most impor-

tant viruses which in recent years has resulted in a huge

increase in the incidence of pneumonia is HIV (Human

Immunodeficiency Virus), the causative virus in AIDS

(Acquired Immunodeficiency Syndrome). Because AIDS

results in a general decreased effectiveness of many aspects of

the host’s immune system, a patient with AIDS is susceptible

to all types of pneumonia, including some previously rare par-

asitic types which would be unable to cause illness in an indi-

vidual possessing a normal immune system.

The elderly have a less effective mucociliary escalator,

as well as changes in their immune system, all of which cause

them to be more at risk for the development of pneumonia.

Various chronic conditions predispose to pneumonia,

including asthma, cystic fibrosis, neuromuscular diseases which

may interfere with the seal of the epiglottis, and esophageal dis-

orders which result in stomach contents passing upwards into

the esophagus (increasing the risk of aspiration of those stom-

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