Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Diphtheria

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thesis. Still other dinoflagellates can do both. The life cycle of

the organisms is also complex, involving forms that are

immobile and capable of movement and forms that are capa-

ble of sexual or asexual reproduction (bacteria, for example,

reproduce asexually, by the self-replication of their genetic

material and other constituents). Dinoflagellates are primarily

asexual in reproduction.

Some dinoflagellates contain plates of cellulose that lie

between the two surface membranes that cover the organism.

These plates function as protective armor.

Dinoflagellates are noteworthy for several reasons.

They are one of the bedrocks of the food chain, particularly in

the oceans and lakes of the world. Their numbers can be so

great that they are evident as a mass of color on the surface of

the water. Sometimes satellite cameras can even visualize

these blooms. This abundant growth can consume so much

oxygen that survival of other species in the area is threatened.

As well, some dinoflagellates can produce toxins that can find

their way into higher species, particularly those such as shell-

fish that feed by filtering water through them. Paralytic shell-

fish poisoning, which harms the neurological system of

humans, is an example of a malady associated with the con-

sumption of clams, mussels, and oysters that are contaminated

with dinoflagellate toxins known as saxitoxin and brevitoxin.

Saxitoxin is extremely potent, exerting its effect on the neuro-

logical system at concentrations 10,000 times lower than that

required by cocaine. Another example of a dinoflagellate-

related malady is a disease called ciguatera, which results

from eating toxin-contaminated fish.

A third distinctive feature of dinoflagellates concerns

their nucleus. The deoxyribonucleic acid shares some features

with the DNAof eukaryotes, such as the presence of repeated

stretches of DNA. But, other eukaryotic features, such as the

supportive structures known as histones, have as yet not been

detected. Also, the amount of DNA in dinoflagellates is far

greater than in eukaryotes. The nucleus can occupy half the

volume of the cell.

As with other microorganisms, dinoflagellates have been

present on the Earth for a long time. Fossils of Arpylorus

antiquushave been found in rock that dates back 400 million

years. And, fossils that may be dinoflagellate cysts have been

found in rock that is almost two billion years old. Current

thought is that dinoflagellates arose when a bacterium was swal-

lowed but not digested by another microorganism. The bacteria

became symbiotic with the organism that swallowed them. This

explanation is also how mitochondria are thought to have arisen.

Dinoflagellates cysts are analogous to the cysts formed

by other microorganisms. They function to protect the genetic

material during periods when conditions are too harsh for

growth. When conditions become more favorable, resuscita-

tion of the cyst and growth of the dinoflagellate resumes.

Dinoflagellates are sometimes referred to as

Pyrrhophyta, which means fire plants. This is because of their

ability to produce biological luminescence, akin to that of the

firefly. Often, these luminescent dinoflagellates can be seen in

the wake of ocean-going ships at night.

See alsoBioluminescence; Red tide; Snow blooms

DDiphtheriaIPHTHERIA

Diphtheria is a potentially fatal, contagious bacterial disease

that usually involves the nose, throat, and air passages, but

may also infect the skin. Its most striking feature is the forma-

tion of a grayish membrane covering the tonsils and upper part

of the throat.

Like many other upper respiratory diseases, diphtheria

is most likely to break out during the winter months. At one

time it was a major childhood killer, but it is now rare in devel-

oped countries because of widespread immunization. Since

1988, all confirmed cases in the United States have involved

visitors or immigrants. In countries that do not have routine

immunization against this infection, the mortality rate varies

from 1.5% to 25%.

Persons who have not been immunized may get diph-

theria at any age. The disease is spread most often by droplets

from the coughing or sneezing of an infected person or carrier.

The incubation period is two to seven days, with an average of

three days. It is vital to seek medical help at once when diph-

theria is suspected, because treatment requires emergency

measures for adults as well as children.

The symptoms of diphtheria are caused by toxins pro-

duced by the diphtheria bacillus, Corynebacterium diphthe-

riae(from the Greek for “rubber membrane”). In fact, toxin

production is related to infections of the bacillus itself with a

particular bacteriavirus called a phage (from bacteriophage;

a virus that infects bacteria). The intoxication destroys healthy

tissue in the upper area of the throat around the tonsils, or in

open wounds in the skin. Fluid from the dying cells then coag-

ulates to form the telltale gray or grayish green membrane.

Inside the membrane, the bacteria produce an exotoxin, which

is a poisonous secretion that causes the life-threatening symp-

toms of diphtheria. The exotoxin is carried throughout the

body in the bloodstream, destroying healthy tissue in other

parts of the body.

The most serious complications caused by the exotoxin

are inflammations of the heart muscle (myocarditis) and dam-

age to the nervous system. The risk of serious complications is

increased as the time between onset of symptoms and the

administration of antitoxin increases, and as the size of the

membrane formed increases. The myocarditis may cause dis-

turbances in the heart rhythm and may culminate in heart fail-

ure. The symptoms of nervous system involvement can

include seeing double (diplopia), painful or difficult swallow-

ing, and slurred speech or loss of voice, which are all indica-

tions of the exotoxin’s effect on nerve functions. The exotoxin

may also cause severe swelling in the neck (“bull neck”).

The signs and symptoms of diphtheria vary according to

the location of the infection. Nasal diphtheria produces few

symptoms other than a watery or bloody discharge. On exam-

ination, there may be a small visible membrane in the nasal

passages. Nasal infection rarely causes complications by

itself, but it is a public healthproblem because it spreads the

disease more rapidly than other forms of diphtheria.

Pharyngeal diphtheria gets its name from the pharynx,

which is the part of the upper throat that connects the mouth

and nasal passages with the larynx. This is the most common

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