Microbiology and Immunology

Anaerobes and anaerobic infections WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Amino acids are also the core construction materials for

neurotransmitters and hormones. Neurotransmitters are chem-

icals that allow nerve cells to communicate with one another

and to convey information through the nervous system.

Hormones also serve a communication purpose. These chem-

icals are produced by glands and trigger metabolic processes

throughout the body. Plants also produce hormones.

Important neurotransmitters that are created from amino

acids include serotonin and gamma-aminobutyric acid.

Serotonin(C 10 H 12 N 2 O) is manufactured from tryptophan, and

gamma-aminobutyric acid (H 2 N(CH 2 ) 3 COOH) is made from

glutamic acid. Hormones that require amino acids for starting

materials include thyroxine (the hormone produced by the thy-

roid gland), and auxin (a hormone produced by plants).

Thyroxine is made from tyrosine, and auxin is constructed

from tryptophan.

A class of chemicals important for both neurotransmit-

ter and hormone construction are the catecholamines. The

amino acids tyrosine and phenylalanine are the building mate-

rials for catecholamines, which are used as source material for

both neurotransmitters and for hormones.

Amino acids also play a central role in the immune sys-

tem. Allergic reactions involve the release of histamine, a

chemical that triggers inflammationand swelling. Histamine is

a close chemical cousin to the amino acid histidine, from

which it is manufactured.

Melatonin, the chemical that helps regulate sleep cycles,

and melanin, the one that determines the color of the skin, are

both based on amino acids. Although the names are similar,

the activities and component parts of these compounds are

quite different. Melatonin uses tryptophan as its main building

block, and melanin is formed from tyrosine. An individual’s

melanin production depends both on genetic and environmen-

tal factors.

Proteins in the diet contain amino acids that are used

within the body to construct new proteins. Although the body

also has the ability to manufacture certain amino acids, other

amino acids cannot be manufactured in the body and must be

gained through diet. Such amino acids are called the essential

dietary amino acids, and include arginine, histidine,

isoleucine, leucine, lysine, methionine, phenylalanine, threo-

nine, tryptophan, and valine.

Foods such as meat, fish, and poultry contain all of the

essential dietary amino acids. Foods such as fruits, vegetables,

grains, and beans contain protein, but they may lack one or

more of the essential dietary amino acids. However, they do

not all lack the same essential dietary amino acid. For exam-

ple, corn lacks lysine and tryptophan, but these amino acids

can be found in soy beans. Therefore, vegetarians can meet

their dietary needs for amino acids as long by eating a variety

of foods.

Amino acids are not stockpiled in the body, so it is nec-

essary to obtain a constant supply through diet. A well-bal-

anced diet delivers more protein than most people need. In

fact, amino acid and protein supplements are unnecessary for

most people, including athletes and other very active individ-

uals. If more amino acids are consumed than the body needs,

they will be converted to fat or metabolized and excreted in

the urine.

However, it is vital that all essential amino acids be

present in the diet if an organism is to remain healthy. Nearly

all proteins in the body require all of the essential amino acids

in their synthesis. If even one amino acid is missing, the pro-

tein cannot be constructed. In cases in which there is an on-

going deficiency of one or more essential amino acids, an

individual may develop a condition known as kwashiorkor,

which is characterized by severe weight loss, stunted growth,

and swelling in the body’s tissues. The situation is made even

more grave because the intestines lose their ability to extract

nutrients from whatever food is consumed. Children are more

strongly affected by kwashiorkor than adults because they are

still growing and their protein requirements are higher.

Kwashiorkor often accompanies conditions of famine and

starvation.

See alsoBacterial growth and division; Biochemistry; Cell

cycle (eukaryotic), genetic regulation of; Cell cycle (prokary-

otic), genetic regulation of; Cell cycle and cell division;

Chromosomes, eukaryotic; Chromosomes, prokaryotic; DNA

(Deoxyribonucleic acid); Enzymes; Genetic regulation of

eukaryotic cells; Genetic regulation of prokaryotic cells;

Genotype and phenotype; Molecular biology and molecular

genetics

AMINOGLYCOSIDE ANTIBIOTICS•see

ANTIBIOTICS

AMYLOID PLAQUES• seeBSE ANDCJD DISEASE

ANAEROBES AND ANAEROBIC

INFECTIONSAnaerobes and anaerobic infections

Anaerobes are bacteriathat are either capable of growing in

the absence of oxygen (referred to as facultative anaerobes) or

that absolutely require the absence of oxygen (these are also

called obligate anaerobes). Among the oxygen-free environ-

ments in which such bacteria can grow are deep wounds

and tissues in the body. Growth in these niches can produce

infections.

Examples of infections are gas gangrene (which is

caused by Streptococcus pyogenes) and botulism(which is

caused by Clostridium botulinum). Other anaerobic bacteria

that are frequently the cause of clinical infections are members

of the genus Peptostreptococcusand Bacteroides fragilis.

There are a number of different types of anaerobic bac-

teria. Two fundamental means of differentiation of these types

is by their reaction to the Gram stain and by their shape. The

genus Clostridiumconsists of Gram-positive rod-shaped bac-

teria that form spores. Gram-positive rods that do not form

spores include the genera Actinomyces, Bifidobacterium,

Eubacterium, Propionibacterium, and Lactobacillus. Gram-

positive bacteria that are spherical in shape includes the gen-

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