Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY AIDS

7

•

cific, with a spectrum of components present. Other media are

defined, with precise amounts of a few set materials included.

Likewise the use of agarose has proved tremendously useful in

electrophoretic techniques. By manipulation of the formula-

tion conditions, the agarose matrix can have pores, or tunnels

through the agarose strands, which can be of different size.

Thus the agarose can act as a sieve, to separate molecules on

the basis of the size. The uncharged nature of agarose allows

a current to be passed through it, which can drive the move-

ment of samples such as pieces of deoxyribonucleic acid

(DNA) from one end of an agarose slab to the other. The speed

of the molecule movement, is also related to molecular size

(largest molecules moving the least).

In the non-microbiological world, agar and agarose

have also found a use as stabilizers in ice cream, instant cream

whips, and dessert gelatins.

See alsoBacterial growth and division; Laboratory techniques

in microbiology

AAgar diffusionGAR DIFFUSION

Agardiffusion refers to the movement of molecules through

the matrix that is formed by the gelling of agar. When per-

formed under controlled conditions, the degree of the mole-

cule’s movement can be related to the concentration of the

molecule. This phenomenon forms the basis of the agar diffu-

sion assay that is used to determine the susceptibility or resist-

ance of a bacterial strain to an antibacterial agent, (e.g.,

including antibiotics.

When the seaweed extract known as agar is allowed to

harden, the resulting material is not impermeable. Rather,

there are spaces present between the myriad of strands of agar

that comprise the hardened polymer. Small molecules such as

antibiotics are able to diffuse through the agar.

Typically, an antibiotic is applied to a well that is cut

into the agar. Thus, the antibiotic will tend to move from this

region of high concentration to the surrounding regions of

lower antibiotic concentration. If more material is present in

the well, then the zone of diffusion can be larger.

This diffusion was the basis of the agar diffusion assay

devised in 1944. A bacterial suspension is spread onto the sur-

face of the agar. Then, antibiotic is applied to a number of

wells in the plate. There can be different concentrations of a

single antibiotic or a number of different antibiotics present.

Following a time to allow for growth of the bacteriathen agar

is examined. If bacterial growthis right up to the antibiotic

containing well, then the bacterial strain is deemed to be

resistant to the antibiotic. If there is a clearing around the

antibiotic well, then the bacteria have been adversely affected

by the antibiotic. The size of the inhibition zone can be meas-

ured and related to standards, in order to determine whether

the bacterial strain is sensitive to the antibiotic.

This technique can also be done by placing disks of an

absorbent material that have been soaked with the antibiotic of

interest directly onto the agar surface. The antibiotic will subse-

quently diffuse out of the disk into the agar. This version of agar

diffusion is known as the Kirby-Bauer disk-diffusion assay.

The agar diffusion assay allows bacteria to be screened

in a routine, economical and easy way for the detection of

resistance. More detailed analysis to ascertain the nature of the

resistance can then follow.

See alsoAntibiotic resistance, tests for; Laboratory techniques

in microbiology

AGGLUTINATION•seeANTIBODY-ANTIGEN, BIOCHEM-

ICAL AND MOLECULAR REACTIONS

AIDSAIDS

The advent of AIDS (acquired immunitydeficiency syndrome)

in early 1981 surprised the scientific community, as many

researchers at that time viewed the world to be on the brink of

eliminating infectious disease. AIDS, an infectious disease

syndrome that suppresses the immune system, is caused by the

Human Immune Deficiency Virus(HIV), part of a group of

virusesknown as retroviruses. The name AIDS was coined in

1982. Victims of AIDS most often die from opportunistic
      infections that take hold of the body because the immune sys-
      tem is severely impaired.
      Following the discovery of AIDS, scientists attempted
      to identify the virus that causes the disease. In 1983 and 1984
      two scientists and their teams reported isolating HIV, the virus
      that causes AIDS. One was French immunologist Luc
      Montagnier(1932– ), working at the Pasteur Institute in Paris,
      and the other was American immunologist Robert Gallo
      (1937– ) at the National Cancer Institute in Bethesda,
      Maryland. Both identified HIV as the cause of AIDS and
      showed the pathogen to be a retrovirus, meaning that its
      genetic material is RNAinstead of DNA. Following the discov-

Staphylococcus colonies showing hemolytic reaction on blood agar.

womi_A 5/6/03 1:06 PM Page 7