Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Agar and agarose

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continuous over a longer time. Secondly, an adjuvant itself can

react with some of the cells of the immune system. This inter-

action may stimulate the immune cells to heightened activity.

Thirdly, an adjuvant can also enhance the recognition and

ingestion of the antigen by the immune cell known as the

phagocyte. This enhanced phagocytosispresents more anti-

gens to the other cells that form the antibody.

There are several different types of antigens. The adju-

vant selected typically depends on the animal being used to gen-

erate the antibodies. Different adjuvants produce different

responses in different animals. Some adjuvants are inappropri-

ate for certain animals, due to the inflammation, tissue damage,

and pain that are caused to the animal. Other factors that influ-

ence the choice of an adjuvant include the injection site, the

manner of antigen preparation, and amount of antigen injected.

One type of adjuvant that has been of long-standing

service in generating antibodies for the study of bacteria is

known as Freund’s Complete Adjuvant. This type of adjuvant

enhances the response to the immunogen of choice via the

inclusion of a type of bacteria called mycobacteria into a mix-

ture of oil and water. Typically, there is more oil present than

water. The oil and water acts to emulsify, or spread evenly

throughout the suspension, the mycobacteria and the immuno-

gen. Sometimes the mycobacteria are left out of the adjuvant.

In this case, it is referred to as “incomplete” adjuvant.

See alsoImmunity: active, passive, and delayed

AAerobesEROBES

Aerobic microorganismsrequire the presence of oxygen for

growth. Molecular oxygen functions in the respiratory path-

way of the microbes to produce the energy necessary for life.

Bacteria, yeasts, fungi, and algae are capable of aerobic

growth.

The opposite of an aerobe is an anaerobe. An anaerobe

does not require oxygen, or sometimes cannot even tolerate

the presence of oxygen.

There are various degrees of oxygen tolerance among

aerobic microorganisms. Those that absolutely require oxygen

are known as obligate aerobes. Facultative aerobes prefer the

presence of oxygen but can adjust their metabolic machinery

so as to grow in the absence of oxygen. Microaerophilic

organisms are capable of oxygen-dependent growth but can-

not grow if the oxygen concentration is that of an air atmo-

sphere (about 21% oxygen). The oxygen content must be lower.

Oxygen functions to accept an electron from a sub-

stance that yields an electron, typically a substance that con-

tains carbon. Compounds called flavoproteins and

cytochromes are key to this electron transport process. They

act as electron carriers. By accepting an electron, oxygen

enables a process known as catabolism to occur. Catabolism is

the breakdown of complex structures to yield energy. The

energy is used to sustain the microorganism.

A common food source for microorganisms is the sugar

glucose. Compounds such as glucose store energy inside

themselves, in order to bond their constituent molecules

together. When these bonds are severed, energy is released. In

aerobic bacteria and other organisms, a compound called

pyruvic acid retains most of the energy that is present in the

glucose. The pyruvic acid in turn is broken down via a series

of reactions that collectively are called the tricarboxylic acid

cycle, or the Kreb’s cycle (named after one the cycle’s discov-

erers, Sir Hans Krebs). A principle product of the Kreb’s cycle

is a compound called nicotinamide adenine dinucleotide

(NADH 2 ). The NADH 2 molecules feed into another chain of

reactions of which oxygen is a key.

The energy-generating process in which oxygen func-

tions is termed aerobic respiration. Oxygen is the final electron

acceptor in the process. Anaerobic respiration exists, and

involves the use of an electron acceptor other than oxygen. One

of the most common of these alternate acceptors is nitrate, and

the process involving it is known as denitrification.

Aerobic respiration allows a substrate to be broken

down (this is also known as oxidation) to carbon dioxide and

water. The complete breakdown process yields 38 molecules

of adenine triphosphate (ATP) for each molecule of the sugar

glucose. ATP is essentially the gasoline of the cell. Electron

transport that does not involve oxygen also generates ATP, but

not in the same quantity as with aerobic respiration. Thus, a

facultative aerobe will preferentially use oxygen as the elec-

tron acceptor. The other so-called fermentative type of energy

generation is a fall-back mechanism to permit the organism’s

survival in an oxygen-depleted environment.

The aerobic mode of energy production can occur in

the disperse cytoplasmof bacteria and in the compartmental-

ized regions of yeast, fungi and algae cells. In the latter

microorganisms, the structure in which the reactions take

place is called the mitochondrion. The activities of the mito-

chondrion are coordinated with other energy-requiring

processes in the cell.

See alsoCarbon cycle in microorganisms; Metabolism

AGAMMAGLOBULINAEMIA WITH HYPER

IGM•seeIMMUNODEFICIENCY DISEASE SYNDROMES

AAgar and agaroseGAR AND AGAROSE

Agar and agarose are two forms of solid growth media that are

used for the cultureof microorganisms, particularly bacteria.

Both agar and agarose act to solidify the nutrients that would

otherwise remain in solution. Both agar and agarose are able

to liquefy when heated sufficiently, and both return to a gel

state upon cooling.

Solid media is prepared by heating up the agar and

nutrient components so that a solution results. The solution is

then sterilized, typically in steam-heat apparatus known as an

autoclave. The sterile medium is then poured into one half of

sterile Petri plates and the lid is placed over the still hot solu-

tion. As the solution cools, the agar or agarose becomes gel-

like, rendering the medium in a semi-solid. When bacteria

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