Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Antibody-antigen, biochemical and molecular reactions

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infections could blossom into life-threatening maladies. In the

decades following the discovery of penicillin, many naturally

occurring antibiotics were discovered and still more were syn-

thesized towards specific targets on or in bacteria.

Antibiotics are manufactured by bacteria and various

eukaryotic organisms, such as plants, usually to protect the

organism from attack by other bacteria. The discovery of these

compounds involves screening samples against bacteria for an

inhibition in growth of the bacteria. In commercial settings,

such screening has been automated so that thousands of sam-

ples can be processed each day. Antibiotics can also be manu-

factured by tailoring a compound to hone in on a selected

target. The advent of molecular sequencing technology and

three-dimensional image reconstruction has made the design

of antibiotics easier.

Penicillin is one of the antibiotics in a class known as

beta-lactam antibiotics. This class is named for the ring struc-

ture that forms part of the antibiotic molecule. Other classes of

antibiotics include the tetracyclines, aminoglycosides,

rifamycins, quinolones, and sulphonamides. The action of

these antibiotics is varied. For example, beta-lactam antibi-

otics exert their effect by disrupting the manufacture of pepti-

doglycan, which is main stress-bearing network in the

bacterial cell wall. The disruption can occur by blocking either

the construction of the subunits of the peptidoglycan or by pre-

venting their incorporation into the existing network. In

another example, amonglycoside antibiotics can bind to a sub-

unit of the ribosome, which blocks the manufacture of protein,

or can reduce the ability of molecules to move across the cell

wall to the inside of the bacterium. As a final example, the

quinolone antibiotics disrupt the function of an enzyme that

uncoils the double helix of deoxyribonucleic acid, which is

vital if the DNAis to be replicated.

Besides being varied in their targets for antibacterial

activity, different antibiotics can also vary in the range of

bacteria they affect. Some antibiotics are classified as nar-

row-spectrum antibiotics. They are lethal against only a few

types (or genera) of bacteria. Other antibiotics are active

against many bacteria whose construction can be very differ-

ent. Such antibiotics are described as having a broad-spec-

trum of activity.

In the decades following the discovery of penicillin, a

myriad of different antibiotics proved to be phenomenally

effective in controlling infectious bacteria. Antibiotics

quickly became (and to a large extent remain) a vital tool in

the physician’s arsenal against many bacterial infections.

Indeed, by the 1970s the success of antibiotics led to the gen-

erally held view that bacterial infectious diseases would soon

be eliminated. However, the subsequent acquisition of resist-

ance to many antibiotics by bacteria has proved to be very

problematic.

Sometimes resistance to an antibiotic can be overcome

by modifying the antibiotic slightly, via addition of a different

chemical group. This acts to alter the tree-dimensional struc-

ture of the antibiotic. Unfortunately, such a modification tends

to produce susceptibility to the new antibiotic for a relatively

short time.

Antibiotic resistance, a problem that develops when

antibiotics are overused or misused. If an antibiotic is used

properly to treat an infection, then all the infectious bacteria

should be killed directly, or weakened such that the host’s

immune response will kill them. However, the use of too low

a concentration of an antibiotic or stopping antibiotic therapy

before the prescribed time period can leave surviving bacteria

in the population. These surviving bacteria have demonstrated

resistance. If the resistance is governed by a genetic alteration,

the genetic change may be passed on to subsequent genera-

tions of bacterial. For example, many strains of the bacterium

that causes tuberculosis are now also resistant to one or more

of the antibiotics routinely used to control the lung infection.

As a second example, some strains of Staphylococcus aureus

that can cause boils, pneumonia, or bloodstream infections,

are resistant to almost all antibiotics, making those conditions

difficult to treat. Ominously, a strain of Staphylococcus

(which so far has been rarely encountered) is resistant to all

known antibiotics.

See alsoBacteria and bacterial infection; Bacterial genetics;

Escherichia coli; Rare genotype advantage

ANTIBIOTICS, HISTORY OF DEVELOP-

MENT•seeHISTORY OF THE DEVELOPMENT OF ANTIBIOTICS

ANTIBODY-ANTIGEN, BIOCHEMICAL AND

MOLECULAR REACTIONSAntibody-antigen, biochemical, and molecular reactions

Antibodies are produced by the immune systemin response to

antigens (material perceived as foreign. The antibodyresponse

to a particular antigenis highly specific and often involves a

physical association between the two molecules. This associa-

tion is governed by biochemical and molecular forces.

In two dimensions, many antibody molecules present a

“Y” shape. At the tips of the arms of the molecules are regions

that are variable in their amino acid sequences, depending

upon the antigen and the antibody formed in response. The

Ciprofloxacin.

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