Microbiology and Immunology

Antibiotics WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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of a bacterium. For example, living bacteria will fluoresce in

the presence of acridine orange, while dead bacteria will not.

These probes combined with the optical technique of confocal

laser microscopy, now enables populations of cells to be

viewed without disrupting them to see if they fluoresce or not

in the presence of an antibiotic of interest.

The ability of living bacteria to fluoresce can also be

exploited by another machine called a flow cytometer. This

machine operates essentially by forcing a suspension of bacte-

ria (or other cells) through an opening so that only one bac-

terium at a time passes by a sensor. The sensor monitors each

passing bacterium and can sort these into categories, in this

case, fluorescing (living) from non-fluorescing (dead). The

entire process can be completely quickly. This provides an

almost “real-time” assessment of the proportion of a popula-

tion that has been killed by an antibiotic. If the proportion of

dead bacteria is low, resistance is indicated.

All the assessments of antibiotic effectiveness need to

be done in a controlled manner. This necessitates the use of

standard test types of bacteria (strains that are known to be

resistant to the particular antibiotic as well as other strains that

are known to be sensitive to the antibiotic). The concentration

of the bacteria used is also important. Too many bacteria can

“dilute” out the antibiotic, producing a false indication of

resistance. Controls need to be included to verify that the

experiment was not subject to contamination, otherwise the

possibility that a finding of resistance was due to a contami-

nating bacteria could not be discounted.

In clinical settings, a finding of resistance would prompt

the search for another antibiotic. Often, identification of the

bacteria will suggest, from previous documented tests of oth-

ers, an antibiotic to which the organism will be susceptible.

But, increasingly, formerly effective antibiotics are losing

their potency as bacteria acquire resistance to them. Thus, tests

of antibiotic resistance grow in importance.

AAntibioticsNTIBIOTICS

Antibiotics are natural or synthetic compounds that kill bacte-

ria. There are a myriad of different antibiotics that act on dif-

ferent structural or biochemical components of bacteria.

Antibiotics have no direct effect on virus.

Prior to the discovery of the first antibiotic, penicillin, in

the 1930s, there were few effective ways of combating bacte-

rial infections. Illnesses such as pneumonia, tuberculosis, and

typhoid feverwere virtually untreatable, and minor bacterial

Antibiotic susceptible and resistant strains of Stapylococcus.

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