Antibiotics
Laboratory studies on antibiotic action
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In the laboratory, susceptibility is most often measured
using a disk diffusion test. Antibiotic solutions of particular concentrations
are dried onto filter paper disks. These are then applied to a lawn of the
microbe under examination which has previously been inoculated onto an appropriate
solid medium.
In the Stokes controlled sensitivity test, a control organism
is inoculated on part of a plate and the test organism is plated on the
remainder. Disks are placed at the interface and the zones of inhibition
are compared. The use of a sensitive control shows that the antibiotic
is active, so that if the test organism grows up to the disk it may safely
be assumed that the test organism is resistant to that drug.
Stokes' sensitivity test
The test bacterium in the diagram is susceptible to drug "x" but resistant to drug "y". The disk containing drug "y" contains active antibiotic as shown by the zone of inhibition it causes in the control bacterium. |
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An alternative measure of susceptibility is to determine
the Minimum Inhibitory Concentration (MIC) and the Minimum Bactericidal
Concentration (MBC) of a drug. A series of broths are mixed with serially
diluted antibiotic solutions and a standard inoculum is applied. After incubation,
the MIC is the first broth in which growth of the organism has been inhibited.
The more resistant an organism is, then the higher will be the MIC.
The MBC is measured by inoculating the broths used for MIC determinations
onto drug-free medium. The MBC is the first dilution at which no growth
is observed. Cidal drugs have MBC values that are close
to the MIC value for particular organisms. With static agents,
the MIC is much lower than the MBC.
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The MIC/MBC test of a moderately resistant bacteriostatic
drug.
Note that once the bacteria are removed from the drug they can grow on drug
free medium at most concentrations.
The MIC/MBC test of a moderately resistant bactericidal drug.
Note that once the bacteria are removed from the drug they cannot grow on
drug free medium apart from the tube representing the MIC of the antibiotic.
One tube difference is allowed in this test.
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Most antibiotics are given as single agents. There are, however, occasions
when two or more drugs are used in combination. Antimicrobial agents may
affect each other when used in combination. The effect may simply be additive.
In some cases the activity of one drug enhances that of a second drug.
This is referred to as synergy. Alternatively, drugs may interfere
with each other - antagonism. Penicillins and bacteriostatic drugs
such as tetracyclines are antagonistic, since penicillins require actively
growing cells and static drugs prevent cell growth. In contrast, aminoglycosides
are synergistic when used in combination with penicillins. This is important
when considering antimicrobial therapy, for example for endocarditis.
In this condition, it is essential that the antimicrobial regime is bactericidal,
since the bacteria become walled off inside vegetations. Synergistic combinations
are typically used to treat this condition.
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The effects of combining antimicrobial agents
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Although, as illustrated above, laboratory tests in vitro can
test for synergy or antagonism this effect is not necessarily apparent
when combinations are used in vivo. Sulphonamides and trimethorpim
both act on folic acid metabolism and show synergistic activity against
bacteria in vitro it is difficult to achieve a synergistic ratio
of these drugs in humans.
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Page edited April 2006
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