Antibiotic susceptibility testing

In order to determine the most appropriate antimicrobial agent to use against an infection, it is necessary to determine the susceptibility of the pathogen. There are several ways of doing this, but here we describe the two most commonly employed techniques.

The tube dilution assay determines the minimum inhibitory concentration (MIC) of the antibiotic, that is, the lowest concentration at which it prevents growth of a given organism. A series of tubes containing increasingly dilute preparations of the antibiotic is introduced into a broth with a standard number of test organisms and incubated. The lowest concentration in the series to show no microbial growth is the MIC (Figure 14.7).

Antimicrobial Agent

9.9ml growth medium

No growth

Growth

Figure 14.7 Minimal inhibitory concentration (MIC). The test organism is incubated with serially diluted antibiotic. The lowest concentration capable of preventing microbial growth is the MIC

Bacterial lawn

Paper disc

Bacterial lawn

Paper disc

Area of clearing paper disc

Figure 14.8 Antibiotic testing by disc diffusion. The bacterium to be tested is spread on an agar plate, then paper discs impregnated with appropriate antibiotics are placed on the surface. Following incubation, susceptibility to an antibiotic is indicated by a clear ring surrounding the disc

Area of clearing paper disc

Figure 14.8 Antibiotic testing by disc diffusion. The bacterium to be tested is spread on an agar plate, then paper discs impregnated with appropriate antibiotics are placed on the surface. Following incubation, susceptibility to an antibiotic is indicated by a clear ring surrounding the disc

In the disc diffusion method, paper discs impregnated with the antibiotic are placed on the surface of an agar plate previously inoculated with the test organism (Figure 14.8). The antibiotic diffuses radially outwards, becoming less concentrated as it does so. A clear zone of inhibition appears where growth has been inhibited. The larger this is, the more susceptible the organism. Conditions may be standardised so that susceptibility (or otherwise) to the antibiotic can be determined by comparing the diameter of the zone of clearing with standard tables of values. From this, a suitable concentration for therapeutic use can be determined.

We have focused our attention up to now on those antimicrobial agents which are directed against bacteria, but of course these are not the only cause of infections. Below, we review the relatively limited repertoire of compounds available for the treatment of fungal and viral infections.

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