Antibiotics

Antiviral Drugs

Enzyme Inhibitors

•Interferons prevent spread of viruses to new cells (Viral hepatitis)

•Natural products of the immune system in viral infections

Antiprotozoan Drugs

•Protozoa are eukaryotic cells

•Many drugs are experimental and their mode of action is unknown

Antihelminthic Drugs

• Helminths are macroscopic multicellular eukaryotic organisms: tapeworms, roundworms, pinworms, hookworms

Antihelminthic Drugs

•Prevent ATP generation (Tapeworms)

•Alters membrane permeability (Flatworms)

•Neuromuscular block (Intestinal roundworms)

•Inhibits nutrient absorption (Intestinal roundworms)

•Paralyzes worm (Intestinal roundworms)

Different microbial species and strains have different degrees of susceptibility to different chemotherapeutic agents. Moreover, the susceptibility of a microorganism can change with time, even during therapy with a specific drug. Thus, a physician must know the sensitivities of the pathogen before treatment can be started. However, physicians often cannot wait for sensitivity tests and must begin treatment based on their "best guess" estimation of the most likely pathogen causing the illness.

Several tests can be used to indicate which chemotherapeutic agent is most likely to combat a specific pathogen.

The Diffusion Methods

Probably the most widely used, although not necessarily the best, method of testing is the disk-diffusion method, also known as the

Kirby-Bauer test.

A Petri plate containing an agar medium is inoculated ("seeded") uniformly over its entire surface with a standardized amount of a test organism. Next, filter paper disks impregnated with known concentrations of chemotherapeutic agents are placed on the solidified agar surface.

During incubation, the chemotherapeutic agents diffuse from the disks into the agar. The farther the agent diffuses from the disk, the lower its concentration. If the chemotherapeutic agent is effective, a zone of inhibition forms around the disk after a standardized incubation. The diameter of the zone can be measured; in general, the larger the zone, the more sensitive the microbe is to the antibiotic. The zone diameter is compared to a standard table for that drug and concentration, and the organism is reported as sensitive, intermediate, or resistant.

Measuring Antimicrobial

Sensitivity: Disk Diffusion