Antimicrobial drugs are produced either synthetically or from natural sources. They inhibit or destroy microbial growth in the infected host. Antibiotic drugs are the subset of antimicrobials produced by the natural metabolic processes of microorganisms.

Antimicrobial drugs are classified by their range of effectiveness. Broad-spectrum antimicrobials are effective against many types of microbes. Narrow-spectrum antimicrobials are effective against a limited group of microbes.

Antimicrobial therapy involves three interacting factors: the drug, the microbe, and the infected host.

Two genera of bacteria and two genera of fungi are the primary sources of most antibiotics. The molecular structures of these compounds can be chemically altered to form additional semi-synthetic antimicrobials.

Characteristic Interactions Between Drug and Microbe
Microorganisms are termed ``drug resistant'' when they are no longer inhibited by an antimicrobial to which they were previously sensitive.

Drug resistance is genetic; microbes develop or acquire genes which code for methods of inactivating or escaping the antimicrobial. Resistance is selected for in environments where antimicrobials are present in high concentrations, such as in hospitals.

Microbial resistance develops through random mutation and through acquisition of resistance genes from other microorganisms.

Varieties of microbial resistance include drug inactivation, decreased drug uptake, decreased drug receptor sites, and modification of metabolic pathways formerly attacked by the drug.

Widespread indiscriminate prescribing of antimicrobials has resulted in an explosion of microorganisms resistant to all common drugs, including the following: Streptococcus, Staphylococcus, all the Enterobacteriaceae, Salmonella, Shigella, and Mycobacterium tuberculosis.

Survey of Major Antimicrobial Drug Groups
Antimicrobials are classified into 18 major drug families, based on their chemical composition, source or origin, and their site of action.

The majority of antimicrobials are effective against bacteria, but a limited number are effective against protozoa, helminths, fungi, and viruses.

Sulfonamides, trimethoprim, isoniazid, nitrofurantoin, and the fluroquinolones are synthetic antimicrobials effective against a broad range of microorganisms. They block steps in the synthesis of nucleic acids.

Beta-lactams, bacitracin, vancomycin, and cycloserines block cell wall synthesis in bacteria, primarily the gram-positive bacteria.

Aminoglycosides and tetracyclines block protein synthesis in procaryotes.

Fungal antimicrobials, macrolide polyenes, griseofulvin, imidazoles, and fluorocytosine must be monitored carefully because of the potential toxicity to the infected host. They promote lysis of cell membranes.

There are fewer antiparasitic drugs compared to antibacterial drugs because parasites are eucaryotes like their human hosts and they have several life stages, some of which may be resistant to the drug.

Antihelminth drugs immobilize or disintegrate infesting helminths, or inhibit their metabolism in some manner.

Antiviral drugs interfere with viral replication by blocking viral entry into cells, blocking the replication process, or preventing the assembly of viral subunits into complete virions.

Many antiviral agents are analogues of nucleotides. They inactivate the replication process when incorporated into viral nucleic acids.

While interferon is effective in vivo against certain viral infections, commercial interferon is not currently effective as a broad-spectrum antiviral agent.

Characteristics of Host/Drug Reactions
The three major side effects of antimicrobials are toxicity to organs, allergic reactions, and problems resulting from suppression or alteration of normal flora.

Antimicrobials that destroy most but not all normal flora allow the unaffected normal flora to overgrow, causing a superinfection.

Considerations in Selecting an Antimicrobial Drug
The three major considerations necessary to choose an effective antimicrobial are the nature of the infecting microbe, the microbe's sensitivity to available drugs, and the overall medical status of the infected host.

The Kirby-Bauer plate identifies antimicrobials that are effective against a specific infectious bacterial isolate.

The MIC (minimum inhibitory concentration) identifies the smallest effective dose of an antimicrobial toxic to the infecting microbe.

The therapeutic index is a ratio of the amount of drug toxic to the infected host and the MIC. The smaller the ratio, the greater the potential for toxic host drug reactions.