Introduction
The world is teeming with microorganisms - bacteria, fungi, parasites, and viruses. Our immune systems have evolved to fight those that threaten us and to co-exist peacefully with countless others. Every person has a balance of normal flora, a mixture of microorganisms that live on the surface of the skin and in the digestive tract. In general, these tiny life forms are helpful, aiding in the digestion of food and serving as a barrier against pathogenic microorganisms. Most cause no problems unless there is a disruption in the balance, the patient’s immune system is depressed, or an injury or stressor creates a breach in the immune system’s protection. If this happens, there may be an opportunistic overgrowth of one type of normal flora, or one or more pathogenic microorganisms may find their way into the body and cause an infection.

When a patient’s immune system cannot rid itself of a pathogen, or restore its normal flora balance, then doctors turn to antimicrobials for assistance. These are drugs (antibacterial, antiviral, and antifungal) that have been developed to combat the offending microorganisms. They target different physical characteristics of the “bug” – such as its ability to multiply, its cell wall, or metabolism. Different antimicrobials are effective against different kinds of microorganisms. Some are narrowly focused, meant to eliminate one specific family of bacteria, for instance, without disrupting the balance of normal flora. Others are broad spectrum, developed to inhibit the growth of many microorganisms. When used, broad spectrum antimicrobials may affect both pathogens and normal flora.

Some microorganisms are resistant to certain antimicrobials, however. Susceptibility testing is often used to determine the likelihood that a particular drug treatment regimen will be effective in eliminating or inhibiting the growth of the infection. The remainder of this article explains how susceptibility (also called “sensitivity”) testing is performed and how drug resistance occurs, focusing exclusively on bacterial infections. (A companion piece focusing on viral resistance and susceptibility testing will soon follow.)