A wide variety of chemical agents display antimicrobial activity to some degree. In considering their application to patient care, we may separate them into two general classes: (1) those that are useful for destroying pathogenic microorganisms in the environment (disinfectants) or on skin (antiseptics), and (2) those that may be administered to patients for treatment of infectious diseases (antimicrobial agents).
Many antimicrobial substances are too toxic to be used for patient therapy but are valuable as environmental disinfectants. These must be chosen carefully for the job to be done, because a given disinfectant usually does not kill all microbial pathogens. Each agent has a limited chemical mode of action, and microorganisms exposed to it may vary widely in their responses. Some microbes or their forms may succumb to its effects (such as vegetative bacterial cells) whereas others may not (such as bacterial endospores). In the experiments of Exercise Disinfectants, we shall study some of the many factors that influence the disinfection process.
Antimicrobial agents are substances that are naturally produced by a variety of microorganisms (primarily fungi and bacteria), or have been synthesized in the laboratory, or a combination of both. For example, scientists in pharmaceutical companies have made many chemical modifications of the penicillin molecule (a product of the fungus Penicillium notatum) to broaden its spectrum of activity. In strict use, antibiotic refers only to those antimicrobial substances produced by microorganisms, but the term is often used interchangeably with antimicrobial agent. Antimicrobial agents have inhibitory or lethal effects on many pathogenic organisms (especially bacteria) that cause infectious diseases. In purified form, they are administered to patients for their antimicrobial effects within the body. In general, each agent has special activity against one or more types of microorganisms (gram-positive bacteria, gram-negative bacteria, fungi, and some viruses).
Like disinfectants, antimicrobial agents have specific chemical modes of action, but the range of activity of antimicrobial agents is narrower. Therefore, as we shall learn in Exercise 15, the diagnostic microbiology laboratory tests the antimicrobial susceptibility of pathogenic bacteria so as to provide the physician with valuable information about the most clinically useful antimicrobial agent with which to treat a patient’s infection specifically. At present, reliable tests for determining fungal and viral susceptibility to antimicrobial agents are not generally available. In addition to the isolation and identification of pathogenic microorganisms that we shall study in sections of Part 3, antimicrobial susceptibility testing is one of the most important functions of the diagnostic microbiology laboratory.
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