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Antimicrobial Agents

Antimicrobial agents, as their name implies, are chemicals that kill or prevent the growth of microbes.  There are agents that are effective against bacteria, others against fungi, others against viruses, others against parasites.

Antimicrobial agents used to sanitize and disinfect surfaces are called antiseptics and disinfectants.  They often are effective against a wide range of microbes, and so can have antibacterial, antiviral, and antifungal activity.  They work in a general way - destroying proteins or membranes or DNA, for example.  Antiseptics can be used on skin while disinfectants are used on inanimate objects and surfaces.  You would use an antiseptic to clean a wound before putting on a bandaid.  You would use a disinfectant for cleaning a floor or toilet or sink. One thing about antiseptics and disinfectants is that they are often as harmful to a patient's cells as they are to microbe cells.  So you would never take an antiseptic or a disinfectant internally to fight an infection.

On the other hand, there are antimicrobial agents that exemplify Paul Ehrlich's concept of a "magic bullet", a substance that can be taken internally to kill microbes while leaving patient cells untouched.  These substances are antibiotics and chemotherapeutic agents.  In contrast to antiseptics and disinfectants, they work not by destroying proteins and membranes, but instead they target specific cell structures and processes.  They may not destroy all proteins they encounter, but they may prevent a key protein from doing its job to keep the cell alive.  So they are much more specific in their activity than are antiseptics and disinfectants.

Antibiotics are chemicals created by microbes (usually) that are toxic to other microbes.  Alexander Fleming found penicillin, a chemical created by a fungus that killed bacteria.  Since that time, scientists have been on an unending quest to find new antibiotics.  The result has been many that are true magic bullets - lethal to bacteria while mostly harmless to humans.  Here is an excellent video at the Howard Hughes Medical Institute website that shows penicillin killing bacteria.

In addition to antibiotics created by microbes to fight microbes, there are chemicals created by scientists that also are effective in killing microbes while leaving human cells untouched.  These are called chemotherapeutic agents (not to be confused with chemotherapy for treating cancer).  

Remember these two properties to distinguish between these four types of antimicrobial agents:

  • Selective Toxicity:  Is the chemical selectively toxic?  That is, does it kill cells of some organisms while leaving the cells of other organisms untouched?  Antibiotics and chemotherapeutic agents are selectively toxic (kill pathogens but leave host cells alone), while antiseptics and disinfectants are not (they destroy any cell, not just the pathogen).
  • Specific Activity:  Does the chemical take a shotgun approach to destroy all proteins or membranes or nucleic acids?  Or does it target a specific cell reaction or destroy a key protein or enzyme?  Antibiotics and chemotherapeutic agents exhibit specific activity rather than general activity as is seen with antiseptics and disinfectants.

In this section we will look at the four categories of chemicals described above and see how they are a common part of our general health and common practices in our homes.

Before leaving, your instructor may wish for you to take the quiz at this link.