Did You Ever Wonder How Antibiotics Work?

by Shawn M. Finch, DVM

Antibiotics are wonderful but narrowly specific tools in the fight against disease.

An Antibiotic’s (Anti)Life Vision

The goal of an antibiotic is to rid the body of disease-causing bacteria without causing harm to the animal.

Not All Bacteria are Mean

Bacteria are single-celled living organisms with a cell wall and without a nucleus.  Most bacteria are harmless, and some are even beneficial to animals.  Trouble starts when disease-causing bacteria invade our pets’ bodies and proliferate at a rate that is too overwhelming for the immune system to handle alone.

Image: Science Class: The inside of a bacteria!

Bacteria Have Achilles’ Heels

Antibiotics tend to target unique components of bacteria, for example, their cell walls.

Animal cells do not have cell walls and thus are not harmed by cell wall-destroying antibiotics.

Specific Mechanisms of Antibiotics Commonly Used in Small Animal Medicine

Penicillin and cephalexin antibiotics interfere with the production of bacterial cell walls.  The cell walls become unstable (somewhat like a soap bubble about to pop) and eventually burst.

Lincosamide, aminoglycoside and tetracycline antibiotics inhibit one of the two subunits of bacterial ribosomes.  Proteins cannot be synthesized without ribosomes, and the bacteria are either unable to replicate or they die.

Metronidazole is taken up by bacteria and is changed to a substance that prevents DNA synthesis, causing the bacteria to die.

Fluoroquinolones prevent bacterial DNA supercoiling and synthesis.

Sulfonamides and trimethoprim interfere with different steps of bacterial folic acid synthesis.  Animals are able to obtain folic acid from their diet, but bacteria must make their own.  Without folic acid, bacteria cannot survive.

Does an antibiotic kill ALL bacteria?

Antibiotics kill certain types of bacteria, based on:
  • Whether the bacteria are gram positive or gram negative
  • Whether the bacteria are aerobic (needing oxygen) or anaerobic (not needing oxygen)
  • What the specific bacteria species is
  • What the specific bacteria (individual) is
  • What body system the bacteria is in
  • What disease the bacteria is causing
  • How fast/actively the bacteria are multiplying
  • What the individual animal's reaction, including the strength of their immune system reaction, is
  • How the individual antibiotic and bacteria and host interact

Should antibiotics be used for suspected viral infections “just in case?”

No!  Antibiotics have no effect against viruses.

How do antibiotics work with the immune system?

The antibiotics kill or stop many of the bacteria while the immune system kills bacteria that the antibiotics have stopped, and also kills bacteria the antibiotics have missed.

Meanwhile, the remaining bacteria are multiplying.  At each “round” (that is, a dose of antibiotics) the antibiotics strike another blow against the bacteria, ideally gaining a bit more ground each time.

Can I stop antibiotics when my pet feels better or his or her clinical signs have improved?

No!  The goal of the antibiotic is to bring the bacterial numbers low enough that the immune system can finish them off.  Your veterinarian will have given you the number of doses that typically reach this goal.

If antibiotics are stopped before bacterial numbers are low enough for the immune system to finish off, only the bacteria that have “outsmarted” both the antibiotic and the immune system so far will be left to multiply, perhaps causing the next generation of bacteria to be stronger and “meaner” than ever.  This contributes to antibiotic resistance and is unsafe for your pet.

Are You Listening, Me?

I prescribe antibiotics more than any other drug class, and I love having such a great arsenal of medication available.  Reviewing how antibiotics work provides a good reminder of why choosing an appropriate antibiotic, giving it at the appropriate dose and for the appropriate duration is so very important!  On behalf of the entire veterinary and medical communities, I will say that we need to hear this as much as anyone!

For Further Reading:
Plumb, D. C. 2011. Veterinary Drug Handbook, Seventh Edition. Ames: Iowa State University Press.

Dr. Shawn is a veterinarian and mom in Omaha Nebraska. She writes for CareFRESH, Life With Dogs (new!) and Omaha.net.

Dr. Shawn graduated from Iowa State University Veterinary Teaching Hospital in 1998 with a Doctor of Veterinary Medicine (DVM) degree.

Her veterinary passions (besides her patients, of course!) are preventative/wellness care, pet owner education, positive reinforcement training and solving pet overpopulation.

You can also connect with Dr. Shawn on Twitter or Facebook

Articles by Dr. Shawn:
Xylitol And The Basset Hound 


  1. This was very informative. I do think that just like some human doctors do, there are vets who over prescribe antibiotics as well.

  2. Yes, antibiotics are overused everywhere. Next thing we know they'll be adding them to our water (if they don't secretly do already LOL)


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