Antibiotics for Dogs: Did You Ever Wonder How Antibiotics Work?

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

Antibiotics for Dogs: Did You Ever Wonder How Antibiotics Work?

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 dogs’ bodies and proliferate at a too overwhelming rate for the immune system to handle alone.

Did You Ever Wonder How Antibiotics Work?
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 cell wall-destroying antibiotics don’t harm them.


All cells have a cell membrane. However, only some cells have cell walls.

Cell membranes contain and protect the cell content. It consists of a bilipid layer and proteins. It is flexible and allows stuff to travel in and out of the cells.

Cell walls are a rigid covering that goes on top of the cell membrane, such as in plants. In plants, the cell membrane consists of cellulose, and n bacteria and fungi of chitin, a derivative of glucose.

Further information: Cell Wall Vs. Cell Membrane

Antibiotics for Dogs: Cell Membrane versus Cell Wall

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)
  • specific bacteria species
  • specific individual bacteria
  • affected body system
  • resulting disease
  • 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 your veterinarian prescribe antibiotics 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 dog 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.

Discontinuing antibiotic treatment 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. This can lead to the next generation of bacteria to be stronger and “meaner” than ever.  This contributes to antibiotic resistance and is unsafe for your dog.

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 appropriate duration is significant!  On behalf of the entire veterinary and medical communities, I will say that we need to hear this as much as anyone!

Categories: AntibioticsTreatmentTreatments

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Dr. Shawn Finch

Dr. Shawn wanted to be a vet ever since she was 9 years old. She went to The University of Nebraska at Lincoln for her Pre-Veterinary Medicine degree and to Iowa State University for her Doctor of Veterinary Medicine degree. Shawn has been a vet since 1998. She is a member of the AVMA (American Veterinary Medical Association) and the Nebraska Academy of Veterinary Medicine. She is a supporter of local pet savers, including Nebraska Humane Society, Taysia Blue Siberian Husky and Malamute Rescue and Basset and Beagle Rescue of the Heartland. She is on the Veterinary Economics Editorial Advisory Board. Shawn and her husband Russ, who owns Russell Finch Construction, have two wonderful daughters, Amanda and Abby. The Finch family also includes Joy the Puppy, a Lab mix, Luna Lovegood, a chihuahua X Westie and Noodle the Poodle, a miniature poodle. Dr. Finch and her family are members of Westwood Community Church. Shawn M. Finch, DVM practices at Gentle Doctor Animal Hospital in Omaha, Nebraska.

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