It Starts With Biology: The Basics in Antibiotic Use Decisions

It starts with biology: The basics in antibiotic use decisions

Disclaimer: This post is part of a series written for Saskatchewan livestock & poultry producers. Content may not be relevant for non-producers.

When was the last time you used antibiotics in your operation? How did you choose what product to use? No doubt, you considered the cost, the ease of administration, and the withdrawal time. But what was your working diagnosis? Did you have culture results? How did you know exactly what you were treating and what the optimal antibiotic was for that disease?

Choosing an antibiotic is complex. In future posts we will talk about veterinary-client-patient relationships, extra-label use, and withdrawal times. Today, let’s look at the basics of how antibiotics work. If your eyes just glazed over – that’s ok –your vet is trained to make these decisions for you. Call them!

Antibiotics are a class of medicines that selectively kill or suppress the growth of bacteria while causing little to no damage to the host. The most important part of that statement is what it is missing: Antibiotics are not effective against viruses or parasites! So before choosing an antibiotic be sure you’re dealing with a bacterial disease.

The basic biology of a bacteria dictates which antibiotics are effective against them. Bacteria are classified as Gram-positive or Gram-negative based on their cell wall. Bacteria are also classified as aerobic if they grow in the presence of oxygen, anaerobic if they grow in the absence of oxygen. Below are the classification of a few common bacterial diseases in each industry.


There are technical differences between antibiotics and antimicrobials but for the purposes of most publications, including this one, they are synonymous. Antibiotics are classified their mode of action (i.e. how they work). A family of antibiotics includes all the drugs that target the same bacterial structure.

The mode of action dictates what bacteria the drug is normally effective against. Broad-spectrum antibiotics are effective again both gram-positive and gram-negative bacteria. Narrow-spectrum antibiotics typically are more effective against Gram-positive or Gram-negative bacteria. Antibiotics also differ in their efficacy against aerobic vs. anaerobic bacteria. Below are general guidelines of which antibiotic families are effective against each type of bacteria. The drug-bug combinations colored black and labeled “not effective” have intrinsic resistance because the bacteria lack the structure that the antibiotic needs to attack.


Below provides a single example of a registered antibiotic in Canada for each family.


Good stewardship starts with knowing the bacteria most likely causing disease and selecting antibiotics that are effective against those pathogens. But stewardship doesn’t end there. Bacteria can acquire resistance. This means that antibiotics that are supposed to work no longer do. Your veterinarian will monitor this and can test when an antibiotic does not work as expected. Finally, having selected the optimum drug for the biology you also must consider if that product is labeled for your species and indication and the withdrawal time.


As a producer, you obviously consider practical things like cost and administration convenience when selecting an antibiotic. But before you get to these seek professional guidance in (1) making a presumptive diagnosis, (2) selecting an antibiotic that is effective against that pathogen, and (3) monitoring resistance in your operation.


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