Antibiotic use, bacterial susceptibility and sensitivity: what does it mean?

Fence Post Article from the April 2023 issue of North Dakota Stockman

Kelli Maddock, MS, MLS(ASCP)M; Senior Research Scholar- NDSU VDL

Heidi Pecoraro, DVM, PhD, Diplomate, ACVP; Director & Pathologist- NDSU VDL

Quynn Steichen, DVM, MS; Post-Doctorate Research Fellow- NDSU VDL

Gerald Stokka, DVM, MS; Associate Professor/Extension Veterinarian- Livestock Stewardship

Antibiotics are chemical substances produced either synthetically or by various species of microorganisms (bacteria, fungi, actinomycetes) that suppress the growth of other microorganisms and may eventually destroy them. The initial discovery of penicillin in 1928 was made by Dr. Alexander Fleming while observing that a mold overgrowing a culture of Staphylococcus bacteria was inhibiting the growth of the bacteria. The compound in the mold growth was named penicillin and ultimately purified, produced in large quantities and produced positive results in the treatment of bacterial infections.

Classes of antibiotics are in categories defined by the mechanism of action (how they work) to inhibit growth or destroy the organism. For example, the class of antibiotics which includes penicillin is effective against bacteria by the destruction of the cell wall which kills the bacteria. Other classes inhibit the ability of bacteria to survive by destroying protein production needed for survival. The tetracycline class of antibiotics were discovered in a soil sample in Columbia, Mo. They had a spectrum of activity against a broader population of bacterial pathogens and are still used today in both human and animal medicine. This class of tetracycline antibiotics contains all of the oxytetracycline products, such as LA 200®, Noromycin®, Agrimycin® and others. More discoveries of naturally derived and synthetically produced antibiotics led to the additional classes available today. The beta-lactam class includes penicillin and amoxicillin, and the cephalosporins, Naxcel®, Excenel® and Excede®. The macrolide class includes Tylan®, Micotil®, Draxxin®, Zuprevo® and Zactran®. The phenicol class includes Nuflor® and Resflor®. The fluoroquinolone class consists of Baytril® and Advocin®. In addition, there are a number of generics of these classes of antibiotics.

All of these antibiotics are approved for use by the U.S. Food and Drug Administration (FDA) to treat specific diseases caused by specific organisms. The label indicates the disease, the bacterial agents associated with the condition, the dose volume associated with weight, the recommended amount delivered to each injection site and the withdrawal time prior to marketing to clear any antibiotic in the meat or milk. Any use of the antibiotics that is not in agreement with the label is considered extra label drug use. This is a privilege afforded to veterinarians in the context of a veterinary client-patient relationship (VCPR). The majority of the antibiotics are retailed through veterinary clinics and animal dealers as prescription only. The prescription is an order for the drug to be used according to label upon the supervision of a veterinarian.

There are antibiotics for use in livestock that have not been sold and used with the oversight of a veterinary prescription, such as those retailed with an over-the-counter (OTC) designation. In June 2023, that designation will change from OTC to prescription status. The
antibiotics in this category are the penicillins, oxytetracyclines, and sulfa antimicrobial drugs. This process began two years ago with a voluntary guidance (#263) from the FDA to provide new labels to these older antimicrobial drugs. Beginning June 11, 2023, you will need a prescription to purchase and use these drugs as well. These changes have been made because veterinarians have the necessary specialized training that will help guide appropriate use of antibiotics, which will result in better treatment outcomes, reduced treatment costs due to fewer antibiotic resistant infections and preserved effectiveness of these antibiotics for future generations.

The North Dakota State University (NDSU) Veterinary Diagnostic Laboratory (VDL) provides test services to the veterinary profession and livestock industry, which allows veterinarians to make educated treatment decisions. This leads to improved treatment outcomes and animal well-being. One such service is the culture and identification of bacteria related to respiratory disease, pinkeye, and other diseases of bacterial origin. The origin of the bacterial organisms may come from swabs or fluid collected and sent to the lab, fresh tissues collected necropsies in the field or from necropsies performed at the diagnostic lab. Upon testing for bacterial growth, the laboratory will perform antibiotic susceptibility testing (AST), if appropriate. The term susceptibility, also referred to as sensitivity, implies that the cultured bacterial pathogen growth may be inhibited, or not, by exposure to specific antibiotics. Results of AST are reported as susceptible, intermediate, and resistant. The interpretation of the test is that successful treatment of the animal is improved when the antibiotic choice is susceptible, or shows inhibited growth. Alternatively, if AST shows an organism is resistant, or not inhibited by exposure to a specific antibiotic, treatment of the animal with that antibiotic is more likely to result in failure of the animal to improve.

Bacterial culture and AST are performed by highly trained medical laboratory scientists that are skilled in interpretation of the culture results in relation to the animal species, body site and bacteria cultured from the sample. Not all antibiotics will work for all bacteria based on the “spectrum of activity” of the antibiotic. Some antibiotics, no matter how high the dose, will not be effective on certain bacteria based on its intrinsic or acquired resistance (for example, the ability to overcome the effects of the antibiotic). Veterinarians are trained to choose the most appropriate antibiotic to treat the infection. They interpret the results that the NDSU VDL provides for AST and choose an antibiotic based on the animal species, health status of the animal and body site where the infection is occurring. Partnership with a veterinarian is critical for appropriate treatment and management of disease.

Not all disease conditions will be improved through use of antibiotics. For example, treatment of diarrhea with antibiotics may lead to prolonged shedding if the cause is bacterial, such as a toxin-producing E. coli or Salmonella species. Subsequently, the animal is potentially passing the bacteria to other animals in the environment. Other causes of diarrhea, such as viruses like bovine coronavirus or rotavirus and parasites such as Cryptosporidium or Eimeria will not be impacted by antibiotic treatment. Therefore, many modern resources suggest that animal separation by age and supportive care such as electrolytes are best for management of diarrhea. In fact, treatment with antibiotics also wipes out the “good” or normal enteric bacteria that promote gut health in humans and animals. In the absence of this good bacteria due to antibiotic treatment, the animal will likely continue to have symptoms of diarrhea and will take longer to recover because the good bacteria needs to re-establish itself in the gut.

Antibiotic use is complicated. To ensure that our antibiotics continue to remain effective requires a stewardship approach. This approach includes producers, the laboratory and veterinarians working together to choose the right antibiotics at the right time for the right animals. This will ensure better treatment outcomes, reduced costs for treatment of disease and, overall, improved animal well-being.

This article was reposted on this website with permission from North Dakota Stockman magazine.