Antibiotics Basics

Posted: February 2, 2008

This is the second in a series that will explore the use as well as discuss some of the controversies concerning antibiotic therapy in dairy cattle.

This is the second in a series that will explore the use as well as discuss some of the controversies concerning antibiotic therapy in dairy cattle. Antibiotics are important tools to help manage infectious disease in dairy cattle. While a very useful aid in the control of disease, antibiotics should never be viewed as a substitute for good management. Prolonged or improper use of antibiotics over time can or will selectively increase the population of resistant bacteria. Prudent use of antibiotics can reduce the risk of antibiotic resistance and minimize residue concerns.

No antibiotic can completely kill all the pathogenic bacteria during therapy for an infectious condition. Antibiotics have different modes of action, penetrate tissues differently, occasionally are inactivated by inflammatory substances or the bacteria themselves, or simply may not be effective against the bacteria in question. Some antibiotics are only effective against Gram positive (+) bacteria (e.g., Streptococcus spp, or Staphylococcus spp.), others only against Gram negative (-) bacteria (e.g., E. coli, Klebsiella, Manheimia, Pseudomonas), while others are considered broad spectrum and could be used against either Gram + or – bacteria. Mycoplasma are unique micro-organisms and can be quite difficult to control. Infection by Mycoplasma are frequently not eliminated with typical antibiotic treatment regiments.

Antibiotics inhibit the growth or kill bacteria by a variety of methods. Some antibiotics stop key bacteria functions (e.g., protein synthesis, energy production, cell well development, or cell wall integrity). For these reasons it is important to understand a few basics about antibiotics in order to make their use, when necessary, more effective. The following table contains a brief outline of some common antibiotic classes.

Gentocin, Amikacin ®

Primarily Gram -

Inhibit protein synthesis

Ceftiofur (Excenel ®)

Broad spectrum

Cell wall synthesis

Cephapirin (Today ®)

Broad spectrum

Cell wall synthesis

Ampicillin (Polyflex®)

Primarily Gram+; Some Gram-

Cell wall synthesis

Penicillin, Cloxacillin

Gram +

Cell wall synthesis


Primarily Gram+; Mycoplasma

Inhibit peptide bonds

Tetracycline,  Florfenicol(Nuflor®)

Broad spectrum; Mycoplasma

Inhibit protein synthesis

Pirsue ®

Primarily Gram +

Inhibit protein synthesis


Broad spectrum

Inhibit thymidine or folic acid synthesis

To further complicate the use and choice of a particular antibiotic, not all antibiotics move uniformly into tissues. As an example, some antibiotics even if known to be effective against a particular bacteria family, when given in the muscle or sub-subcutaneously may not enter the mammary gland at levels sufficient to be effective. Further some bacteria may be present in tissue spaces, like joints or abscesses, again effectively isolating the bacteria from certain antibiotics. Viruses are not controlled by antibiotics. The use of antibiotics to treat purely viral infections is expensive, wasteful, and increases the development of antibiotic resistance.

This should help to emphasize the critical need for each farm to work closely with their veterinarian in designing therapeutic plans. To be effective a plan should include a diagnosis of type of bacteria likely to be involved, as narrow a choice of antibiotic as possible, an understanding of the target tissue, likelihood of success, adequate dose level given for sufficient time, and good records to ensure proper withdrawal and/or withholding.

The next article in this series will discuss briefly some key points in the development of therapeutic plans. 

David Wolfgang, Extension Veterinarian, Department of Veterinary and Biomedical Sciences