Do Antibiotics Work as Well as They Used to?

Posted: December 1, 2007

While controversy continues as to the exact role of antibiotic use in animals and linkage to increased resistant populations of bacteria, many producers and veterinarians feel antibiotics do not work as well as they did years ago.

Much interest and concern about antibiotic resistance has been generated by the highly publicized cased of methacillin resistant Staphylococcus aureus (MRSA) recently discovered in increasing numbers in people. While controversy continues as to the exact role of antibiotic use in animals and linkage to increased resistant populations of bacteria, many producers and veterinarians feel antibiotics do not work as well as they did years ago.  Certainly continued use of antibiotics does put selective pressure on bacterial populations and over time does tend to select the strains that survive or are more resistant to therapy.  That being said, several factors can guide antibiotic selection and help ensure that antibiotic use in cattle is prudent and as efficacious as possible.

Obviously when a clinical condition is caused by a bacterial infection, it is important to choose a product which is believed to have low toxicity, high efficacy, reasonable cost, and minimal residues. Once a species of bacteria is isolated, sensitivity testing may be requested to try to match the isolate with an appropriate antibiotic. Undoubtedly, one of the most misunderstood tests done in veterinary medicine is sensitivity testing.  While the use of sensitive (S), intermediate (I), and resistant (R) status is often reported and used in choosing an antibiotic for a clinical case, such information alone is really not sufficient as the sole criteria for selection in therapy.  Nor is sensitivity testing always the most useful criteria necessary for predicting success or failure of a particular antibiotic for a clinical case.  This article will be one in series designed to help producers better understand why antibiotics seem to work or not work in particular cases. 

One of the first areas of confusion begins with the term antibiotic testing. Just what constitutes antibiotic sensitivity testing? In the simplest terms, these are tests designed to be quick and consistent methods of determining what antibiotic level will inhibit the growth of certain bacteria isolates.  While the terms sensitive, intermediate, and resistant get tossed around as if they were always the same, this just isn’t the case.  There are tremendous differences between testing methods and if strict standards aren’t followed the end results may have little or no bearing on what might be observed in clinical cases.

Some important definitions: Minimum Inhibitory Concentration (MIC) is the lowest concentration of drug that prevents visible growth of bacteria in agar or broth dilution tests (usually in µg/ml),   Sensitive levels (S) imply an effective amount of drug can be obtained at the target tissue using normal label dosages (relative to the MIC of the bacterial agent),   Intermediate (I) levels imply that MIC levels can be reached at maximal dose ranges or in tissues where drugs may be physiologically accumulated,  Resistant (R) levels imply that concentration of drug needed can not be reached reliability with normal dosage schemes, or that the drug can not consistently reach the target tissue at sufficient MIC levels. Key point to remember is that all antibiotic sensitivity tests are guides, not absolutes. In order to make accurate statements about particular antibiotics and particular bacteria strains, very exacting standards for testing must be followed. 

Testing standards are set by the National Committee on Clinical Laboratory Standards/Veterinary Antimicrobial Susceptibility Testing Subcommittee (NACCLS/VASTS).  NACCLS sets breakpoints (MIC levels that would be effective for most strains) for certain antibiotics and bacterial strains.   These values are highly dependant on the type of disease, bacteria, type of animal, site of infection, and mode of therapy.  Change in any of these factors alters the predictive value of the sensitivity testing in question.  It is not well know by many in animal agriculture but, nearly all early sensitivity test data and criteria were generated to predict antibiotic usefulness in cases of human pneumonia or urinary tract infections.  While there may be some value in using sensitivity criteria standards generated in this way for predicting the best antibiotic for pneumonia cases in cattle, this information will have much, much less predictive value in other cattle diseases.  Even infections caused by the same bacteria but in different types of tissues, i.e., mastitis vs. pneumonia.

Antibiotic sensitivity tests are usually done in one of two methods.  The first is serial dilutions in agar or broth.  In these tests serial dilutions in 1:2 steps are made. A standardized number of bacteria are incubated in the presence of increasingly more dilute antibiotics.  In commercial or large diagnostic labs these tests are performed in 96 well plates.  Using good lab practices and standard operating procedures, these tests are very repeatable and consistent.  Normally these tests give S, I, R and MIC values.  Breakpoints can be calculated when large numbers of isolates are subjected to this type of testing and extended ranges of dilutions are also included.  Breakpoints allow practitioners to fine tune dosages to match effective range of the product vs. bacterial agent. In nearly all cases the NACCLS data is obtained using these criteria.

The other common susceptibility testing method is the Agar-Disk Diffusion method. This is commonly referred to as the Kirby-Bauer method.  A standardized bacterial innoculum is streaked onto a specific type of agar and antibiotic impregnated disks are placed on the surface of the agar.  After 16-18 hours of incubation, the size of the zone around the disk which is free of bacterial growth is measured.  This clear zone, zone of inhibition, has specific interpretive criteria based on type of antibiotic, innoculum, growth characteristics, and media.  This testing strategy is simpler, cheaper, and quicker. However, if testing procedures are not completed in a very consistent and standardized method, results tend to be highly variable and interpretation can be more difficult.  Whenever possible, it is important to ensure that NACCLS procedures have been followed before comparing susceptibilities of bacteria isolates or comparing the relative value of test data in regards to clinical responses.

The next article in this series will discuss common antibiotic groups and how they affect bacterial growth. 


Apley, M. D., Susceptibility testing for bovine respiratory and enteric disease, Vet Clinics of NA, Vol 3, No 3., Nov. 2003, pp 625-646

Erskine, R.J.,, Mastitis therapy and pharmacology, Vet Clinics of NA, Vol 19, No 1, March 2003, pp 109-138

David Wolfgang and Bhushan Jayara, Penn State, Extension Veterinarians