This is due to their own defense mechanism. Therefore, antibiotic treatment is not the only, or necessarily the best, defense. Treatment frequently aids in providing time for the immune response to reach protective levels. Additional information concerning past history of the herd and environmental factors provide a basis for medical decision making. Similarly, laboratory work is necessary at some point in the course of many problems if you are to expect effectiveness of treatment procedures.
Confusion seems to exist about antibiotic susceptibility tests.
There is an apparent lack of understanding about both their interpretation and the role such testing plays in a treatment program. In the laboratory, isolated bacteria are exposed to a variety of antibiotics. If the organism is killed or its growth restricted by one of these antibiotics, that bacteria is assumed to be susceptible to that drug. If the organism continues to multiply, it is assumed to be resistant to that antibiotic. A single species of bacteria, such as Staphylococcus aureus , has many strains or types that may vary in their susceptibility to a given antibiotic.