Unit Animal Science

Problem Area Growth and Development of Animals

Lesson Effect of Antibiotics on Bacteria

Student Learning Objectives 1. Understand the function and purpose of antibiotics. 2. Explain the way that bacteria can cause infections. 3. Understand how bacteria can be spread from host to host.

Terms Antibiotics Bacterialcidal Effect Broad Spectrum Mastitis Selective Toxicity Zone of Inhibition

What is the function and purpose of antibiotics? A. Antibiotics are antimicrobial drugs that slow down or inhibit the growth of some kinds of microorganisms (microbes) in animals. Some microbes are beneficial while others are harmful. Antibiotics have a bacterialcidal effect, meaning they kill or inhibit the growth of other organisms. In our lab, the antibiotic discs will create a zone of inhibition, or area surrounding each disc where bacteria will be unable to grow.

B. For an antibiotic to inhibit the growth of bacteria, it must meet several criteria:  1. The antibiotic must not interfere with host’s immune system. Some antibiotics kill beneficial as well as harmful bacteria, which will slow the animal’s recovery.  2. The antibiotic should cause no direct or irreparable damage to the host. This refers to selective toxicity, or the ability of an antibiotic to kill an infection without harming the host.  3. The antibiotic should have a broad spectrum, or be effective against several types of bacteria.

C. There are several types of bacteria that cause a mastitis infection, or infection of the udders of milking animals. Since there are several types of bacteria involved, it is best to use a broad spectrum antibiotic to control the infection initially. Once the specific bacteria has been controlled, the antibiotic should be changed to a narrow spectrum antibiotic to control the infection without endangering the beneficial bacteria necessary for milk production.

How does bacteria cause infection? A. Some forms of bacteria invade host tissues directly and congregate near weak cells. Once weakened cells have been infected, the disease can spread to other body tissues and areas. Thus, it is important to keep all wounds and body injuries clean, disinfected, and bandaged to reduce the risk of bacterial infection. B. Some bacteria employ the use of microscopic “capsules” to protect them from the environment. These bacteria can survive longer on surface areas and inside the animal body before becoming active and causing infection.

C. Scientists have now discovered that some bacteria may use tiny, thread-like apparatus to actually “pull” themselves into position in order to congregate and infect weak cells. D. Most bacteria rely on the method of adhesion to body tissues in order to gain a foothold for infection. Inside the urinary tract of the animal, urine flow is the primary way to rid the body of harmful bacteria. If bacteria are not adhered firmly to the urinary mucosa, they will be flushed out with the urine flow.

How can bacteria spread from host to host? A. Bacteria can be spread by using non-sterile instruments on animals, orally (by ingesting the bacteria from contaminated food or surfaces), by direct contact between an animal carrying the bacteria and a healthy animal, and through air and water.

B. The most common way that bacteria can be spread from host to host is through contaminated feces. Waste management, especially in confinement operations, becomes crucial to control and minimize the spread of bacteria and disease in livestock herds.

Review/Summary What is the function and purpose of antibiotics? How does bacteria cause infection? How can bacteria spread from host to host?