Written by: Dr. Jacquie Jacob, University of Kentucky
NOTE Antibiotics may not be used in organic poultry production. However, organic producers may not withhold medical treatment from a sick animal in an effort to preserve its organic status. All appropriate medications must be used to restore an animal to health when methods acceptable to organic production fail. Livestock treated with a prohibited substance must be clearly identified and shall not be sold, labeled, or represented as organically produced.
An antibiotic is a drug that kills or slows the growth of bacteria. (Drugs that kill bacteria are referred to as bacteriocidal; those that slow the growth of bacteria are referred to as bacteriostatic.) Antibiotics are chemicals produced by microorganisms such as bacteria and fungi.
There are many different kinds of antibiotics, and they destroy bacteria in different ways. The antibiotics within a class generally have similar effectiveness and mechanisms of action and resistance and they tend to attack the same types of bacteria. Some antibiotics, referred to as broad-spectrum antibiotics, treat a wide range of infections. Others, called narrow-spectrum antibiotics, are effective against only a few types of bacteria. Although antibiotics are sometimes used in conventional animal feeds, some of the antibiotics discussed below can be used only under the supervision of a veterinarian.
Major classes of antibiotics include the following:
Aminoglycosides are derived from various species of Streptomyces bacteria. These antibiotics act by blocking the synthesis of proteins vital to bacterial growth. The aminoglycosides remain in the digestive tract so are effective in the treatment of enteric infections. Examples include gentamycin, neomycin, spectinomycin, and streptomycin.
Bambermycins are derived from Streptomyces bambergiensis. These antibiotics act by inhibiting the synthesis of the bacterial cell wall. Examples include bambermycin and flavophospholipol, which are effective against gram-positive pathogenic bacteria and do not affect Lactobacillus, Bifidobacterium, and other protective bacteria.
Pencillins and cephalosporins are the two major types of beta-lactams.
Penicillins, the first antibiotics to be discovered, are produced by the mold Penicillium notatum. Penicillins are bacteriocidal, and they kill bacteria by inhibiting formation of the bacterial cell wall. The wall breaks down, releasing the contents of the cell. Amoxicillin and ampicillin are examples of penicillins. Penicillins are effective in the treatment of sinusitis and chronic respiratory disease in poultry.
Structurally, cephalosporins are closely related to penicillins. This class of antibiotics is further divided into first, second, and third generation. Each generation has a broader spectrum of activity than the one before. Like penicillins, cephalosporins interfere with the formation of bacterial cell walls. The Food and Drug Administration (FDA) issued an order January 4, 2012, that prohibits certain extra-label uses of the cephalosporin class of antimicrobial drugs in cattle, swine, chickens, and turkeys. The order became effective on April 5, 2012. (The term extra-label refers to the use of a drug in a way that is not listed as an approved use on the label.)
In its order, the FDA prohibitied extra-label or unapproved uses of cephalosporins, specifically including
Glycopeptides act by interfering with cell-wall formation as well as the production of proteins. The only glycopeptide antibiotic available in the United States is the human product vancomycin (Vancocin). Vancomycin is often the treatment of last resort for methicillin-resistant Staphylococcus aureus (MRSA) infections in humans. The FDA Center for Veterinary Medicine (FDA-CVM) issued an order in 1997 prohibiting the extra-label use of all glycopeptides in food animals.
Ionophores are feed additives used in the control of coccidiosis, primarily when raising broilers, broiler breeders, and replacement pullets. Coccidiosis is a disease in poultry worldwide and is caused by a protozoan parasite (Eimeria) that invades the cells of poultry intestines. Ionophores are used primarily as an anti-microbial but can control some bacteria so is often grouped with antibiotics. Ionphores are not used in human medicine.
For more information refer to the article The Use of Ionophores in Poultry Production
Lincosamides are structurally different from macrolides, but they have the same antimicrobial effect. Lincosamides are produced by Streptomyces lincolnensis. Lincomycin is a lincosamide that penetrates most tissues well, including bone. Lincomycin is effective against bone and joint infections, as well as necrotic enteritis caused by Clostridium perfringens.
Macrolides, which are derived from the Streptomyces bacteria, are bacteriostatic and act by interefering in protein production. An example is tylosin, which is produced by Streptomyces fradiae. Tilmicosin is a semisynthetic macrolide. Macrolides are effective against Mycoplasma and Orhnithobacterium rhinotracheale and can be used to treat necrotic enteritis.
Polypeptides have bactericial activity against bacilli. Examples of polypeptides include bacitracin, which is effective against Clostridium bacteria, and polymyxin E, which is effective against E coli, Salmonella, Pasteurella, and Pseudomonas aeruginosa.
Fluoroquinolones are synthetic antibiotics not derived from bacteria or fungi. They are broad-spectrum bacteriocidal drugs. Fluoroquinolones prevent bacteria from making DNA, thus preventing the bacteria from multiplying. Examples include enrofloxacin, danofloxacin, flumequine, norfloxacin, and difloxacin. Fluorquinolones are effective against salmonellosis, colibacillosis, fowl cholera, and Pseudomonas aeruginosa infections.
Streptogramins are produced by Streptomyces species and consist of two structurally unrelated molecules. One inhibits protein synthesis, and the other, cell-wall formation. Individually the molecules are bacteriostatic, but the combination of the molecules is bacteriocidal. The drug virginiamycin is a streptogramin that is effective against necrotic enteritis.
Sulfonamides are produced by chemical synthesis. They have bacteriostatic activity against a broad spectrum of pathogens. They interfere with RNA and DNA, which are necessary for cell growth and replication. Sulfonamides, such as trimethoprim, are effective against Staphylococcus species, Streptococcus species, Pasteurella, Salmonella, and E coli.
Tetracyclines are derived from the bacteria Streptomyces. They are broad-spectrum bacteriostatic agents. Tetracyclines prevent bacteria from multiplying while the host animal's immune system deals with the original infection. Examples include chlortetracycline (Aureomycin) and oxytetracycline (Terramycin). Doxycycline is a semisynthetic tetracycline. Tetracyclines are effective against Mycoplasma, Chlamydia, Pasteurella, Clostridium, Ornithobacterium rhinotracheale, and some protozoa.
NOTE Brand names appearing in this article are examples only. No endorsement is intended, nor is criticism implied of similar products not mentioned.