Antibiotics Used in Animal Feed Create
Long Term Health Hazards for
Humans.
Authors: James Moore, Acacia Strachan, Whelton Miller, and Cristina Ricardo.
Antimicrobial resistance was quickly recognized after the
introduction of penicillin nearly 50 years ago when
penicillin-resistant infections caused by Statphylococcus aureus
quickly appeared. With the development of new antibiotics
hospitals have many more resistant strains, some demonstrating
resistance to more than one antimicrobial agent.
Antibiotics used in human therapy and animal feed
Penicillin
The discovery of penicillin was due to the research of Alexander Fleming in 1928. Penicillin, an important antibiotic derived from mold, is effective against a wide range of disease-causing bacteria. It acts by killing bacteria directly or inhibiting their cell wall growth. Penicillins are bactericidal, inhibiting formation of the cell wall. There are four types of penicillns: the narrow-spectrum penicillin-G types, ampicillin and its relatives, the penicillinase-resistants, and the extended spectrum penicillins that are active against pseudomonas. Penicillin-G types are effective against gram-positive strains of streptococci, staphylococci, and some gram-negative bacteria such as meningococcus. Penicillin-G is used to treat such diseases as syphilis, gonorrhea, meningitis, anthrax, and yaws. The related penicillin V has a similar range of action but is less effective. Ampicillin and amoxicillin have a range of effectiveness similar to that of penicillin-G, with a slightly broader spectrum, including some gram-negative bacteria.
The penicillinase-resistants are penicillins that combat bacteria that have developed resistance to penicillin-G. The antipseudomonal penicillins are used against infections caused by gram-negative Pseudomonas bacteria, a particular problem in hospitals. They may be administered as a prophylactic in patients with compromised immune systems, who are at risk from gram-negative infections.
Side effects of the penicillins, while relatively rare, can include immediate and delayed allergic reactions-specifically, skin rashes, fever, and anaphylactic shock, which can be fatal.
Tetracyclene
Tetracyclines are bacteriostatic, inhibiting bacterial protein synthesis. They are broad-spectrum antibiotics effective against strains of streptococci, gram-negative bacilli, rickettsia (the bacteria that causes typhoid fever), and spirochetes (the bacteria that causes syphilis). They are also used to treat urinary-tract infections and bronchitis. Because of their wide range of effectiveness, tetracyclines can sometimes upset the balance of resident bacteria that are normally held in check by the body's immune system, leading to secondary infections in the gastrointestinal tract and vagina, for example. Tetracycline use is now limited because of the increase of resistant bacterial strains.
Erythromyicin
The macrolides are
bacteriostatic, binding with bacterial ribosomes to inhibit
protein synthesis. Erythromycin, one of the macrolides, is
effective against gram-positive cocci and is often used as a
substitute for penicillin against streptococcal and pneumococcal
infections. Other uses for macrolides include diphtheria and
bacteremia. Side effects may include nausea, vomiting, and
diarrhea; infrequently, there may be temporary auditory
impairment.
Erythromycin is
synthesized by Streptomyces erthraeus. Erythromycin is
usually bacteriostatic and binds with the 23S rRNA of the 50S
ribosomal subunit to inhibit peptide chain elongation during
protein synthesis.
References
"Discovery of Penicillin," Microsoft Encarta
Encyclopedia 99. 1993-1998
Microsoft Corporation. All rights reserved.
"Antibiotics," Microsoft Encarta Encyclopedia
99. 1993-1998 Microsoft
Corporation. All rights reserved.
J.P Harley, D. A. Klein, L. M. Prescott:
Microbiology 1999 McGraw-Hill
Companies, Inc.
Development of resistance to antibiotics
Additional Link
Antibiotics can be defined as substances produced by living organisms, which in low concentrations are able to inhibit growth of other organisms.
In any population that is sensitive to a given antibiotic, resistant forms can arise through genetic change. This genetic change can be either mutation or gene transmission from other cells. Mutations to drug resistance have been found for most antibiotics and these mutations occur spontaneously. Mutations that confer a high degree of resistance are referred to as single one- step. These mutants are least common and have a 1000- fold greater resistance than there parents and include streptomycin and erythromycin. The more common way for antibiotics to build up resistance is through several mutations. These antibiotics are referred to as multi-step.
In genetic transmission, the genes for drug resistance are carried on not only the chromosomes but also on circular, extrachromosomal elements called plasmids. Plasmids are independent replicons and thus control their own replication. The plasmids of enteric bacteria are referred to as R factors and are found in pathogenic staphylococci and thus confers resistance to penicillin, tetracycline, streptomycin, erythromycin, and sulfonamides. These genes are found in a variety of combinations on different R factor, thus a given R factor may carry several resistance genes.
One circumstance that has contributed to the spread of R factors is the use of antibiotics in animal feeds. With this practice resistant strains may be transferred to humans through the eating of raw meats or neat products. It has been found that with widespread of any clinically used antibiotic the proportion of resistant strains within a group may rise rapidly. For example, penicillin which was the first antibiotic discovered showed a rapid increase in resistant forms over a four year period:
Year
Percent of total strains resistant
1946
5
1947/8
17.8
1949
29.1
1950
43.5
To prevent the spread of resistant forms only those antibiotics not used for clinical use should be given to farm animals. Another way to reduce resistance is if hospitals rotate the use of antibiotics being employed. For example alternate Groups I and II in three- to- four year cycles, and those in Group III as special reserves using them only for special cases.
Group I
Tetracylines, Macrolides, & Sulfonamides
Group II
Penicillin, Streptomycin, & Chloramphenicol
Group III
Vancomycin, Ristocetin, Neomycin, Colistin , & Polymyxin
Some of the antibiotic resistant human diseases
Salmonella
Urinary Tract infections
Gonorrhea
Pneumonia
Staphylococcus Aureus
Malaria
Examples of illnesses caused by food
#1 - WebMD Health page titled 'Staph aureus food poisoning'
#2 - Case studies of infections caused by antibiotic resistant bacteria.
Costs of treating antibiotic resistant infections
The Institute of Medicine, a part of the National Academy of Sciences, has estimated that the annual cost of treating antibiotic resistant infections in the United States may be as high as $30 billion.
New antibiotic research
Conclusion
Antibiotic resistant disease causing bacteria develop
resistance as a direct result of antibiotic use in living
organisms either through mutation or plasmid transfer from other
resistant bacteria. Sub-therapeutic use of antibiotics for
other than treatment of disease promotes the development of
resistant strains of bacteria. Use of antibiotics as a
growth supplement at subtherapeutic levels is currently at
~25,000,000 pounds annually in the United States. This
25,000,000 pounds contributes directly to the development of
antibiotic resistant bacteria without any significant benifit
other than the economic benifit to the grower. Conversely,
the cost of treating the resultant antibiotic resistant bacterial
diseases is estimated at $30,000,000,000 per year. If
the current trend continues, even more resistant bacteria will be
produced - possibly resistant to all known therapies.
Write to us if you have any questions or comments.
James Moore, Acacia Strachan, Whelton Miller, and Cristina Ricardo.
Additional References
1. Zaehner, Has and Mass, Warner; "Biology of Antibiotics" Springer-Verlag 1972
2. "The Effects on Human Health of Subtherapeutic Use of Antibiotics in Animal Feeds", National Academy of Sciences, Washington D. C. 1980
3. "Remington's Pharmaceutical Sciences", Mack Publishing Co. 1970
This website was created as part of a project for Senior Seminar in the Department of Chemistry and Biochemistry at the University of Delaware.
Last updated December 1, 2000
