So, what are Antibiotics?

Several different definitions of antibiotics are used; the Joint Expert Technical Advisory Committee On Antibiotic Resistance (JETACAR) report has adopted the widest meaning possible, not confining itself to therapeutic substances derived from naturally produced antimicrobial chemicals. The earliest antibiotics, penicillin, the tetracyclines and streptomycin, were produced by moulds to control competing bacteria or fungi.

About twelve families of natural antibiotics have been identified. Members of the same family have related chemical structures and usually are active against a similar range of organisms. Other chemicals have been discovered, the sulphonamides for example, that do not occur naturally but have anti-microbial properties similar to natural antibiotics. These are sometimes included in discussions of antibiotics even though they differ in derivation.

As well as the naturally occurring antibiotics, there is a wide range of new chemicals, developed using knowledge of the structure and effects of natural antibiotics. These retain the significant chemical features of natural antibiotics. Ampicillin is an example, derived from penicillin.

Families of Antibiotics

The poultry industry has at one time or another used seven of the twelve families of antibiotics on a regular basis. The earliest, penicillin and the tetracyclines, are now rarely used, the tetracyclines sometimes for therapeutic purposes.

Virginiomycin was originally used as a growth promoter. This was thought to be safe as none of its family was used for disease treatment in humans. However some relatives are now used against infections with multi-resistant bacteria. Because of the importance of these “drugs of last resort”, the use of Virginiomycin in animal feed has been voluntarily discontinued.

Three families of antibiotics are regularly used by the poultry industry.

Macrolides, including Tylosin and Erythromycin, are used to control some respiratory diseases.

Ionophores, mainly Monensin and Salinomycin to modify the gut flora and to control the group of parasites that cause a disease called coccidiosis; so far they have no apparent value in human medicine.

Polypeptides, particularly Bacitracin, are used to modify the gut flora. These were the antibiotics of choice in layer feeds as they were believed not to be absorbed from the intestine, hence did not produce problems with antibiotic residues in eggs or egg products.

Until recently, a Glycopeptide antibiotic, Avoparcin, was used in poultry meat production to modify the gut flora and enhance growth rate, but not in egg production. Because it is related to Vancomycin, a drug of last resort in cases of multiple resistant bacterial infections in humans, production of Avoparcin have been discontinued and its use stopped.

Controls on antibiotic use

In recent years, there has been a move towards banning the non-therapeutic use of antibiotics in animal production in Europe. Sweden was first in 1986. The European Community banned the use of Avoparcin in 1999 as resistance could be transferred to bacteria causing disease in humans, via intestinal bacteria, and these then would also be resistant to Vancomycin. Manufacture of Avoparcin has now ceased. Subsequently use of Spiramycin, Tylosin, Virginiomycin and Bacitracin was also banned in Europe. Britain has followed suit and has recently suggested that the Ionophores should also be banned.

At present Australia places no restrictions on the use of low levels of antibiotics in poultry feed. As a result of a unilateral voluntary action by feed manufacturers Avoparcin was withdrawn from sale and Virginiomycin is no longer used.

Antibiotics in egg production

Antibiotics are used in egg production in two ways – either for the prevention and treatment of specific diseases or to improve performance when added to the feed or water at low levels. In this second role, the antibiotics are used to alter the range of bacteria growing in the gut without necessarily targeting any specific disease-causing organism.

In 1999, JETACAR, a committee of the Commonwealth Department of Health and Age Care published a report "The use of Antibiotics in food-producing animals: antibiotic resistant bacteria in animals and humans". In this report, the risks associated with using antibiotics for non-therapeutic or prophylactic purposes were analysed.

Download PDF (Help)

The therapeutic use of antibiotics is a valid, sometimes vital, application of these precious chemicals as disease can spread rapidly through flocks of birds. However, their use to stimulate egg production without any therapeutic justification has been widely condemned. This use risks reducing the effectiveness of these drugs for disease control. It also unnecessarily risks contaminating a highly nutritious product with traces of antibiotics and their metabolic breakdown products. Antibiotic residues in prepared food, leading to selection of resistance in bacteria associated, not with the birds that laid the eggs, but with consumers.

Antibiotic residues and consumer health

A significant proportion of consumers of poultry products is sensitive to, or even allergic to, one or more types of antibiotic. There can be sufficient antibiotic present in eggs from birds fed or injected with antibiotics to trigger an unfavourable reaction in these sensitive people. This reaction might be as mild as feeling slightly off-colour to an outbreak of hives or extreme distress with respiratory failure.

Antibiotic resistant bacteria and poultry

There is always the possibility, if not ultimate certainty, that continued use of an antibiotic either for the control of disease or for performance enhancement will select a population of bacteria in the birds’ intestine that is resistant to that antibiotic. This resistance is genetic and is easily and quickly transferred from one species of bacteria to another. Harmless antibiotic resistant bacterium present on or inside eggs could quickly introduce that resistance to pathogenic bacteria in the human intestine so that these bacteria become resistant to that antibiotic before they were exposed to it.

Antibiotic residues and selection for resistance in consumers

Antibiotic residues present in eggs or egg-based products may lead to resistant bacteria being selected within the intestines of humans consuming them. This could aid in maintaining population of resistant bacteria even though the person had not knowingly been exposed to the antibiotic.