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By Dr Jim Robinson for the Pig Vet Society

One Monday morning in 1928, so the story goes, Alexander Fleming came into his laboratory in Oxford where he was working with cultures of Staphylococcus aureus, the common pus-forming bacterium. He found that one of the Petri dishes with a flourishing staph culture had been contaminated by a bread mould and, just as he was about to throw it away, he noticed that a clear area surrounded the green mould colony.

Fleming put the dish aside and observed that, as the fungal colony grew, the staphylococcal growth retreated. What was more, the drops of moisture exuded by the mould had a similar inhibiting effect on fresh Staphylococcal growth.

The mould was identified as Penicilium notatum, the fluid was christened "penicillin" and that was the birth of the modern antibiotic industry. Fleming was given the Nobel prize and a knighthood and had the foresight to warn his colleagues of two problems – not all bacteria (and no viruses) were susceptible to penicillin, and there was a likelihood that resistance would develop in sensitive organisms.

Since then, as we know, a huge industry has developed, with dozens of different antibiotics with different modes of action and varying spectra of effectiveness, dozens of resistant strains of bacteria and, all in all, a fantastically successful track record in saving millions of human and animal lives, not to mention increasing food production and preservation.

The prudent use of antibiotics

Much attention has been given this aspect of treatment in both animals and humans and caution is necessary, particularly in the animal industry where low doses of antibiotic are fed as growth stimulants, theoretically increasing the risk of resistance.

In some European countries the use of all antibiotic growth promotants has been banned and in others it is restricted to those that are not used in human medicine.

So far as day to day use by pig producers is concerned, we need to look at therapeutic uses of sulphonamides and antibiotics as the manager or stockman is often left to his own devices when deciding on which drug to use at a particular moment.

Important tips to remember

• Antibiotics may be long or short-acting, broad or narrow spectrum, bactericidal or bacteriostatic, and there are certain antibiotics that are specific for certain bacteria. For this reason it is essential that animals are treated under supervision of your veterinary consultant, who is in any case the only person who may legally prescribe or supply Schedule 4 drugs for you herd.

• As a result of a particular stain reaction, most bacteria fall into groups known as Gram positive or Gram negative. To some extent this grouping is linked with sensitivity to certain groups of antibiotics.

• In general, the intestinal pathogens such as E coli, Lawsonia, salmonellas, Brachyspira and other spirochaetes are Gram negative, along with Pasteurellas, Haemophilus and Actinobacillus species.

Examples of Gram positive bacteria are: Erysipelas, Staphylococci, Streptococci, Clostridium spp.

• If in doubt about the diagnosis, and an antibiotic seems to be indicated, choose a broad spectrum one;

• Do not use a bactericidal and a bacteriostatic antibiotic together – they can interfere with each other’s action;

• Avoid using bactericidal drugs in advanced Gram negative infections: the release of endotoxins from dying bacteria may aggravate the condition;

• Avoid bacteriostatic drugs in immuno-compromised patients: they do not have the natural weapons such as serum antibodies, macro and microphages to tackle the immobilised pathogens;

• Never under-dose antibiotics; it leads to emergence of antibiotic resistant strains

• Antibiotics are sometimes grouped by their chemical structures but this does not affect their action on bacteria or their interaction with each other.

It is more important to know whether they are bactericidal or bacteriostatic and whether they are broad or narrow spectrum.

Table 1 will put some of the antibiotics commonly used on pigs into categories.

Table 1. A grouping of commonly used antibiotics according to their structure and action.

(With thanks to the Pharmacology Dept of the Veterinary Faculty at Onderstepoort)