The Rise and Rise of Resistance
Pyper, Wendy, Ecos
Only 50 years since antibiotics were hailed as miracle drugs against potentially fatal bacterial infections, a new breed of `superbugs', resistant to many antibiotics in our arsenal, is on the rise.
Multiple drug resistant (MDR) strains of Staphylococcus aureus, Mycobacterium tuberculosis, Pneumococcus, Enterococcus, Salmonella, Klebsiella and Pseudomonas, are appearing in hospitals and communities worldwide, thanks to years of antibiotic use and abuse in human medicine, animal husbandry and agriculture.
Concern over this trend was expressed by a panel of speakers at the International Congress of Bacteriology and Applied Microbiology in Sydney last August.
One speaker, Professor Julian Davies from the Department of Microbiology and Immunology at the University of British Columbia, said more than one million tonnes of antibiotics had been released into the environment in the past 50 years, exerting a selective pressure on microbes to adapt or die.
But after nearly four billion years on earth, microbes are experts at adaptation. Antibiotic resistant genes, for example, originally evolved to counteract natural antibiotic production, a key defence in many a microbe's survival strategy. Fifty years of human intervention, and the development of synthetic antibiotics, is a small hurdle in microbial evolution -- a `minor crisis' from which many bacteria are emerging victorious.
The rise of these so-called `superbugs' has been aided by our inability to internationally regulate antibiotic use in animal husbandry and human medicine.
Of particular concern are antibiotics used for `growth promotion' in the intensive production of cattle, poultry and pigs. These are given in low or `subtherapeutic' doses to improve feeding efficiency so that marketable weights are reached sooner, and with less food.
Growth promoters are thought to work by controlling low levels of pathogenic bacteria, which normally would divert the animal's energy from weight gain to fighting infections. But they also create a selective pressure for resistant bacteria in the animal gut and it is feared these bacteria may pass their resistance genes on to human bacteria, via the food chain.
As far back as 1968, the Swann Committee to the British Parliament found compelling evidence that resistance genes could move from animal bacteria to human bacteria. The committee recommended that antibiotics used in human medicine should not be given to animals as growth promoters.
Later European studies also found evidence of resistance gene transfer, as well as cross-resistance of animal bugs to antibiotics important in human therapy. But Australia, the United States and most countries in the European Union (EU) continued to use human antibiotics as growth promoters, arguing that any risk to human health was largely theoretical and exceedingly small.
In 1997, the EU banned the use of the controversial glycopeptide growth promoter avoparcin and in 1999 suspended the use of four others, based on the `precautionary principle'. Given the money and livelihoods invested in the production and sale of antibiotics, this move had many critics who believed the evidence for resistance transfer was insufficient to justify such a ban.
The debate has now moved to Australia. In 1998 the Australian Federal Government set up the Joint Expert Technical Advisory Committee on Antibiotic Resistance (JETACAR), made up of health, veterinary, molecular biology and primary industry representatives, to review antibiotic use in the Australian livestock industry (see story on page 19)
Here too, opinion is divided over the risk growth promoters pose to human health. Medical and scientific representatives say that our potential to treat life-threatening human infections is diminishing as a result of resistance transfer from animal bacteria to human bacteria. Veterinary and industry representatives say the evidence for this transfer is weak and that the real culprit is years of antibiotic misuse in humans. …