Pigs and pathogens: understanding antibiotic resistance

Pigs could be the key to understanding how antibiotic resistant bacteria persist in intensive care units in hospitals.

NSW Department of Primary Industries (NSW DPI) Immunology & Molecular Diagnostic Research Unit team leader, Dr James Chin, said it was commonly believed that each time an antibiotic is used only pathogens or disease-causing bacteria will be killed.

"Antibiotic use in hospitals is often perceived to be solely directed against only bad bacteria," Chin said.

"In reality, antibiotics also act against entire microbial communities, including the good bacteria which can protect patients from pathogenic bacteria.

"Antibiotics do not just eliminate bad bacteria. They also maintain a pool of antibiotic resistance genes within the microbial community of patients treated with antibiotics."

Using pigs as a model, Chin and Dr Toni Chapman at NSW DPI's Elizabeth Macarthur Agricultural Institute have examined how E.coli bacteria - a common cause of diarrhoea in pigs and humans - respond to treatment by antibiotics.

"The current theory of antibiotic resistance is that the 'fittest' bacteria, that is, those carrying genes for resistance, are the most likely to survive," Chin told the 2007 Australian Society for Microbiology's annual conference in Adelaide this month.

"Because antibiotic treatment will never kill all bacteria, bad or good, there will always be a pool of antibiotic resistance bacteria that can potentially transfer resistance to incoming pathogens.

"It is important to identify the antimicrobial resistant gene pool in entire microbial communities before antibiotic treatment."

This has been tested with E. coli in pigs.

"Our research shows clearly that use of one antibiotic to treat E. coli not only increases resistance against that antibiotic but also increases the carriage of resistance genes against other classes of antibiotics.

"This creates a real problem because subsequent therapy with a second antibiotic may be ineffective because resistance against the second antibiotic had already been increased by the first antibiotic."

Chin said the use of antibiotics for disease prevention is critical in patients admitted to intensive care. However there is currently a major bottleneck when it comes to deciding which antibiotics to use.

"Current protocols require pathogens to be cultured, leading to delays of some days before the bacteria to be targeted can be accurately identified," he said.

For this reason it is important to develop a molecular detection method that can identify antibiotic resistance signatures of entire microbial communities, he said.

"Our hope is that this kind of information will equip clinicians to better manage prescribing of antibiotics."

This latest research is being planned in collaboration with clinical microbiologists and intensive care specialists at Westmead Hospital.

Joanne Finlay is a science communication specialist with the NSW Department of Primary Industries.