Tuesday May 18 2010
Bacteria in animals transferred antibiotic resistance to humans
Urinary tract infections may become resistant to antibiotics through the “overuse of antibiotics in the farming industry”, BBC News has said.
The news is based on research on the bacterium E. coli, a common cause of urinary tract infections. The researchers looked at E. coli grown from the urine and stool samples of humans and from the stool samples from various animals.
They tested the urine and stool’s resistance to an antibiotic called gentamicin. Animal and human samples that proved resistant were found to have specific genetic sequences in common, suggesting that the strains had transferred genes for resistance between each other.
The study did not look at how antibiotic resistance in the bacteria could be transferred between animals and humans, therefore it does not indicate whether it is possible to transmit resistance through eating meat. Regardless, it is well known that when dealing with infections patients should correctly use prescribed antibiotics and take their whole course of treatment to help prevent bacteria from developing antibiotic resistance.
Where did the story come from?
The study was carried out by researchers from the University of Hong Kong, and it was funded by the university and the Hong Kong government. The study was published in the peer-reviewed Journal of Medical Microbiology.
What kind of research was this?
Urinary tract infection in women is often caused by the bacteria E. coli. Some strains of E. coli are resistant to antibiotics such as gentamicin, tobramycin and nitilmicin. The researchers suggested that some of the antibiotic-resistant E. coli may have acquired their resistance while living in food-producing animals that had been given these antibiotics.
This was a Hong Kong-based laboratory study that used isolates (samples of bacteria) from humans and animals gathered from previous studies of antimicrobial resistance. It used these bacterial isolates to assess the distribution of antimicrobial resistance among the samples, and to identify which specific genetic changes in the bacteria had enabled them to become resistant to antibiotics.
What did the research involve?
The researchers grew 249 bacterial isolates taken from various human and animal sources; 103 isolates from the urine of women with uncomplicated urinary tract infections, 82 isolates from the faeces of food-producing animals and 64 isolates from the faeces of children and adults.
They assessed antimicrobial resistance by examining whether the bacteria was able to grow on an agar medium containing an antibiotic called gentamicin. They also looked for specific genes that are associated with antibiotic resistance, including four genes that produce AAC(3) enzymes – types of enzyme which in turn cause resistance to these antibiotics.
Bacteria can transfer parts of their DNA called plasmids through cell-to-cell contact. This is called ‘conjugation’ or ‘horizontal gene transfer’. To see whether E. coli could pass genetic resistance to gentamicin between themselves, the researchers mixed gentamicin-resistant bacteria with bacteria sensitive to the antibiotic in a ratio of 1:10. They measured this transfer by analysing the bacteria’s DNA.
What were the basic results?
Of the 249 isolates tested, 160 were gentamicin-resistant and 89 gentamicin-sensitive. They found that 84.1% of the human samples and 75.5% of the gentamicin-resistant animal isolates possessed resistance gene aacC2. However, none of the 89 gentamicin-sensitive isolates contained the gene.
They found that gentamicin-resistant E. coli from 10 animal samples and 10 human samples had two aacC2 gene alleles (different versions of a gene). The presence of these alleles was equal between the animal and human samples. One of the alleles, called AAC(3)-II, had a genetic sequence identical to the published sequences of various types of bacteria from around the world. In light of this, the researchers suggested that different species of bacteria can transfer mobile genetic elements containing this gene between each other.
When they looked at the dynamics of transferring antibiotic resistance between bacteria, they found that for each 10,000 resistant donor cells the resistance would in turn be passed on to between one and 100 non-resistant cells.
How did the researchers interpret the results?
The researchers suggested that a substantial proportion of the gentamicin resistance in E. coli found in outpatient urinary samples was attributable to resistance genes that are widespread among faecal samples from food-producing animals.
They say that this observation provides further support to concerns over the transmission of antibiotic resistance between food-producing animals and humans.
This was a small study, which found that resistance to the antibiotic gentamicin was granted by the same gene sampled from both animals and humans. However, it did not look at the possible routes by which this resistance may be transmitted between animals and humans. For example, it could not say whether consuming animals with antibiotic-resistant E. coli in their guts is a possible route of transmission. It did however highlight the way that antibiotic resistance can be transferred between bacteria.
This study was carried out in Hong Kong where meat consumed by the population is produced by Chinese farms. It is not clear whether the antibiotic use in Chinese farms would differ from antibiotic use in British farms. It is well known that antibiotic resistance is a great public health concern, and antibiotics should be carefully prescribed by doctors and vets. It is also important that if patients are prescribed antibiotics, they should take the whole course of treatment in order to avoid pathogenic bacteria in their bodies developing antibiotic resistance.