Creating indomitable superbugs

Antibiotic resistance is a growing threat

Published: Thursday 30 September 2010

Creating indomitable superbugs

imageAntibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic. If even at a large dose, the antibiotic is not effective in treating an infection, then the microorganism that is responsible for the infection is declared resistant to that antibiotic.

Antibiotic resistance is a global public health concern today. Its primary cause is long-term over-exposure to antibiotics through their use as medicines in humans, as well as in animals, horticulture and for food preservation. Antibiotics are used in animals not only to treat or prevent diseases, but also to promote growth. The types of antibiotics used in animals are frequently the same as, or closely related to, those used in humans.

Antibiotic resistance in bacteria evolves via natural selection through random mutation. When a bacterium is exposed to an antibiotic it starts making changes in its DNA to withstand the effects of the medicine. Once it acquires a specific antibioticresistant gene, it quickly passes it on to its next generation. Resistant genes also pass from one type of bacteria to another. Nature has developed different systems for transfer of genes between bacteria (conjugation, transformation, transduction and transposition) and these mechanisms have proved effective in the promotion of resistant genes. If a bacterium carries several resistant genes, it is called multi-resistant or a superbug.

Several WHO consultations and expert bodies have identified links between antibiotic use in animals and the emergence of mainly food-borne bacteria which are resistant to important antibiotics used in treating infectious diseases in humans.

In December 2003, an expert workshop was jointly convened by UN’s Food and Agriculture Organisation (FAO), the World Organization for Animal Health and WHO to make a scientific assessment of resistance risks arising from non-human use of antibiotics/antimicrobials. The workshop concluded “there is clear evidence of adverse human health consequences due to resistant organisms resulting from non-human usage of antimicrobials. These consequences include infections that would not have otherwise occurred, increased frequency of treatment failures (in some cases death), and increased severity of infections”.

In recent years, more evidence has emerged of an association between use of antibiotic agents in animals and antibiotic resistance among bacteria isolated from humans. An outbreak of human nalidixic acidresistant Salmonella typhimurium DT104 infection in Denmark was traced to a pig farm. Another outbreak of the same infection, reported in the UK, was traced to a dairy farm where fluoroquinolones had been used on the cattle a month before the outbreak. In the US, there was a marked increase in the proportion of domestically acquired Campylobacter infections that were fluoroquinolone- resistant, following the first approved use of fluoroquinolones in animals in 1995.

WHO has recommended that antibiotics which are licensed in human medicine should not be used any more as growth promoters in animals. An EU resolution to this effect was put in place in 1999. Since then, studies from Denmark, Germany and Italy have shown a significant reduction in vancomycin-resistant enterococci bacteria from poultry and poultry- derived food products. Some EU member states (such as Denmark) have, with insignificant or no consequence either on disease rates in animals or on meat market prices, voluntarily suspended the use of all growth promoters, irrespective of their significance to human health.

The major challenge in combating antibiotic resistance lies in the development and implementation of methods for their prudent use. There is also no method for safety assessments of antibiotics intended for animal use. There is a significant difference between traditional chemical residuebased determination of safety of animal drugs and the determination of safety in the context of antibiotic resistance.

The traditional method is found to be inadequate in addressing antibiotic resistance aspect. This is one of the reasons why countries like Denmark have suspended the use of antibiotics as growth promoters.

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