Antibiotic resistance: Are we done with ‘magic bullets’?

Antibiotics are often hailed as ‘magic bullets’, but widespread use of these compounds in recent years has become a cause for concern globally, as their excessive use has resulted in antibiotic resistant bacteria in environment

Antibiotics are compounds produced by microorganisms’ bacteria, fungi and actinomycetes, which have an inhibitory effect against other bacteria and fungi. The first antibiotic penicillin was discovered by Alexander Fleming in 1920. Since the discovery of penicillin, many other antibiotics with inhibitory actions against a wide variety of microorganisms have been discovered. Antibiotics have often been hailed as “magic bullets” for their ability to control infections.

In addition to natural antibiotics, many semi-synthetic derivatives and synthetic antimicrobial agents also have been developed for the treatment of infections. Antibiotics act on bacteria by a variety of mechanisms like inhibition of protein synthesis and interference with DNA (deoxyribonucleic acid) replication to disrupt the microbial metabolism.

Antimicrobials while are used for treating humans, they are also effective in animal health and production. However, widespread use of antibiotics in recent years has become a cause for concern globally, as excessive use of antibiotics has resulted in antibiotic resistant bacteria in environment.

Soon after their use in human health, antibiotics were introduced to veterinary medicine to treat microbial infections in animals. Since then antibiotics have formed a formidable part of the veterinarian’s arsenal for the treatment of various types of infectious diseases and enabling in part to meet the food requirement of the increasing human populations. Almost all major antibiotics that are prescribed for humans are used in veterinary practice. In animals, antibiotics are used not only to treat infectious diseases but also to augment animal production as growth promoters. As growth promoters, antibiotics help in rapid weight gain up to five per cent or more without increasing in feed consumption. How antibiotics exactly help in weight gain is not clear but it is assumed that these compounds interfere with gut microflora, leading to better absorption of food.

To apply them as growth promoters, antibiotics are used on animals quite below recommended therapeutics levels and are supplied mainly through feed and occasionally through water. When administered through feed, more than 80 per cent of the antibiotics are excreted out through faeces or urine. The sub-therapeutic use of antibiotics as growth promoters has emerged as a major factor contributing to development and spread of antibiotic resistant bacteria. This resistance could spread as a reservoir of resistant bacteria. This resistance could spread through commensal microflora of food animals, serving as a reservoir of resistance traits spreading to commensals and pathogenic microbes of human beings

When administered at sub-therapeutic levels, antibiotics are not at a concentration high enough to inhibit disease causing bacteria, but rather by selection pressure, this leads to emergence of antibiotic resistance as well as spread of resistance among bacterial population.

Selective pressure leads to development of resistance because bacteria are capable of changing their genetic make-up in adverse physiological conditions to adjust to new environment. Antibiotic resistance genes then spread to other bacteria through all known means of genetic exchange most commonly by conjugation, naked uptake and mobile genetic elements.

Antibiotic resistance bacteria then find their way to the environment, when manure from farms using antibiotics as growth promoters is applied to agricultural land. Once in the soil, antibiotic resistance bacteria exchange their genetic information with native microflora including pathogens.

From such agricultural land these antibiotics and antibiotic resistance bacteria find their way to nearby streams or rivers by leaching or through surface runoff. Antibiotic resistance bacteria then find their way to food chain, posing a grave threat to human and animal health.

Development of antibiotics resistance bacteria is a matter of grave concern from both human as well as animal health point of view. Diseases caused by antibiotic resistance bacteria are difficult to treat because of longer diagnosis time and high-treatment costs.

Besides cross-resistance, microbes which cause diseases in humans are also acquiring resistance genes from these environmental gene pools of bacteria. Infections caused by these antibiotic resistant microbes are already on the rise in human beings. Hospital-borne multiple drug resistant Staphylococcus aureus infection is now very common. Methicillin-resistant Staphylococcus aureus (MRSA) infection is very difficult to treat. Similarly emergence of vancomycin-resistant Enterococci is also a cause for concern among scientific communities as vancomycin is the last known antibiotic to treat infections by Enterococcus faecalis.

Besides human pathogens, many disease-causing microbial pathogens have acquired resistance genes. Emergence of antibiotic resistance bacteria that infect human directly or by transfer of resistant genes to human pathogens is a major concern across the globe, with scientists wondering whether antibiotics should be continued in animals. It is important therefore to have discussions among medical and veterinary practitioners on the proper use of antibiotics in animals as well as humans.