The Theory behind the Spread of Resistant Strains of Bacteria
The rapid spread of the resistant forms of bacteria relies is a perfect example of the natural selection theory that was pioneered by Darwin. This theory states that those organisms adapted to the surrounding environment the best (both in feeding habits and body form, such as having thick fur in a cold place or being camouflaged correctly for a particular background) will survive to reproduce whilst those less well adapted die before having a chance to mate and reproduce. In this way genes that increase the likelihood of survival and so reproduction increase in frequency and so become better established in the gene pool from generation to generation. This process happens in all species but takes place particularly quickly in bacteria.
Bacteria multiply by a process known as binary fission. This is when a single celled organism divides into two smaller ‘daughter’ cells. It is a form of asexual reproduction and can take place in bacteria as regularly as every twenty minutes. Therefore if only one in a large colony of bacteria is resistant to the drug, then all the others will be destroyed leaving the single bacterium to multiply and produce a completely new strain that will no longer respond to antibiotic treatment. The over use of antibiotics has meant that more and more bacteria are mutating and becoming resistant to a wider range of limited antibiotics.
The Economic Reasons behind the Problem
Traditionally, Britain has been a farming nation with large part of its economy being drawn from this source. During the Second World War, when import channels from the colonies but also from other places such as those from America and Europe were greatly reduced due to attack from the Germans, the British relied heavily on what farmers could produce for the entire population. The government realised that were the country not to starve the efficiency of home agriculture must be bettered. It became imperative to increase return to the maximum point. To this end measures were taken to boost the fertility of the soil (use of fertilizer) and the yield of the livestock. It was at this point that intensive farming became common practice. Before this animals were given plenty of space and allowed to roam free in large fields, foraging for food themselves. However the land would not be able to support the numbers of animals being introduced so new techniques were adopted. Now animals were kept in close confinement and fed at regular intervals. Traditionally chickens had been allowed to roam over the farmyard but this limited egg production and it was found to be far more economically sound to keep them in batteries. More eggs were produced and less manpower was needed to gather them. However, this collection of large numbers of farm animals in one place for long periods of time had its disadvantages. Disease was able to spread quickly throughout the herd so farmers needed a way of controlling this.
In the 1950’s the discovery was made that animals could be made to grow more quickly if antibiotics were fed to them. This method had the advantage of helping to control the rising spread of disease and so seemed like a perfect option. The increase in growth can be explained in the following way: all mammals harbour bacteria in their gut. Just like any organism, bacteria needs nutrients and in the gut the only way it can get them is to be a competitor for nutrients that would otherwise go to the host. In a bacteria-free environment (such as was developed in the animals that were fed the antibiotics every day) all nutrients are available to the animal (host) and so it is able to grow faster. The theory also exists that the presence of the antibiotics in the bloodstream of the animal prevents any number of “sub-clinical” diseases. It is thought that these diseases may cause periods of malnutrition absorption and so stunted growth will arise. Amyes, SGB. (2001) Magic Bullets, Lost Horizons. The Rise and Fall of Antibiotics. Taylor and Francis, London. At this point there was no restriction on which antibiotics should be used for intensive farming and so all were tried and most were found to be effective, with the concentrations being used increasing as the benefits were realised.
The Problems Caused by the Over-Use of Antimicrobial Drugs in Farming
The extent of the problem was demonstrated very effectively when Danish farmers undertook a voluntary ban on the use of growth promoter drugs such as avoparcin in chickens and pigs. They cut their use of antibiotics by 54% overall and the antibiotic resistance in the animals concerned dropped by 90%. In this experiment the only factor altered was the halting of the use of growth promoters. Antibiotic use for specific infections was not observed to rise and neither did the levels of animal disease. The cost incurred by the farmers in the production of the chickens did not change and increased by just 1% for the production of pigs. MacKenzie, D. (13th August 2003) Antibiotic ban cuts drug resistant bugs. New Scientist, http://www.newscientist.com/article.ns?id=dn4054
Another example of the gain to the human population by the removal of antibiotics from vetereinary practice can be seen in the ban of the use of avoparcin. This antibiotic awas used as a growth enhancer mostly in cattle, chickens and pigs. Shown in figure I is the structure of vancomycin. This is a bacteriocidal drug used in humans to eliminate the Enterococci bacteria. However, as you can see below the structure of avoparcin is very similar to that of vancomycin. This means that when the bacteria residing in the gut of cattle, chickens and pigs become resistant to avoparcin they also become resirtant to vancomyicn giving rise to a new form of the enterococci bacteria called vancomycin-resistant enterococci (VRE). This is particularly serious as this strain can be passed across the species divide to humans in two main ways – handling the animals and eating their meat. In order to restrict the spread of VRE the use of vancomycin-like drugs must be kept to a minimum, hence the ban of avoparcin. Hawkes, N. (1998) Antibiotics in Catlle Feed Linked to Superbug. The Times of London (UK), http://www.mindfully.org/Farm/Antibiotics-Cattle-Superbug24mar98html; Anonymous (1997) Vancomycin-resistant enterococci (VRE). Association of Medical Microbiologists,
Figure I
Figure II
Bibliography
-
Amyes, SGB. (2001) Magic Bullets, Lost Horizons. The Rise and Fall of Antibiotics. Taylor and Francis, London
-
Campbell and Reece. (2005) Biology, Seventh Edition. Pearson Education, Inc.Californis, USA
-
Wessels and Hopson (1988) Biology. Random House, USA
-
Standing Medical Advisory Commitee (1998) The Path of Least Resistance. The Department of Health, London
-
Hine, RS and Martin, E. (2004) Oxford Dictionary of Biology. Oxford University Press, Oxford
-
Anonymous (1997) Vancomycin-resistant enterococci (VRE). Association of Medical Microbiologists,
-
Hawkes, N. (1998) Antibiotics in Catlle Feed Linked to Superbug. The Times of London (UK), ;
-
MacKenzie, D. (13th August 2003) Antibiotic ban cuts drug resistant bugs. New Scientist, http://www.newscientist.com/article.ns?id=dn4054
