Darwin and the Antibacterial Arms Race: antibiotics versus bacteria
Is there evidence of evolution by natural selection today? Where should we look for it? Which living creature should we consider? There are numerous problems in this quest.
For example, evolution takes place relatively slowly, covering many generations. To monitor evolution in large animals would take much too long. It would also be difficult to decide on what parameter to study in complex ‘higher’ animals such as mammals. There are breeding programmes in dogs and cats, but these are conducted with specific aims in mind and do not represent a truly ‘natural process’.
Bacteria on the other hand do offer opportunities. They are tiny – over a million can be present on the tip of a needle. They reproduce quickly, with generation times of 20 minutes under ideal conditions. They can also be grown readily in the laboratory, as seen in the figure below, which shows bacteria growing on a jelly-like material called agar. This agar is red because it also contains blood to help the bacteria grow.
Bacteria have another major advantage, their genetic material (DNA) is in two locations, in the nucleus and as extra-nuclear circular bits of DNA termed plasmids. This means two locations for possible mutations. Bacteria also can exchange DNA between cells enabling rapid transmission of genetic material.
Bacteria, therefore are suitable organisms for our quest, but what property/ies should we look for?
Arms races offer a prime opportunity for evolution. During arms races, technology can advance rapidly. Witness the development of ballistic missiles, anti-ballistic missiles, multiple independent warheads during the ‘cold war’. However the cost of an arms race is considerable and the benefit achieved must be great for it to be maintained. Survival itself is a great benefit.
The battle between mankind and bacteria in the control of bacterial infections is a classic arms race. Mankind develops antibiotics to control infectious disease and the bacteria can develop defence mechanisms to enable their survival, the anti-bacterial arms race.
A bacterium called Staphylococcus aureus is a good candidate for study. It is carried by around 30% of the population usually in the nose and it causes a wide variety of infections ranging from pimples and boils to pneumonia and septicaemia. In simplistic terms it is born troublemaker, which causes a lot of morbidity and mortality in humans. Man has waged war against Staphylococcus aureus for decades. The hallmarks of osteomyelitis, an infection of the bone caused by Staphylococcus aureus, have been found in skeletons of people who died hundreds of years ago.
A significant progress in the battle against this organism and the infections it causes was the development of penicillin. Penicillin was discovered by chance by Professor Alexander Fleming in his laboratory at a London hospital (see figure below).
Penicillin was introduced in the early 1940s for clinical use and its impact was dramatic, particularly after mass production became possible. Patients who would have died in the pre-penicillin era now survived and others who suffered from debilitating chronic infection such as boils and carbuncles had their lives transformed. So began the golden era of antibacterial chemotherapy.
However, it wasn’t long before some strains of Staphylococcus aureus began to appear which were not killed by penicillin. These strains became rampant throughout hospitals in the 1950s and 1960s, causing closure of hospital wards. A return to the pre-penicillin era was on the cards. These strains – called penicillin resistant Staphylococcus aureus – are now so numerous that they constitute the most common type of Staphylococcus aureus found today.
What had happened to produce this phenomenon?
Penicillins, and a closely related group of antibiotics called cephalosporins, kill bacteria by interfering with the metabolism of the bacterial cell wall.
They mimic one of the building blocks of the cell wall, so inhibiting the function of cell-wall enzymes (termed Penicillin-Binding Proteins or PBPs for short). It soon became apparent that the ‘resistant bacteria’ had developed an enzyme which was capable of breaking part of the penicillin structure, making it inactive (see figure 1). This enzyme, which was for obvious reasons given the name of penicillinase, was found to be present on extra-chromosomal DNA (on a plasmid) which could be transferred readily between bacteria, enabling its rapid spread. The bacteria had fought back!
Pharmaceutical companies, after isolating the main component of penicillin (and cephalosporins), added new side chains and developed new agents, which were stable to the action of penicillinase. The first of these was a penicillin called methicillin. This and other agents which followed restored the ability to control infections caused by Staphylococcus aureus and remained the mainstay of antibiotic therapy against this bacterium for 40 years. A few strains did appear resistant to the newer penicillins and cephalosporins but were not a major problem in most countries. Such strains were given the name of ‘methicillin resistant Staphylococcus aureus’ abbreviated to MRSA. Over the past decade this bacterium increased in frequency and has become the scourge of hospitals.
What had happened to produce this phenomenon?
There had been a small but highly significant change in the gene (DNA) for one of the PBPs to which all penicillins bind to inhibit bacterial cell wall metabolism. This had occurred in PBP 2.
DNA is primarily composed of four different ‘chemicals’, which are referred to by the abbreviations A, T, G, and C. A single mutation, whereby a C had been replaced by T, resulted in the incorporation of the amino acid leucine instead of proline at position 458 in the long chain of amino acids which make up the PBP enzyme. This resultant ‘new PBP’, termed PBP2a, was not inhibited by methicillin and other penicillins and cephalosporins. Strains of Staphylococcus aureus carrying this mutation were resistant (not killed) to these antibiotics. Their survival would be encouraged in any environment where these antibiotics were being widely used, for example in the hospital. Natural selection dictated that they would become the dominant type of Staphylococcus aureus.
Staphylococcus aureus has over a period of 50 years adapted to changes in its environment, caused by the introduction of antibiotics. There have been small but significant changes to those aspects of its metabolism which were the targets of penicillins and cephalosporins. The most significant of these being the appearance of PBP2a which enables it to survive and multiply in the presence of these agents.
Random mutation in the DNA coding for PBP2, resulted in PBP2a with different properties giving enhanced survival in an environment where penicillins and cephalosporins are widely used, resulting in the ‘survival of the fittest’.
Staphylococcus aureus’ has demonstrated the basic tenet of evolution by natural selection as it ‘responds’ to mankind’s attempt to control it. It is a clear example of Darwin’s theory of evolution by natural selection.
Fortunately, Staphylococcus aureus’ has not won the arms race. There are other antibiotics in addition to penicillins which are effective against MRSA. There are even some new penicillins which are active against MRSA. However, this bacterium has shown an alarming ability to mutate and acquire resistance to every antibiotic ever developed to combat it. It will no doubt continue to do so and there will be a constant need for new agents to control it.
Survival of the fittest is still the prize.