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It is hard for us to imagine now how the emergence of antibiotic drugs such as penicillin have changed our lives over the last 50-60 years.
In the 1930s, a typical hospital ward would be full of patients with pneumonia, meningitis, typhoid fever, syphilis, tuberculosis and rheumatic fever. All are caused by bacterial infections and the patients could expect to die, often very quickly, from such infections - many at an early age.
The discovery and use of the myriad of antibiotics1 now available has changed all this and a typical hospital ward now will be full of patients of old age suffering from the big killers of today, cancer, heart disease and complications of hypertension (high blood pressure) or diabetes.
It should be noted that both pneumonia and meningitis have viral forms that remain fairly untreatable by drug therapy - in the case of meningitis the bacterial form meningococcal meningitis is much the most serious form, however.
The Discovery and Use of Antibacterial Agents
The earliest recorded use of an antimicrobial agent is the use of extracts of cinchona bark2 to treat malaria in Peru. Mercury, which was used to treat syphilis, was the first substance not of plant origin to be used as a medical drug. These were the only therapeutic drugs largely available until the beginning of the 20th Century. Their effectiveness was very limited and mercury of course is now known to be toxic.
The discovery of the first antibiotic drug is attributed to Ehrlich. In 1909, as a result of research into dyes and differential staining of various tissues he discovered salvarsan, an arsenic-based drug used to treat syphilis. Work on dyes as pharmaceutical agents continued leading to the discovery of prontosil rubrum3 in 1935. This year saw the effective treatment of bacterial infections begin.
In 1928, Sir Alexander Fleming had noted in St Mary's Hospital in London the partial death of colonies of a bacterium called Staphylococcus aureus on a petri dish that had been contaminated by the mould Penicillium notatum. He called the active agent penicillin but did not pursue his discovery as:
The trouble of making it seemed not worthwhile.
However, in 1940, a group of chemists in Oxford demonstrated the unprecedented potency of penicillin against bacterial infections. A huge scientific project of unprecedented scale was undertaken in both the UK and USA4 to discover the chemical structure of penicillin and then to make it on a bulk scale.
Today, many of the bacterial diseases that used to afflict us have been eradicated or are at least routinely treated. The array of antibiotics now available to us is huge. There are now 50 penicillins in clinical use along with 70 cephalosporins, 12 tetracyclines, eight aminoglycosides, one monobactam, three carbapenems, nine macrolides, two streptogramins and three dihydrofolate reductase inhibitors (see below).
How Do Antibiotics Work?
Typically, antibiotics are molecules produced naturally5 by other organisms to combat the threat to themselves of bacteria, some are even made by bacteria to kill other, competing bacteria. They work by inhibiting some essential function of the bacterial cell's life cycle. Bacteria have a very different type of cell to higher organisms6 and so often antibiotic drugs are lethal to the bacterium but have no effect on us at all. This makes some antibiotics very safe drugs for us to use indeed. To see how the different antibiotics work it is simplest to list some of the different methods of operation.
Cell wall synthesis inhibitors - All bacteria possess a cell wall called peptidoglycan that is both essential for their survival and is unique to bacteria. Any drug that interferes with the biosynthesis of this structure will be lethal to bacteria and is unlikely to cause any damage to the patient. Cell wall synthesis inhibitors include
- Vancomycin (an aminoglycoside antibiotic)
DNA gyrase inhibitors - DNA gyrase is an enzyme that twists or untwists DNA strands. Even in the small genome of bacteria, if the DNA was stretched out to its full length, it would be several milimetres long - clearly much longer than a typical cell. The cell therefore requires enzymes to twist DNA into bundles so that it can compactly fit inside the cell. If this process is prevented then the cell cannot translate its DNA into proteins and so therefore dies. DNA gyrase inhibitors include:
- Quinolones (eg, ciprofloxacim)
Protein synthesis inhibitors - These drugs act to prevent the part of the cell that makes proteins (the ribosome) from working. Protein synthesis inhibitors include
- Tetracyclines (eg oxytetracycline)
- Erythromycin (a macrolide)
- Gentamycin (another aminoglycoside)
Folic acid metabolism inhibitors - In order for bacterial cells to make some essential amino acids and the bases of DNA they need to make a molecule called tetrahydrofolate (THFA)7. This molecule is made by turning a molecule called PABA (para-amino benzoate) into dihydrofolate (DHFA) and then into the required THFA using an enzyme called dihydrofolate reductase. There is a class of antibiotics that can inhibit either of these two biosynthetic steps and they include:
- Trimethoprim (a dihydrofolate reductase inhibitor)
- Sulphonamides (eg, prontosil rubrum)
It is dangerous, however, to become complacent over the successful treatment of these infections. Bacteria have become resistant to many of these agents and the situation is growing serious...
The Growth of Bacterial Drug Resistance
Why Do they Become Resistant?
Bacteria, like all other life forms, are subject to the driving force of evolution, of natural selection.
Imagine being on a course of antibiotics. If one takes a dose of penicillin, it will typically kill a high percentage of the bacterial colony in the first dose. The bacteria left are naturally just that little bit harder to kill with penicillin than those that have already died. If no further doses are taken, then the colony will re-grow and all the colony will now be slightly resistant. If this is repeated through many generations of bacteria the resistance can become very high indeed. A so-called 'selective pressure' on the bacteria to be resistant to penicillin has been introduced.
Such selective pressure on higher forms of life can take thousands of years to have an effect but with bacteria it happens much more quickly. Humans typically reproduce every 25 years or so but under ideal conditions, bacteria reproduce every 20-30 minutes. Consequently, drug resistance in bacterial colonies can develop very quickly.
In addition, bacteria possess the ability to swap some of their DNA with others, even with those of other species, and so a drug resistance gene in say Escherichia coli (E. coli) can be introduced into another species such as Bacillus or Streptococcus very easily. The method by which this is done is not known for sure but it is known that bacteria possess some non-genomal circular DNA strands called plasmids. Now bacteria have been observed to give off small blobs of their cell membrane and these form small compartments that can contain material usually found inside the cell. It is postulated that these so-called 'blebs' could be absorbed by another bacterium, and should the bleb contain a plasmid bearing a drug-resistance gene, then drug resistance has been passed from one bacterium to another. This swapping of DNA fragments is also suspected to increase the rate of mutation of the DNA within bacteria. This could also accelerate the rate at which drug resistance is attained.
The Emergence and Spread of Drug Resistance
The ability of bacteria to adapt to a hostile environment has been known about for a long time. The first report of bacterial drug resistance was in 1887. A scientist called Kossiakoff described the acclimatisation of Bacillus subtilis to mercuric chloride and boric acid when the organism was grown in a medium containing these chemicals.
While working on penicillin, Florey and Chain in Oxford noted that some strains of E coli produced a 'penicillinase' enzyme that inactivated the drug. This 'penicillinase', quickly renamed 'ß-lactamase'8 was soon to spread to other organisms.
In 1941, all strains of Staphylococcus aureus - the most common post-operative infection in hospitals - were susceptible to penicillin V. By 1944, some strains of S aureus were capable of destroying Penicillin V by means of ß-lactamase. Today, within hospitals and other medical facilities, in excess of 95% of S aureus is resistant to penicillin and all the other ß-lactam antibiotics. The response of the pharmaceutical industry was to produce a ß-lactamase resistant penicillin called methicillin. By the early 1960s, resistance to methicillin also began to emerge. This was the first emergence of the so-called 'superbug' MRSA (methicillin resistant Staphylococcus aureus). Today MRSA is resistant to all ß-lactam antibiotics and in addition some strains have been reported to be resistant to erythromycin (the antibiotic of choice for tuberculosis), fusidic acid, tetracycline, monocycline, streptomycin, sulphonamides, disinfectants and toxic metals such as mercury and cadmium.
Epidemic MRSA infections within hospitals are now endemic and the situation has become so serious that only one antibiotic - the glycopeptide vancomycin - is capable of destroying this bug. Vancomycin is the so-called antibiotic of last resort and in some countries has been withdrawn from use for any other purpose than for the treatment of MRSA infections.
In the last couple of years, the nightmare scenario of a totally untreatable bacterium has been discovered. Vancomycin resistance has emerged in some strains of MRSA. Thankfully, at the moment VRSA (vancomycin resistant S aureus) is quite rare but undoubtedly it will spread.
These infections are normally not too serious as S aureus is not a very dangerous bacterium in healthy people but in the sick and those who are recovering from surgery it can be serious. Also, since so much of the organsism's energy is devoted to being drug resistant it tends to be slow growing so there are less subtle ways of dealing with it. Surgery to cut the infection out has been used, along with the gut-wrenching use of fly larvae to simply eat it out.
S aureus is of course not the only bacterium to have gained drug resistance; it is merely the most famous. Drug resistant tuberculosis is also well known and this is indeed deadly. Some patients in New York have recently (at the time of writing) been effectively imprisoned in order to ensure they complete their course of antibiotics and limit the spread of drug resistance in this lethal bacterium. Streptococcus pneumoniae - responsible for many throat and lung infections - has also been reported to be gaining drug resistance and again there are more.
Methods of Resistance
There are a variety of methods bacteria use to prevent an antibiotic from destroying them:
Destroy the drug - It has already been mentioned that many drug resistant bacteria produce an enzyme called ß-lactamase. This destroys the ß-lactam ring structure of penicillin-like antibiotics and hence destroys their toxicity toward bacteria. The original ß-lactamase enzymes found were quite poor at this and the pharmaceutical industry found it easy to slightly modify the structure of penicillins to get around this problem. Unfortunately, today, the ß-lactamase enzymes have evolved to be highly efficient at destroying any ß-lactam antibiotic and are generally faster at adapting to new drugs than we are at making them.
Remove the drug - All cells possess pumping mechanisms to pump essential chemicals across the cell membrane into the cell and to pump toxic or waste materials out of the cell. Many drug resistant species of bacteria have simply evolved a response to a toxic antibiotic environment. Upon detection of an antibiotic they simply pump it all out of the cell again before it can do irreparable damage. This is how most drug resistance appears to take place and it explains why toxic metals such as palladium and mercury can be ineffective.
Change cell structure - Perhaps the most ingenious method and the most disheartening to scientists trying to deal with this problem. This has been observed with drug resistance to the last resort drug vancomycin. Vancomycin acts at the final step of the cell wall biosynthesis. It binds to two specific amino acids on the cell wall precursor therefore blocking them from performing the final step in this process. To get around this, the bacteria have simply changed the identity of one of these amino acids that bind vancomycin - this prevents vancomycin from binding to the cell wall precursor and renders it inoperative.
This method effectively means the bacteria have changed their cell wall structure - a pretty major change in itself - and also have changed the biosynthetic route to the cell wall. This is something no one expected to see and makes it pretty clear that no matter what drug we come up with, bacteria will always find a way around it.
So what Can We Do about This?
There are a number of ways around the problem of drug resistance. Some have been already been applied and used with a mixed level of success, while some are still in the realms of theory. Just a few of the possibilities are mentioned here:
Limit the use of antibiotics - In the past antibiotics were overused and this is one of the causes of the rapid spread of drug resistance. They were formerly given routinely in hospitals mainly to prevent the outbreak of dangerous infections in already sick patients. Also, in farming, some antibiotics were put into animal feed to again prevent disease among animals. The drawbacks have clearly outweighed the benefits on this issue and so the overuse of antibiotics is now frowned upon. There is a growing trend to use them only when a clear bacterial infection has taken place and this should slow the spread of resistance.
Development of new drugs - The oldest of the methods and the most often used. If bacteria become resistant to our current array of antibiotics, simply make some more. The development of methicillin to get around ß-lacatamase-based resistance is the most famous case of this.
Unfortunately, this is more easily said than done. To develop and make a new drug is an expensive and long drawn out process as well as being of high financial risk. A pharmaceutical company typically has to put a drug through 5-10 years of trials before it is given a safety certificate to go on the market. A drug can fail at any stage of this process and more importantly, drug resistance now tends to develop more quickly than this. Bacteria can be resistant to a drug before it is available on the market.
Pharmaceutical companies are still researching into new antibiotics, however. The financial rewards of finding a new 'penicillin' with no resistance yet in place are potentially enormous.
Combination therapy - Another method that has already been successfully employed - although again, bacteria have found ways around it. There are a class of molecules known that prevent the action of ß-lactamase enzymes. If such a drug (clavulanic acid is a well known example of this type of drug - a ß-lactamase inhibitor) is used in combination with a penicillin then the ß-lactamase inhibitor stops the ß-lactamase enzyme from destroying the penicillin which can then get on with its job of killing the bacterium freely. Unfortunately, some bacteria can simply pump out clavulanic acid and this means that the penicillin may be destroyed. Other combination therapies may also be of use however.
Let some antibiotics lie 'fallow' - It has been observed that when the selective pressure to become drug resistant is removed, some bacteria will lose the DNA that leads to drug resistance. In other words they return to a more native state. It has therefore been proposed that it might be possible to use a rotating regime of antibiotics. Some antibiotics will be used for a period of time while others are not used at all. Hopefully, the bacteria would become resistant to those in use but would lose their resistance to those not in use. One could then switch the therapy and the situation would reverse, the bacteria would gain resistance to the new drug but would lose it to the old one. The cycle could then start again. This is analogous to the old three-field system in farming.
While possible in theory, this has yet to be tested in practice. Also, it is known that under certain conditions, bacteria can form spores and these spores can lie dormant for thousands to millions of years. Bacillus 2,9,3 is an example illustrating this. This allows the possibility that a bacterium that is multi-drug resistant could resurface at any time, rendering this method useless.
Use of bacteriophages - Bacteriophages are viruses that specifically attack bacteria. It is proposed to use these as a way of killing drug resistant bacteria. These viruses have probably been around for as long as bacteria (billions of years) and so are excellent at their exploitation of them. Bacteriophages are specialised in invading bacterial cells only and so cannot affect our own cells. This is potentially, therefore, an exceptionally safe therapy for us.
One potential problem with this idea is that bacteriophages may trigger an immune response in us even though they are not pathogenic. It is possible that our own defences might work against us and kill these viruses before they can destroy the target bacteria.
Although this all looks rather scary, one should keep it all in perspective. Drug resistant bacteria are only generally found in places where they are constantly attacked by disinfectants and antibiotics. Hospitals, old people's homes, hospices etc, are the places you will find them. Outside these places the drug resistant bacteria tend to be disadvantaged; they devote so much of their energy to drug resistance that they are slow-growing and so are swamped out of the bacterial population by wild type bacteria. Penicillin is therefore still a perfectly viable drug for your ordinary patient outside of a hospital.
However, the problem may eventually spread outside hospitals so it is up to us all to help prevent this by treating antibiotics with respect and not to misuse or abuse them. Doctors, at least in the UK, will no longer prescribe antibiotics for just any old infection. You will not get a course if you have a cold or the flu, as they are viral infections and so will not be affected by antibacterial drugs. It is important to completely kill the bacterial infection to prevent the regrowth of a drug resistant strain, so if one is given a course of antibiotics complete the course! This cannot be stressed enough. Not only is there a chance of becoming sick again if the full course of antibiotics is not taken, but it also contributes to the growth of drug resistance in bacteria.