IT IS October. A 43-year-old policeman from Oxford arrives in hospital with a
stubborn sore at the corner of his mouth. Four months later the infection has
engulfed his whole face, lungs and the bones in his right arm. Doctors know that
unless they try the new drug he will die.
So they give it, with dramatic results. For four weeks the patient makes a
rapid recovery鈥攗ntil the doctors run out of the drug. On 15 March, he
dies.
The year in which these events took place was 1941, the infection was caused
by Staphylococcus aureus, and the new drug was penicillin. The
policeman was the first person in Europe reported to have received penicillin.
And although the drug arrived too late to save him, for millions of others since
it has been a godsend. For fifty years, in developed countries at least,
antibiotics have given doctors virtually complete control over bacteria. But not
any more.
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Today, bacteria have evolved to resist antibiotics. The full impact of this
counterattack is only now becoming clear, warns Stuart Levy, director of the
Center for Adaptation Genetics and Drug Resistance at Tufts University in
Boston. 鈥淢ultidrug-resistant bacteria are emerging and increasing in numbers,鈥�
he says. 鈥淧atients are failing in therapy and, in some cases, dying. That would
have been unheard of five or ten years ago.鈥�
The rise of bacteria that can cope with antibiotics is 鈥淒arwinism at its
best鈥�, Levy says. The big question is, what can be done to stem this tide of
untreatable infection? Some physicians have increased calls for tighter
restrictions on the prescribing of antibiotics, and pharmaceuticals companies
have redoubled their efforts to find new ones. But these approaches are likely
to have only a limited effect on the spread of antibiotic resistance.
In the past few years, another way to beat this resistance has been gaining
ground. Rather than scrap all the old antibiotics that bacteria can resist,
scientists are trying to revitalise them. They have found that by assigning a
molecular 鈥渂odyguard鈥� to an old antibiotic, they can give it a new lease of
life. This is an approach that can be adapted as new forms of resistance emerge,
so the same drug can be used indefinitely. According to Julian Davies of the
University of British Columbia, Vancouver, this 鈥渄ual agent鈥� approach offers an
opportunity to slow down or even stop the development of antibiotic
resistance.
No room for complacency
There is no doubt that resistance is soaring. Take Streptococcus
pneumoniae, which causes a range of diseases. According to the Centers for
Disease Control and Prevention in Atlanta, every year in the US the bacterium is
responsible for at least 7 million cases of glue ear, 500 000 cases of pneumonia
and 3000 cases of meningitis. Last year, when Jo Hofmann and colleagues at the
CDC took samples from people in Atlanta with serious pneumonia, they found that
25 per cent harboured strains of S. pneumoniae that were resistant to
penicillin. Less than a decade ago the national figure for resistant strains was
0.02 per cent.
A similar trend is found in Britain, according to Alan Johnson and his
colleagues at the Central Public Health Laboratory in northwest London. During
two weeks in March 1990 and again in March 1995, they collected all samples of
S. pneumoniae sent to public health laboratories in England and Wales.
In 1990, 1.5 per cent of these isolates were resistant to penicillin. By 1995,
that figure had risen to nearly 4 per cent. Although the situation is not as bad
as in the US, says Johnson, Britain has nothing to be complacent about. Many
microbiologists believe Britain faces a crisis just as great. And other
countries have a resistance problem worse even than that in the the US.
Both the American and British research found that many other drugs are also
threatened. For example, the American study discovered that 15 per cent of
S. pneumoniae samples were resistant to another widely used antibiotic,
erythromycin. In England and Wales, erythromycin-resistance was found in 8.6 per
cent of samples.
One response to the rapid growth of resistance has been demands for stricter
control over the use of antibiotics. The hope is that if bacteria are exposed to
lower amounts of antibiotics, resistance to them will develop more slowly (see
Box).
In the US, for example, antibiotics can be bought over the counter. Mitchell
Cohen, director of the division of bacterial diseases at the CDC, believes they
should be available only on prescription. And even then, doctors need to think
about how they prescribe. For example, when patients come in with 鈥渇lu
symptoms鈥�, doctors often prescribe antibiotics because there is a 3 per cent
chance that they have pneumonia. 鈥淭he physician鈥檚 first responsibility is
towards the patient,鈥� says Cohen. But in some cases, doctors should put the
health of society first. 鈥淒octors need the strength to say they won鈥檛 prescribe
unnecessary antibiotics,鈥� says Cohen.
This tactic may sound straightforward but Johnson points out that it is not
always practical for GPs to send swabs to a laboratory to find out if an
antibiotic is appropriate. If they did, he says, some patients would be 鈥渂etter
or dead鈥� before the results came back. Levy reckons the answer is to develop
faster ways to diagnose different infections.
Another response to resistance is to create entirely new antibiotics to
replace those that bacteria have conquered. Several pharmaceuticals companies
have new antibiotics in the pipeline. One of the nearest to market is a class of
antibiotics called the oxazolidinones, which are being developed by Upjohn. 鈥淲e
believe oxazolidinones interfere with protein synthesis before the bacterial
cell starts to divide, thus stopping the bacteria from producing proteins
essential to its replication,鈥� says Charles Ford of Upjohn.
In laboratory studies, the first of the oxazolidinones has killed strains of
Staphylococcus and Enterococcus that are resistant to other
antibiotics. Human trials have now started, and Ford believes the drug will go
on sale in 1998. But even if the compound makes it over the licensing hurdles,
experience tells us that bacteria will inevitably learn to live with it.
One of the big problems with searching for new antibiotics is that the
relative simplicity of bacteria means there are only a limited number of known
targets for antibiotics to hit. These targets must not be shared by humans, or
the drug could be toxic, and ideally they should be targets that other
antibiotics do not already hit鈥攐therwise resistance to this method of
destruction may already be developing. With all these constraints, it makes
sense not to throw away antibiotics that bacteria have learnt to live with. If
only there was a way to counteract the resistance mechanisms that bacteria have
evolved.
Indefinite lifetime
This is exactly the idea behind dual agents. Bacteria use well-understood
mechanisms to evade antibiotics, says Davies. Some produce chemicals that
destroy or inactivate antibiotics, while others have developed molecular pumps
that flush the drugs out of their cells. The idea behind dual agents is to give
an antibiotic a bodyguard that will keep the bacteria busy while the antibiotic
kills them. If bacteria eventually develop ways to overpower the bodyguard, as
seems certain, researchers can simply fashion a new one to take its place. The
antibiotic, however, stays the same.
The best example of a dual agent is a drug called Augmentin, made by
SmithKline Beecham (SKB). It contains the antibiotic amoxycillin, which is
widely used against common diseases such as pneumonia, urinary tract and dental
infections. At the heart of amoxycillin is a chemical structure called a
&bgr;-lactam ring, which interferes with cell wall production in bacteria, making
the wall weak and prone to bursting.
This mode of destruction is not unique to amoxycillin: all
penicillins鈥攐f which amoxycillin is just one鈥攈ave &bgr;-lactam rings. In
fact, by the early 1970s, when amoxycillin was launched, resistance to the ring
had already emerged as a clinical problem. Many species of bacteria had started
to produce enzymes known as &bgr;-lactamases, which hydrolyse the &bgr;-lactam ring and
draw its sting (see
Diagram
).
Researchers at Beecham鈥攐ne of the companies that later merged to form
SKB鈥攂egan searching for a compound to give in combination with amoxycillin
to counteract the &bgr;-lactamases. Eventually, they identified clavulanic acid, a
molecule that has all the features of amoxycillin recognised by &bgr;-lactamase.
When given with amoxycillin, clavulanic acid forms an irreversible bond with
&bgr;-lactamase, leaving amoxycillin free to do its worst. Augmentin is a better
antibiotic than amoxycillin ever was: it is active against a wider range of
bacteria, and clavulanic acid has even been found to have some antibiotic effect
of its own.
Augmentin鈥檚 success led to a handful of 鈥渃opycat鈥� drugs that contain a
penicillin in combination with a bodyguard to combat &bgr;-lactamase. In 1987,
Pfizer launched a combination of ampicillin and a molecular bodyguard called
sulbactam, while in 1993 Lederle launched a cocktail containing piperacillin and
its protector, tazobactam. Levy says Beecham deserves credit for going out on a
limb with the dual-agent approach. It was a totally new idea and a big gamble,
he says. Other companies have been too cautious. 鈥淭hey don鈥檛 seem to like the
idea of using two drugs at once,鈥� he says. This may account for their apparent
reluctance to look for molecular bodyguards for drugs other than
penicillins.
Levy鈥檚 own work on bacteria that are resistant to the antibiotic tetracycline
was, until very recently, one of the rare exceptions. Last year he described how
bacteria use 鈥渆fflux pumps鈥� to defeat tetracycline. These are molecules that
straddle the cell membrane and literally pump tetracycline out of the cell
before it has a chance to do any damage. Since the discovery, Mark Nelson at
Tufts has been designing molecules that bind to the pumps and block them,
allowing tetracycline to kill the cell. In the test tube, tetracycline in
combination with pump blockers killed resistant strains of Escherichia
coli, Staphylococcus and Enterococcus. 鈥淲e are about to
start studies on animals and have new and better inhibitors,鈥� he says.
Last line of defence
Since Levy discovered the efflux pump, other types of pump have been found
behind bacterial resistance to important classes of antibiotics including the
macrolides鈥攕uch as erythromycin鈥攁nd quinolines, which include
ciprofloxacin. At Wayne State University in Detroit, Michigan, Shahriar
Mobashery has focused on another class, the aminoglycosides. Besides being
powerful antibiotics, the aminoglycosides can cause kidney and nerve damage, so
doctors usually hold them in reserve, as a last line of defence against
life-threatening infections. Nevertheless, resistance to aminoglycosides such as
kanamycin A has started to emerge.
Kanamycin binds to ribosomes, the organelles in bacteria that produce
proteins, and stops them working. Mobashery has concentrated on an enzyme,
phosphotransferase, which adds phosphoryl groups to kanamycin, preventing it
from locking on to ribosomes. He has found several aspiring bodyguards based on
kanamycin, that mop up the phosphoryl groups and stop phosphotransferase in its
tracks.
Dual agents seem to be coming of age at a time when researchers can
crystallise enzymes and scrutinise their three-dimensional structure in
minute detail, allowing them to customise drugs that bind to them. But that is
not to say that this approach is without problems. George Miller of the
Schering-Plough Research Institute in New Jersey, for example, points out that
bacteria have not one, but several genes that endow resistance to
aminoglycosides. Targeting one gene will simply select for the others, giving
them free rein. 鈥淏ut if all the resistance genes could be targeted then it would
be a good approach,鈥� he says. A similar problem faces Levy, since tetracycline
can be flushed from cells by a number of efflux pumps.
Another stiff test for the new approach is looming. Fifteen years after being
launched, resistance to Augmentin is emerging. A new type of &bgr;-lactamase that
carries a metal atom has appeared, and clavulanic acid does not protect against
it. Researchers at the University of Oxford have already begun searching for
molecular bodyguards that will out-smart metallo-actamases. Help may also come
from Canadian researchers who have designed a new drug, based on boronic acid,
that binds strongly to &bgr;-lactamase (快猫短视频, Science, 10 August,
p 19). Such compounds should reveal whether changing molecular bodyguards at
intervals really can work.
Davies is in no doubt. Indeed, he believes we can do more than just target
existing forms of resistance. 快猫短视频s should be able to predict the ways in
which bacteria will foil antibiotics. 鈥淪ay a pharmaceuticals company found a new
antibiotic tomorrow,鈥� he says. 鈥淭hen we could predict the resistance mechanism
and design inhibitors alongside the antibiotic. I would go and get a handful of
soil, I would expose the microbes in that soil to the antibiotic, and pick out
the ones that grow. And in a fortnight I could tell you the mechanism of
resistance that would eventually be found in the clinic.鈥� New antibiotics could
then be launched complete with a molecular bodyguard that would have resistance
beaten from the word go.
* * *
The ebb and flow of resistance
BACTERIA overcome an antibiotic by making proteins that block the drug鈥檚
action. Some of these proteins arise from mutations in genes on the bacterial
chromosome, but most are produced from genes on plasmids鈥攃ircular pieces
of DNA that are not attached to the chromosome. The importance of plasmids lies
in their ability to shuttle between bacteria of the same or different species.
In this way, they transfer resistance between bacteria. 鈥淭hey talk to each
other, they exchange genes,鈥� says Julian Davies of the University of British
Columbia, Vancouver. 鈥淢icrobes have more promiscuous ways of taking care of gene
exchange than we can possibly imagine.鈥�
Widespread use of antibiotics has created an environment weighted heavily
in favour of bacteria that carry resistance genes. To tip the balance back
towards nonresistant bacteria, many microbiologists advocate reducing the
frequency with which antibiotics are prescribed. This tactic is supported by
research which shows that the more antibiotic bacteria are exposed to, the
faster they develop resistance to it.
There is also controversial evidence that if an antibiotic is withdrawn from
use, bacteria will eventually lose their resistance to it altogether. If this is
confirmed, it could make sense to rest antibiotics periodically to flush out
resistance in bacterial populations. Numerous experiments have shown that in the
absence of an antibiotic, nonresistant bacteria thrive and resistant strains do
less well. The reason for this is unclear, but it seems likely that keeping
cells resistant requires energy, which bacteria must borrow from the energy they
would normally use in other ways to keep fit.
However, Richard Lenski from Michigan State University in East Lansing says
these experiments only look at the first few generations after bacteria have
acquired a resistance gene. His studies show that after many generations,
bacteria adapt to maximise their competitiveness while keeping their resistance
genes in place. If this is confirmed, it could mean that it will become
increasingly difficult to wipe out bacterial resistance simply by suspending
antibiotic use. Resistance that has emerged in fifty years may take millennia to
disappear, says Lenski.