Encyclopedia of Evolution.pdf - Online Reading Center

esistance, evolution of
ineffective and was replaced by fluoroquinolines, which are
now becoming ineffective also.
• Food poisoning. Forty percent of the samples of the food
poisoning bacteria of the genus Campylobacter in Finland
could resist fluoroquinolones in 1996, but 60 percent were
resistant in 1999. In America antibiotic-resistant Salmonella
was present in only 5 percent of samples in 1997 but in
10 percent of samples in 2001.
• Spinal infections. In 1996, 10.4 percent of samples of the
meningitis bacterium Streptococcus pneumoniae resisted
penicillin, but by 2001, 51.5 percent of the samples were
resistant to penicillin; resistance to macrolide antibiotics
increased from 16.5 percent to 30.0 percent during that
time.
• General infections. Over 90 percent of the strains of
“staph” (Staphylococcus aureus), a common cause of infections,
now resist penicillin and related antibiotics.
Another reason that antibiotic resistance can spread
rapidly through bacterial populations is that bacteria can
transfer pieces of DNA from one to another, and this transfer
can occur even from one bacterial species to another (see
bacteria, evolution of; horizontal gene transfer). In
eukaryotic species that are totally separate, resistance cannot
evolve until the resistance mutations occur within their separate
populations; but in bacteria, one resistant species can
donate resistant genes to another species!
This is why dozens of species of bacteria have become
resistant to one kind of antibiotic; many are resistant to more
than one. Some strains of bacteria resist all antibiotics except
vancomycin, an antibiotic rarely used because it is difficult to
administer and because of its severe side effects. But even vancomycin
resistance has occurred in bacteria. At first, it was
only in the harmless intestinal bacterium Enterococcus faecium.
When these bacteria transferred their resistance genes
to more harmful bacteria, the result was what public health
officials refer to as a superbug. This happened in 1996, when
the first case of intermediate-level vancomycin resistance was
reported in staph bacteria in Japan. As of 2002 the United
States had eight confirmed cases. The year 2002 also saw the
first case of staph bacteria that were completely resistant to
vancomycin, rather than having intermediate resistance. Six
cases have now been reported in the world.
When a bacterium resistant to penicillin (or a similar
drug such as the widely used amoxicillin) infects a person, the
physician can administer a different antibiotic, saving the life
of the patient if the second antibiotic is administered in time,
and if the bacterium does not resist it also. Because the trial of
each new antibiotic on a patient wastes precious time, a multidrug-resistant
bacterial strain threatens the patient’s life.
Because hospitals represent a fertile breeding ground
for the evolution of antibiotic resistance in bacteria, patients
often become infected with these bacteria when they are in
the hospital. As one observer noted, a hospital is a good
place to go to get sick. About two million Americans experience
nosocomial (hospital-acquired) infections each year,
and more than half of these infections resist at least one antibiotic.
Medical scientists Richard P. Wenzel and Michael B.
Edmond estimate that between 17,500 and 70,000 people die
from nosocomial infections each year in the United States.
Antibiotic resistance has also resulted from the routine
addition of antibiotics to livestock feed. Some of the Salmonella
bacteria, normally present in livestock, have evolved
resistance to the antibiotics used in the feed. In some cases,
these bacteria have spread to people and caused infections,
as a result of contact or improper food handling. The Campylobacter
mentioned previously is also a common contaminant
in supermarket meats. The use of antibiotics in livestock
feed has contributed to the evolution of resistance to some
antibiotics that may be crucial for human health. The heavy
use of the antibiotic Baytril in poultry feed encouraged the
evolution of bacteria that could resist Cipro, since the two
antibiotics are biochemically similar to one another. Cipro
is considered one of the most important antibiotics that can
be used in response to bioterrorist attacks, for example with
anthrax. The U.S. federal government banned the use of Baytril
in poultry feed in August 2005.
Antibiotic resistance can even evolve within a population
of bacteria in a single host individual. This frequently happens
during infections, such as tuberculosis, that require a
long period of antibiotic treatment. This explains why it is
often more difficult to treat a relapse than to treat the original
infection in a patient. HIV can evolve resistance to the
drugs that are used against it, even within the body of a single
host (see AIDS, evolution of).
Bacteriologist Stuart Levy has brought together a set of
recommendations for the judicious use of antibiotics, for the
protection of individuals, and to maintain the effectiveness of
antibiotics:
For individuals:
• Wash fruits and vegetables before consuming them.
• Avoid raw eggs and undercooked meat.
• Use antibacterial soaps only when needed to protect
patients with immune deficiency.
• Complete the full course of prescribed antibiotics.
For doctors:
• Wash hands thoroughly between patients.
• Do not prescribe antibiotics unnecessarily, e.g., for viral
infections.
• Prescribe antibiotics that target the narrowest possible
range of bacteria.
• Isolate patients with multiple-drug-resistant strains of infectious
bacteria.
Pesticide and herbicide resistance. The extensive and heavy
use of pesticides (see photo on page 349) has selected pesticide-resistant
insects and rats; the extensive and heavy use of
herbicides has selected herbicide-resistant weeds. The first DDTresistant
mosquitoes in Pakistan were detected in 1965, just five
years after DDT use began in the region. The swift evolution
of resistance occurs with each new pesticide. In the last half
decade, more than 520 species of insects and mites, 273 weed
species, 150 plant diseases, and 10 rodent species have developed
genetic resistance to at least one pesticide or herbicide.