In the 1990s, we have special concern for two problems: (1) Enterococci have emerged that are resistant to the drug vancomycin. There are no reliable alternative treatments, and there is fear that the resistance gene in the enterococci could spread to more common bacteria, such as staphylococci. (2) The appearance of multidrug resistant tuberculosis. Tuberculosis can spread easily and it is deadly; it was the number one cause of death in the U.S. in 1900, and it is still the leading cause of death by infectious disease worldwide. Treatment success drops from nearly 100 percent for the older strains to barely half that for the drug-resistant ones.
"Any population of bacteria may contain some resistant individuals. Taking a partial treatment course will kill off the susceptible bacteria at first, but it leaves behind the tougher ones. If you take a long course and kill more bacteria, only a few will be left. The patient's own immune system can mop them up. If you take a short course and do not kill many of the bacteria, however, there will be a lot left--too many for the patient's immune system to finish off--so the resistant ones have a chance to grow. Also, a partial course leaves more bacteria alive at the end of therapy. As the antibiotic levels in the body decline, they pass through a phase in which the concentrations are high enough to kill some bacteria but not high enough to stop the tougher ones. At this point, those remaining tougher germs have an advantage because they do not have to compete with the weaker (now dead) ones. This opening gives the resistant bacteria a chance to grow and flourish."
And Abigail Salyers in the department of microbiology at the University of Illinois has provided this extensive overview and a somewhat different perspective on the problem:
"One answer to the first part of this question is that the use of antibiotics has not been constant over the period since antibiotics were discovered, but it has increased drastically in recent years. Antibiotics were first introduced into clinical use in the 1940s and 1950s. At first, they were used mainly to treat existing infections. Clinicians soon learned, however, that administering antibiotics before surgery (prophylactic use) could prevent infections, and prophylactic use of antibiotics became widespread. Even in cases where prophylactic use has been shown not to be efficacious, the tendency has been to continue the practice 'just in case.'
The uses of antibiotics continued to multiply. Dermatologists discovered that long-term, low-dose administration of certain antibiotics was beneficial in the treatment of acne and other skin conditions. Animal scientists discovered that some antibiotics promoted growth of livestock animals. They are also used extensively in aquaculture to prevent infections that arise readily in crowded fish or crustacean populations. Because the selective pressure for antibiotic-resistant strains increases as antibiotic use increases, it is not surprising that the incidence of antibiotic strains has increased more rapidly in recent years.
"The type of selection has also changed. Bacteria exposed to concentrations of antibiotics too low to kill them, especially if exposure occurs over long periods, are much more likely to develop and fix the mutations that make them resistant than are bacteria exposed to high concentrations of antibiotics, which kill them before they can mutate. As the use of antibiotics has increased, opportunities for long-term, low-dose exposure have also increased. In the early days of antibiotics, when antibiotics were used primarily for therapy of existing infections, hospital staff members and hospital patients who were not using antibiotics had very little exposure to antibiotics.
Today, especially in intensive care wards, the amount of antibiotics in the environment can become high enough that people in the vicinity of patients receiving antibiotics are exposed continuously to low levels of antibiotics. If these people are asymptomatic carriers of bacterial pathogens (which is not uncommon in hospital settings), the bacteria they carry experience the type of antibiotic exposure that is conducive to development of resistant strains. This phenomenon may explain why the most resistant strains are now arising in hospitals. A similar situation probably occurs in agricultural settings in which antibiotics are added daily to animal feed.