Breeding an Epidemic Antibiotics and Meat

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HOW BACTERIA BECOME RESISTANT

Part of the reason that it took so long to recognize the significance and extent of bacterial resistance to antibiotics is that no one anticipated the means by which resistance is passed from one cell to another. If resistant strains developed only through the reproduction of strong survivors and chromosomal mutation—as was initially assumed—it would take a long time for large cultures of resistant bacteria to develop.

Unfortunately, as Japanese researchers discovered in the late 1950s and early 1960s, bacteria have a much more efficient means of transmitting resistance. Dr. Tsutomo Watanabe found that, in the case of some microbes, drug resistance could be passed from one bacterium to another in the form of R (resistance) plasmids, pieces of DNA not directly linked to or affecting the chromosome. These plasmids, which can number up to 2,000 per cell, are directly transferred from cell to cell, through a connection called a pilus, without otherwise affecting the donor or recipient. Thus, instead of facing the extended process of mutation (a probability of less than 1 in 10,000,000) and natural selection, resistance DNA can be passed from one group of cells to another in a matter of minutes. What's more, the bacteria involved need not be of the same species; for example, Escherichia coli (a prevalent intestinal bacterium) readily becomes resistant and transfers that resistance to salmonella or Shigella dysenteriae.

CONCERN SPREADS

In the December 1967 issue of Scientific American, Dr. Watanabe wrote, "Unless we put a halt to the prodigal use of antibiotics and synthetic drugs, we may soon be forced back into a pre-antibiotic era." It was an outcry that few heeded at the time. But by 1982, the Lancet reported that 90% of Staphylococcus aureus bacteria (which infect surgical incisions) were resistant to penicillin and that 35% of E. coli were resistant to ampicillin.

Though doctors and scientists don't agree about the severity of the problem, many physicians are now exercising restraint in prescribing antibiotics. For example, the use of antibiotics for cold and flu viruses, against which they are entirely ineffective, has practically ended. Doctors have recognized that antibiotics are a depletable resource that needs to be saved for really serious health problems. Meanwhile, new (and much more expensive) antibiotics are under development to replace those that have become ineffective. But trying to keep up with the expanding inventory of resistant bacteria is a constant battle.

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