The number of drug-resistant bacterial strains has increased in part because of the indiscriminate use of antibiotics, which have sometimes been overprescribed. Such misuse speeds the process by destroying bacteria that would compete with resistant strains. In addition, patients sometimes stop treatment when they start to feel better, leaving a residual population of bacteria that is likely to be more resistant to drug treatment. Another source of resistance is the routine use of antibiotics in animal feed to enhance growth, a practice that has led to resistant strains of Escherichia coli and salmonella that have been passed on to consumers. Resistance is due to random genetic mutations in the bacterial cell that alter its sensitivity to a single drug or to chemically similar drugs through a variety of mechanisms.
Many bacteria can transfer their resistance to other bacteria of the same or different species.
It has also occurred in such diseases as malaria and tuberculosis . Although drug companies are again concentrating on antibiotic research, no new products are expected until the turn of the century, and many infectious-disease experts are urging that doctors consider the public health risk before prescribing antibiotics and that the government regulate the use of antibiotics in agriculture.