When antibiotics are given again the resistant bacteria survive, and as their proportion of the bacterial population increases over time, the drugs become less effective. The more antibiotics we use, the more likely it is that bacteria will become resistant. With thousands of animals crammed into the unhygienic, crowded quarters of a typical factory operation, antibiotics are dispensed constantly through the animals' feed. Scientists do not understand how or why the drugs promote growth. Evidence is increasing that the emergence of antibiotic resistance, caused by overuse of antibiotics, threatens public health.
Many scientists attribute the problem to the misuse of antibiotics in humans and animals.
Although it is not the only source of the problem, the use of antibiotics to promote livestock growth is a major contributor to antibiotic resistance. Drug resistance in campylobacter bacteria, the most common known cause of bacterial food borne illness in the United States, increased from zero in 1991 to 14 percent in 1998. A Harvard University study showed that antibiotic-resistance genes found in bacteria infecting humans were identical to some of the same bacteria infecting animals. Scientists at the Centers for Disease Control linked an outbreak of antibiotic-resistant salmonella in humans to beef cattle that had been fed subtherapeutic doses of chlortetracycline for growth promotion. Staph bacteria, which cause skin, blood, heart valve, and bone infections that can lead to septic shock and death, are becoming increasingly resistant to the chief antibiotic that has been used to treat staph infections, methicillin.