Salmonella, which causes what we sometimes call “food poisoning,” can live more than 400 days in soil. And when dried on a laboratory slide, salmonella survived for almost three years, says Barak, who studies salmonella contamination on leafy greens, a growing cause of gastrointestinal illness.

However, E. coli, another resident of the intestinal tract, tends to die sooner in the environment.

Many bacteria form spores — tough, durable “seeds” that can withstand extreme abuse. Spores of respiratory anthrax, like that used in the 2001 bio-terrorism attacks, can survive for many years.

Environmental also conditions affect survival, Barak adds. For example, the bacterium that causes tuberculosis can be killed by full-spectrum lights, which contain ultraviolet light. In contrast, bacteria that live on plants have pigments that block ultraviolet rays, allowing them to thrive in sunlight.

Finally, bacteria can form communities called “biofilms” that greatly increase their ability to survive adverse conditions. Biofilms can be a major problem on catheters and other medical devices, because measures that kill the outer layers of bacteria may not affect those located deeper inside the biofilm.