Of all the microorganisms, bacteria are the greatest threat to food safety. Bacteria are single-celled, living organisms that can grow quickly at favorable temperatures.
Some bacteria are useful.
We use them to make foods like cheese, buttermilk, sauerkraut, pickles, and yogurt. Other bacteria are infectious disease-causing agents called pathogens, that use the nutrients found in potentially hazardous foods to multiply. Some bacteria are not infectious on their own, but when they multiply in potentially hazardous food, they eject toxins that poison humans when the food is eaten. Food handling practices are risky when they allow harmful bacteria to contaminate and grow in food. If you touch a potentially hazardous food during preparation, you may transfer several thousand bacteria to its surface.