Pathogenic bacteria fall into two categories:

1. Those that grow without oxygen: Anaerobic

2. Those that grow in the presence of oxygen: Aerobic

The majority of pathogens we are concerned about in food safety are aerobic and require oxygen to grow. This is a constant concern during most stages of food production, as food is always being exposed to oxygen and so there is a continuous opportunity for pathogens to grow if other factors are present, and controls are not in place. Since we cannot completely avoid this exposure, it is important we use time and temperature as well as other safe food handling practices to minimize the risk of food contamination in the first place. Because, once a bacteria is there, it will continue to grow.