Preservatives in the makeup may break down over time, allowing bacteria to grow.

Sewage flows slowly through these ponds, allowing bacteria to break down the organic material.

This process allows natural aerobic bacteria to break down the waste into fertile topsoil.

In this process, ammonia is provided to allow beneficial bacteria to colonize.

Others, including military experts, insist that it is best to let the weapons degrade in the water, allowing time and bacteria to break them down.

The fluid becomes trapped in the middle ear, allowing viruses or bacteria to grow and cause infection.

This allows other bacteria to survive, which will be discussed in the next section.

Improper cleaning (or none at all) could allow mold and bacteria to grow inside.

There could be a leak around the crown that is allowing bacteria to re-infect the area.

The inside of a car can get warm quickly, allowing bacteria to multiply.