Can bacteria grow in a hot tub?

Yes, improper sanitizer levels will allow bacteria to grow

The higher temperatures in a spa creates an environment that speeds the reproduction rate of bacteria that are called mesophiles. Mesophiles grow very rapidly at temperatures of 68°F to 113°F. The normal operating temperatures of spas as well as the normal human body temperature falls within this range. Some bacteria can reproduce in as little as fourteen minutes in spa water at 98.6°F. Bacteria that prefer this temperature range are often those that cause disease in humans.

Maintaining proper sanitizer levels with Chlorine Concentrate or FreshWater Salt System will minimize the chances of bacterial growth.