We learn from the passage that microorganisms .............. .

Food preservation systems in general slow down or stop
the growth of microorganisms, but not all microorganisms
are harmful. Some, in fact, are commonly used in
preservation. In the process known as fermentation,
microorganisms convert carbohydrates to acid. Salt, or a
brine solution made with salt, is also used. The amount
of salt used determines whether or not any organism can
grow and what types will grow. Fermented products are
left in containers with salt solutions for several weeks in
rooms with carefully controlled temperatures. Each group of
microorganisms has an optimum temperature for growth
which the processor must maintain so that correct
fermentation action will take place. Fermented foods
include pickles made of fruits and vegetables-, wine and
beer, cheeses, vinegar, and pickled meats.