Very few bacteria are capable of growing in beer.  These types are generally termed "Beer Spoilers".  They are not hazardous to human health, but are tremendously important because of their impact on the appearance, aroma and flavor of beer. 

When a beer spoiler organism grows in beer it multiplies.  This eventually causes beer turbidity or cloudiness.  While the organism is alive, it metabolizes, producing chemical "by-products" which can result in strong "off" tastes and odors.

Below are four of the most common types of bacteria found in beer.

Lactobacillus

This type of bacteria is anaerobic (can grow with or without the presence of oxygen).  When they grow in beer, they make the beer cloudy.  Large amounts of lactic acid and acetic acids are produced.  This causes the beer to become sour tasting.

Pediococcus

This type of bacteria is anaerobic (can grow with or without the presence of oxygen).  When they grow in beer, they produce diacetyl.  This produces a compound that is described as having a popcorn, buttery aroma.

Pectinatus

This type of bacteria is an obligate anaerobe (can not grow in the presence of oxygen).  When they grow in beer, they produce acetic, lactic and propionic acids, methyl mercaptan, dimethyl sulfide and hydrogen sulfide.  Beer which has been broken down by this organism is cloudy and smells like rotten eggs.  Also has a foul taste.

Acetobacter

This type of bacteria is an obligate anaerobe (can not grow in the presence of oxygen).  When they grow in beer, they produce large quantities of acetic acid (vinegar), and can cause beer to become sour, ropy, turbid and visibly discolored.

Note: The information in this section was provided by and reproduced with the permission of The Coors Brewing of Golden, Colorado.