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Acetobacter aceti bacteria are a widespread group of gram-negative, obligate aerobic rods which oxidize ethanol (alcohol) into ethanoic (acetic) acid. They occur mainly in sugary, acidic and/or alcoholic habitats and they play a positive, neutral, or detrimental role in foods and beverages. They are benign, non-pathogenic microorganisms ubiquitous in the environment, particularly in alcoholic ecological niches such as flowers, fruits, fruit flies, honey bees, and also in water and soil. They are generally found as spoilage organisms in alcoholic beverages. A. aceti has a long history of safe use in the fermentation industry for the production of acetic acid (vinegar) from alcohol. Acetic acid bacteria are classified into the genera Acetobacter, Gluconacetobacter, Gluconobacter, Acidomonas, and the recently described genus Asaia.

Lactobacillus acidophilus and Lactobacillus casei from a probiotic soy protein preparation advertizing the presence of 50 billion live cells. The lactobacilli were grown as colonies on an agar gel plate to remove the interference from the soy proteins. L. acidophilus has long cells whereas L. casei has short curved cells (shown in yellow). Lactobacilli are probiotic lactic acid bacteria, the presence of which protect humans from some bacterial pathogens.