AA = strict anaerobes: Clostridium, Sarcina, and many genera from

the rumen of cattle, intestines and similar sites. Strict anaerobes grow only were oxygen is absent. Some are more sensitive to oxygen than others. Some species, especially those from the rumen and intestines, die rapidly when exposed to oxygen. Most Clostridium are not killed by such brief exposure but can't grow in oxygen. Some species of Clostridium can grow slowly in the presence of air. No species of Clostridium is able to produce spores when free (uncombined) oxygen is present.

FA = facultative anaerobes: Escherichia, Citrobacter, Enterobacter, and Proteus grow best in oxygen but can grow in the absence of oxygen by stealing oxygen from foods such as nitrate, sugars, and other "honorary oxygens". The result of this is production of nitrite, organic acids (lactic acid, formic acid, etc.), and other substances which are often foul-smelling. Tubes containing facultative anaerobes contain growth throughout when the bacteria are evenly distributed, but there is usually a heavier growth on the surface of the agar because they grow best in air.

MA = microaerephilic: Azospirillum, Aquaspirillum, Cytophaga

require oxygen but grow best just below the surface of the agar where oxygen is reduced. This type of bacteria is relatively uncommon in laboratories because some won't grow on GST and other common media. It is difficult to find a species which will grow in a well defined band a millimeter or so below the surface to produce a mice band below the surfarce as seen here. Often the band is so near the surface that you can confuse these for aerobic species, however, they do not grow profusely on the surface of the agar like aerobic species.

A = strict aerobes: Acetobacter, Arthobacter,

Azomonas, Bacillus, Micrococcus, Pseudomonas, Xanthomonasgrow only on the surface of the agar where they get plenty of oxygen. Most produce a heavy growth above the agar which may be a liquid. Some bacteria produce slimes or capsules and these produce exceptional profuse growth above the agar and may burrow into the agar slightly. The liguid containing cells may run down between the walls of the tube and the agar plug, but you should not confuse this with growth.

I = indifferent: Lactobaccilus, some Streptococcus, and most other milk organisms grow equally well on the surface and within the agar because they are indifferent to the oxygen level. Notice that the bacteria grow uniformly on the surface and to the bottom of tube provided the cells are distributed uniformly. The growth is similar to that of facultative anaerobes except FAs have a heavy growth of bacteria at the surface. At the surface indifferent bacteria may have barely noticeable growth of cells. We know they can grow on the surface because they do so when spread on a pertri plate. Actually, many of these bacteria require vitamines, amino acids, and other growth factors and the colonies may be tiny even on the media most suitable for them.