Bacteriology 102: A Review of Reasons for Anaerobic Growth
The following summary may help to explain how media formulations can allow anaerobic growth for organisms capable of doing so for one reason or another. The same organism – a typical strain of E. coli – was inoculated into tubes 1, 2 and 3, and a "facultative phototroph" was inoculated into tube 4.
In Tube #1, we have a medium containing peptone and agar plus other nutrients a "typical Bacteriology 102 organism" (i.e., a commonly-found, easy-to-grow chemo- or photoheterotroph) might require for metabolism and replication – except that nothing is included which would support anaerobic growth such as glucose (or something else that could be fermented) or nitrate (or some other electron acceptor/"oxygen substitute" that could be used in anaerobic respiration). After inoculation of this medium and incubation in the dark, any growth would be due to aerobic respiration with the growth only at the top of the medium. There would be no anaerobic growth except for some rare, exceptional organisms which can ferment amino acids; these we do not consider in Bact. 102.
Tube #2 is the same medium as in #1, but glucose has been added. After incubation (in the dark), any anaerobic growth would be due to fermentation of the glucose. Thus the medium can be used to detect whether or not an organism can respire (aerobically) or ferment. An example of such a medium is the Thioglycollate Medium we use to test common chemoheterotrophs for "oxygen relationships" (discussed here and more fully here).
Tube #3 is the same medium as in #1, but potassium nitrate has been added. After incubation (in the dark), any anaerobic growth would be due to anaerobic respiration where the organism is using nitrate as the electron acceptor. In Bact. 102 (Exp. 7), we do a test in a broth medium for nitrate reduction; with reagents we can detect nitrite formation, and with the Durham tube we can detect N2 gas formation. One can probably see why we would not want to include nitrate in the Thioglycollate Medium above.
Tube #4 is the same medium as in #1, but we have incubated the tube in the presence of light. With light as the ultimate energy source, anaerobic growth would be due to anoxygenic phototrophy. This is the basis for the test we do in Bact. 102 (Exp. 11.1) to see if our isolates of purple non-sulfur bacteria are either "strict phototrophs" (just anaerobic growth in the light) or "facultative phototrophs" (anaerobic growth in the light, plus aerobic growth due to aerobic respiration whether in the dark or the light). Click here for a summary of this test.
A relevant quiz/exam question which we repeatedly give in Bact. 102 is No. 3 on this page.