Brain Heart Infusion Tubes for Oxygen Classification
Brain heart infusion (BHI) agar can be used to create an oxygen gradient to determine the oxygen requirements of a microorganism. The microbes being tested are inoculated into a melted BHI agar deep tube, mixing well to evenly distribute the bacteria throughout the liquefied agar. Once the agar solidifies, an oxygen gradient is created, with atmospheric oxygen concentrations at the top of the tube and little to no oxygen present at the bottom of the tube. The bacteria will only be able to grow where their oxygen requirements are met, and will localize to the area(s) of their oxygen requirements in the tube.
Tube 1: Obligate Aerobe -- note the growth is only at the surface of the tube where atmospheric oxygen is present.
Tube 2: Microaerophile -- note the growth indicated by the arrow, just below the surface of the agar where lower concentrations of oxygen are present.
Tube 3: Facultative -- note the uneven distribution of growth from top to bottom (more growth at the top).
Tube 4: Aerotolerant -- note the uniform growth from top to bottom.
Tube 5: Obligate Anaerobe -- note the absence of growth in the top portion of the tube where oxygen is present. Also note the large cracks in the agar as a result of gas production by the anaerobic microorganism.