How do you inoculate agar butt?

Answer 1

In microbiological testing, a "butt" refers to the bottom of a tube filled with agar. Its most common use is in conjunction with the term "slant". These agar-filled tubes are typically allowed to solidify at an angle so that a larger amount of surface area is available to inoculate by streaking. After incubation, both the slant surface and the butt have to be evaluated because growth on the slanted surface (next to air/oxygen) will indicate one kind of information and the bottom (butt) will indicate additional information about the bacterial growth--usually indicated by a difference in color. For example, in the TSI test (Triple Sugar Iron), a combination where the slant is red and the butt is yellow means that fermentation of glucose has occurred.

In a TSI test, the butt is not actually inoculated.

However, in a motility test, which does not require a slant, the agar is inoculated by stabbing a straight wire covered with an inoculum of sample down to the bottom of the tube. If the bacteria can grow away from the stab into the agar, they are motile. If growth is confined to the stab length, they are non-motile.

Answer 2

To inoculate an agar butt, use a sterile inoculating needle or loop to pick up a small amount of the microbial culture, then gently stab the needle into the center of the agar butt making sure not to touch the sides of the tube. After inoculation, carefully flame sterilize the needle before putting it away.