How to Calculate the Volume of
Liquid Held by an Inoculating Loop

If you have a liquid culture of bacteriophage, bacteria, radioisotope or a suitable chemical of known concentration, you can calculate the volume of liquid held by an inoculating loop. This experiment can be done most easily a culture of bacteria, but you must also determine the number of bacteria per ml by plating at the same time the loop was used to plate some bacteria.

Drawing of an Inoculating Loop

An inoculating loop is a piece of stainless steel, nichrome, platinum, or other wire mounted in a handle. Most people use 26 guage platinum or 25 guage nichrome wire. To make a loop, bend the tip of the wire into a small circle having an inside diameter of 1 mm or whatever size you want. When you dip the loop into a liquid, it should retain a droplet of water. The loop shown on the left will not hold a droplet of liquid bacteria culture because the open section of the loop is too wide. The tip of the loop must bend around and touch the metal or it will not hold water.

A Quick Simple Example

Suppose you have a liquid culture of bacteria which has 10,000 bacteria per mL (1 x 10⁴ cells per mL). You know there are 1 x 10⁴ cells per mL because you assayed the number of bacteria per mL using the methods taught on page b038.htm. You must do that assay at the same instant you test the loop because bacteria may be multiplying.

Method:

• dip the loop into the bacteria suspension
• then vigorously mix a tube of melted agar with the loop
• pour the agar into a sterile petri plate
• incubate the plate and count the colonies 20 to 30 hours later.

Suppose you count 100 colonies in the plate. Calculation: 100/10,000 reduces to 1/100 or 0.01. The volume of the loop is 0.01 mL. If you can't understand this, send an e-mail to the address below and this example will be improved.

This method assumes bacteria are not adsorbed to the loop.

An Easy Example Using Radioisotope

Suppose you have a solution of a radioisotope which gives 50,000 counts per minute per mL.

• Dip the loop into the isotope solution.
• Dip the filled loop into count-free water and mix vigorously with the loop to wash isotope off the loop into the water.
• Count the water sample.

Suppose you get 500 counts per minute. 500/50,000 = 0.01 indicating the volume of the loop is 0.01 mL. The two methods gave the same volume for the loop.

Method for Testing a Loop

• Flame the loop
• Dip into the bacteria suspension in the same manner you expect to use the loop. Say 2 cm above loop.
• Dip into the melted agar 45C and mix
• Pour agar into a sterile petri plate.

ice chilled bacteria and shaken

Calculation of the Volume of the Loop