Cell Biology Laboratory Manual Table of Contents

  Techniques Manual Table of Contents

  Lab Safety Sheet


  Related Topics

Aseptic Technique

Benedict's Test

Petri Dishes

Streaking for Isolation

Spread plates


Broth and Semi-solid Media

Staining Bacteria

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Sampling and Inoculation

We frequently use sterile cotton-tipped swabs to collect bacteria from the environment. The swab is then used to transfer bacteria to an agar plate and a loop used to spread the bacteria over the remainder of the plate. For some experiments, we will inoculate media with a need. These tools are described below.

The Swab. To remove the swab from the tube, grasp the tube in the left hand and the cap with the small and ring fingers of the right. Pull the cap off. By using these fingers, the thumb and remaining fingers are free to grab the swab and remove it from the tube. Return the cap to the tube. If the source to be sampled is dry, you will want to moisten the swab before collecting the sample. A moist swab will much more effectively pick up bacteria than a dry swab. Open a tube with sterile water as described above (still holding the swab with the thumb and forefinger) and dip the swab into the water. Raise the swab above the level of the water and push it against the walls of the tube. Rotate the swab to force out excess water. You want the swab to be damp, but not dripping wet. To collect bacteria, rub the swab over the surface being sampled while you rotate the swab handle between thumb and forefinger. You want to expose all surfaces of the swab to the surface, not just one spot. If you don't transfer the bacteria to an agar plate immediately, return the swab to it's original tube for holding and transport.


The loop. Loops are extremely useful tools for moving bacteria. Before acquiring bacteria, the loop is sterilized by holding it into a flame until the wire glows red from loop to holder. (See illustration below). Once the loop has had few moments to cool, bacteria in suspension are picked up by dipping the instrument into the broth and picking up a single loop full of broth. Bacteria in colonies are picked up by touching the end of the loop to the colony. You do NOT need to pick up the whole colony. Each colony contains billions of bacteria so simply touching the colony will almost always give you plenty of bacteria to work with.

The needle. Needles are straight wires (no loop) used to pick up bacteria from closely packed colonies or to inoculate in a very defined area. We commonly used needles to inoculate semi-soft media. The needle is first sterilized as shown above for loops. The needle is then touched to the desired colony. Do not try to stir the needle around. The bacteria are transferred to the target media by a single stab, straight down and back up with minimal wobble.