Determine if you will use petri dish media or a tube of broth to detect bacterial growth. The petri dish method allows individual bacterial colonies to be counted, giving a quicker representation of how many bacteria resided on the bottle at the time it was swabbed.
If you use a tube of broth, you will know if there is bacterial growth by how turbid (cloudy) the solution is. To get a more accurate bacterial count, use a spectrophotometer. If one is available, follow the lab manager's or instructor's directions on how to use it.
Whichever method you choose, the medium you use should be nutrient agar media. This substance allows for a large number of bacteria to grow.
Open the packaging for the sterile swab and gently rub it against the bottle you are testing. You can swab different areas of the bottle, from the opening to even the label. If the area you choose is dry, dip the swab in the broth or run it gently over the petri dish media to get it damp before swabbing the bottle area.
If using a petri dish, rub the swab over the media. Once the sample is dry, place on a shelf or open area or in an incubator for 12 to 48 hours, depending on how much bacterial growth you want.
If using a broth, break off the wooden stick end and insert the swab end in the broth. Put the tube cap on and place it in the incubator or open area as well.
After the incubation period, check for growth. You should be able to see colonies on the petri dish or the broth should appear cloudier than seen with just the original media. If you swabbed multiple areas of the bottle, determine which area had the most colonies or caused more cloudiness in the broth tube -- that area will be the part of the bottle that had the most bacteria at the time of the swabbing.