Isolating Bacteria on a Spread Plate and Streak Plate {Lab 2 - May 13, 2015}

     For this experiment we practiced aseptic techniques to transfer bacteria from our environmental samples onto plates to isolate a single strand of bacteria and grow a pure culture. As our environmental culture had not grown yet, we used a sample provided by our professor.

Materials: 

• Agar plates
• Bunsen burner 
• Ethanol 
• Glass spreader 
• Pipettor 
• Inoculating loop
• Bacterial culture sample

Procedure:  

     We started with the spread plate. This required taking a pipettor and transferring 200 µm of our sample bacteria onto our plate. We then soaked the short end of a glass spreader, a L glass tube, in ethanol and burned it to sterilize it. Once it cooled, Clare spread the bacteria with the short end of the glass spreader all over the spread plate.

     Next we prepared the streak plate. To do this we took the inoculating loop and ran it through the flame to sterilize it. Then taking our broth sample, we uncapped it, ran the mouth through the flame and dipped the loop in. Running the mouth through the flame one more time we recapped the broth and returned it to the rack. I had outlined the pattern of the streaks we want to make on paper and placed our plate on top so Clare could then take the loop and make the first streak. We then passed the loop through the flame again and did the second streak. Before the 3^rd and 4^th streak we again sterilized the loop each time to ensure a pure culture would grow. After this, we placed each sample in the incubator for the night.

Spread plate culture

Conclusion/application: 

     The next day when Clare and I checked the plates our spread plate had grown a good pure culture while our streak plate had not grown as much. Perhaps we did not streak the plate enough; or, we may have not let the loop cool down enough after sterilizing so many of the bacteria may have been killed. Streak and Spread plates allow colonies to be observed and some colonies isolated so that the bacteria may be determined. These techniques will be done in medical laboratories in order to determine the pathogen causing an illness.