Week 3, Day 1

May 28, 2013

Today in lab we began by inoculating differential and selective agar plates. We started with the blood agar plate, which is used to isolate and support the growth of fastidious bacteria and differentiate among bacteria based on their ability to lyse red blood cells (hemolysis). We incubated the blood agar plate with a sample from our slant.

Blood Agar Plate

 The second plate we inoculated was the mannitol salt agar plate, which is used for isolating bacteria based on their salt tolerance and differentiating among these isolates for mannitol fermentation.

Mannitol Salt Agar Plate

 The third was the phenylethyl alcohol (PEA) agar, which is used to isolate gram-positive bacteria from a specimen containing a mixture of gram-positive and -negative bacteria. 

PEA Agar Plate

The final plate we inoculated was the MacConkey agar, which is used to detect and differentiate among gram-negative enteric bacilli, based on their ability to grow on the medium and to ferment lactose.

MacConkey Agar Plate

After inoculating all of the agar plates, we began work on the Kirby-Bauer technique. For this we took a Mueller-Hinton agar plate and used a swab to inoculate the plate with our bacteria.

Inoculating the Mueller-Hinton Agar Plate

 We then placed five different antibiotic disks on the surface of the agar. We did this by dipping our forceps in an ethanol solution and sterilizing it with a flame before carefully taking a disk between the forceps ends and moving it onto the surface of the agar plate. 

Alcohol Solution

Performing Aseptic Technique with Forceps

The five antibiotic disks used were penicillin, chloramphenicol, neomycin, tetracycline, and erythromycin.

 The Kirby-Bauer technique is used to determine the sensitivity of a bacterium to several antibacterial medicines. We will incubate all of our inoculated samples prepared today and examine them after the bacteria has had time to grow.

Next we moved on to the throat and nasal swab tests. For the throat swab test we took a cotton swab and ran it over the surface of the back of a lab-partners  mouth, just behind the uvula. We then swiped the cotton swab over the surface of a blood agar plate. We placed a bacitracin antibiotic disk on the surface of the blood agar plate as well, which will help in determining if the bacteria from the throat swab contains the "strep throat" bacterium, Staphylococcus progenies.

Throat Swab Test with Bacitracin Antibiotic Disk

For our nasal swab, we dipped a cotton swab in saline solution before running it over the inner surface of a lab-partners nostril. The cotton swab was then swiped over the surface of a mannitol salt agar plate. Both swab test plates were incubated at 37°C, body temperature, so the colonies would have time to grow.

Nasal Swab Plate

Nasal and Throat Swab Plates