Week 3, Day 2

 May 29, 2013

Today in lab we started by retrieving our blood, mannitol-salt, MacConkey, and PEA samples prepared in our last lab section. Our mannitol salt agar plate tested negative, we had very little growth on the plate, and it had no yellow appearance to indicate the fermentation of mannitol. Our MacConkey agar plate was also negative, indicating a lack of ability to ferment lactose. This is consistent with our previous tests for lactose. Our blood agar plate tested for gamma (γ) hemolysis (lack of hemolysis), this means that our bacteria lacks the ability to lyse red blood cells.

Mannitol Salt Agar Plate

MacConkey Agar

Blood Agar Plate (Bottom)

 The PEA (phenylethyl alcohol) agar we will be studying tomorrow, but so far it suggests that our bacteria is gram-negative (which is consistent with our previous gram stain test). The PEA agar is a selective medium; it promotes the growth of gram-postive bacteria and slows/inhibits the growth of gram-negative bacteria.

PEA Agar (Bottom)

We also examined our throat and nasal swab samples today. Our nasal swab had been incubating at body temperature on a mannitol salt agar (a selective media). We did have some growth on our plate, indicating the presence of a halophilic bacterium, but it appeared to all be of the same type of bacterium. There was no yellow coloring of the sample, indicating that no fermentation of mannitol was taking place.

Nasal Throat Swab on Mannitol

Our throat swab had also been incubating at body temperature but had been inoculated onto a blood agar plate (differential media) with a bacitracin disk placed on the surface. We did have some different types of bacteria growing on our plate, but they were all sensitive to the bacitracin (did not grow around the disk). The bacteria observed preformed alpha (α) hemolyses (incomplete hemolysis), indicating an ability to only partially lyse red blood cells.

Throat Swab on Blood Agar with Bacitracin Disk

We also studied our Kirby-Bauer agar plate that had been inoculated with our bacterium and set with 5 different antibiotic disks - penicillin, neomycin, erythromycin, tetracycline, chloroquine. We measured the diameters of each circle surrounding the disks (the clear zones where bacteria did not grow in order to avoid the antibiotics). There were no clear zones surrounding penicillin (cell wall attacker) or erythromycin (substitute for penicillin for people with allergies; binds to 50S ribosomal subunit), indicating that the bacteria is resistant to both. The diameter of the clear zone around neomycin was ~1.78 cm. Around tetracycline was ~1.5 cm. And around chloroquine was ~1.0 cm.

Kirby-Bauer Agar Plate

We also attempted to identify our bacterium today, but some of our experimental tests were not consistent and we are going to repeat the tests for indole, oxidase, and TSIA. Tomorrow we will review our data again and identify our bacterium.