Selective / Differential Media Inoculations and Collection of Specimens {Lab 9 - May 22, 2015}

     These tests were done on selective and differential medias which allow only certain kinds of bacteria to grow on them. We inoculated MacConkey, Phenylethyl Alcohol (PEA), Mannitol Salt, Eosin Methylene Blue, and Blood Agar.

     Before we inoculated the agar plates, we first collected specimens: urine, nose swab, and throat swab.

Urine sample

Elisabeth preparing to
have her throat swabbed

     After collecting these specimens, we inoculated them on agar plates. 

MacConkey Agar:

     We inoculated our unknown and the urine sample on the MacConkey agar plate (selective and differential). This plate differentiates between gram-negative bacilli based on there ability to grow on the plate and to ferment lactose. MacConkey Agar plates are often used to process specimens that contain or are contaminated with fecal matter. Lactose-fermenting bacteria will produce pink-red colonies. Non-lactose-fermenting bacteria are colorless. Our Unknown (on left) was gram-negative and a strong lactose fermentor. The urine sample (on right) did not grow any colonies.

PEA Agar:

     We inoculated our unknown and a skin sample on the PEA agar plate. PEA agar plates are selective only allowing gram-positive bacteria to grow. Since our unknown bacteria is gram-negative, we shouldn't expect any growth on the PEA plate. Both our unknown and the skin sample did not grow.

Mannitol Salt Agar:

      We inoculated our bacterial unknown and the nasal swab sample from Elisabeth onto the mannitol salt agar plate. Mannitol plates are both selective and differential. They allow only salt tolerant bacteria to grow; they differentiate between bacteria that will ferment the mannitol changing it from red to yellow. Our unknown (left) did not grow on the mannitol; however, the nasal sample (right) grew some colonies. 

EMB Agar:

     We inoculated our unknown onto the EMB agar plate along with the urine sample. EMB agar plates are both selective and differential because the dyes will inhibit many gram-positive bacteria from growing and will differentiate between bacteria that will ferment lactose and/or sucrose and those that will not. Bacteria that grow on EMB plates and react with the lactose/sucrose and produce acid will produce dark blue with green metallic sheen colonies. Lower acid production from fermentation of the sugars produce pink colonies because less dye is precipitated. Some colonies form but are colorless due to their inability to ferment the sugars. Our Unknown (left) produced dark blue colonies with green metallic sheen. The urine sample (right) did not produce any colonies.

 

Blood Agar:

     We inoculated our unknown and our throat swab sample onto the blood agar plate. Blood agar is an enriched media and is also differential because it distinguishes among bacteria and their ability to lyse RBCs. Beta-hemolysis completely dismantles the RBCs. Alph-hemolysis only partially disrupts the RBCs and results in a greenish hue around the colonies. The unknown (right) and throat sample (right) resulted in alpha-hemolysis on the blood agar. 

Results and Applications:

     We learned how to work with differential, selective, and enriched media plates. These are important for us to know as certain bacteria will or will nor grow on certain plates. By growing bacteria on these, we were able to get just that much closer to discovering what our unknown is.