Post #3

During lab this week we Gram stained several different bacterium. First we made two control stains one Gram positive, B. megaterium, and one Gram negative, K. pneumoniae.  After viewing them under the microscope the Gram positive sample is clearly purple and the Gram negative sample is pink.

Gram positive control

Gram negative control

 We then Gram stained our mystery microbe to determine what category it fell into. When viewed under the microscope our mystery microbe was purple indicating that it is Gram positive. It also appeared rod shaped or Bacillus.

soil microbe

soil microbe under microscope

 After checking out our microbe under the microscope we streaked our soil microbe on a MacConkey agar plate along with B. megaterium, K. pneumoniae, and P. aeruginosa. We also streaked these bacterium on a Rose-Bengal plate to determine if their was any fungi present in our sample.

Our MacConkey agar confirmed our Gram staining. B. megaterium and our mystery microbe both did not grow on the agar indicating that they’re Gram positive. K. pneumoniae and P. aeruginosa successfully grew on the plate demonstrating that they are Gram negative. Also, there was no growth for our microbe on the Rose-Bengal plate. 

There are several differences between the cell wall structure of Gram positive and Gram negative bacteria. Gram positive bacteria have a cytoplasmic membrane with a thick peptidoglycan outer layer. Gram negative bacteria has a thin peptidoglycan layer sandwiched between two cytoplasmic membranes.  The outer membrane of the Gram negative  bacteria contains phospholipids, proteins, and lipopolysaccharide (LPS). LPS is an endotoxin that contains Lipid A.  When the Gram negative bacteria is killed the endotoxin is released causing negative effects in the host. This feature of the cell wall can cause issues when treating Gram negative bacterial infections. The cell wall structure also is resistant to many antibiotics making the infections difficult to treat.