Question 2 of 15

Which sentence uses a comma correctly to set off coordinating adjectives?

A. The stale curdled milk, made LaShawna wrinkle her nose.

B. Meg breathed a sigh of relief to be back in Aunt Sylvia's cheerful, homey kitchen.

C. Bernard waved furiously at the first, yellow car he saw.

D. Through the dense, fog they could just make out the figures approaching.

In this activity, you will review Gram stains and apply your knowledge of this process to draw conclusions from images of Gram-stained bacteria. You will then answer some discussion questions about bacteria.

 

Gram Staining is a process of using different stains to identify bacteria based on the thickness of their cell walls. These stains stick to the peptidoglycan in the cell wall. After fixing bacteria to a slide there are four different stains used in this process. Crystal Violet is added to the slide and absorbed by the cell walls. Iodine is then added and binds with the crystal violet in the cell wall, making it insoluble in water. Alcohol is used as a decolorizer so that any extra stain is removed. For bacteria with thin cell walls, this causes all the colored pigment from the crystal violet to run off. For bacteria with thick cell walls, the pigment becomes dehydrated and crystalizes in the cell. Since bacteria with thin cell walls now have no color, safranin is added to give the cell a pink color and make it visible. If the bacteria have a thick cell wall the Gram stain will turn out purple and if they have a thin cell wall it will be pink. After staining, bacterial cells can be classified as Grampositive or Gram-negative.

 

Gram-positive bacteria have a thick cell wall and stain purple from the crystal violet. Streptococcus, staphylococcus, and S. epidermidis are examples of Gram-positive bacteria.

 

Gram-negative bacteria have a thin cell wall and stain pink from safranin. E. coli, pneumonia, and cholera are examples of Gram-negative bacteria.