Discuss why Gram-positive and Gram-negative bacteria stain differently with the Gram stain? How is
this difference reflected in antibiotic sensitivity/resistance?
What will be an ideal response?
Gram-positive bacteria are those that retain crystal violet, the primary dye of the Gram stain, and thus
appear purple. Gram-negative bacteria do not retain the crystal violet dye, and stain pink because of
safranin, the second dye. The staining difference reflects differences in the cell wall structure. Grampositive
bacteria have a thick peptidoglycan layer, which absorbs and retains the crystal violet
pigment. On the other hand, Gram-negative bacteria have a thin peptidoglycan layer, and therefore
lose the crystal violet pigment during the alcohol wash. This difference in cell walls has a bearing on
how certain antibiotics affect these two bacterial types. Gram-positive bacteria are very susceptible to
penicillin, which blocks new cell wall formation by inhibiting peptidoglycan cross-linking. The
weakened cell wall soon leads to the death of the bacterium. Penicillin is less effective against Gramnegative
bacteria because their outer membrane inhibits entry of the antibiotic. Interestingly, this outer
membrane makes Gram-negative bacteria much more sensitive than Gram-positive bacteria to
antibiotics that disrupt the cell membrane (e.g., polymyxin).
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How many genes does a human X chromosome carry?
a. 100
b. 1,400
c. 2,800
d. 14,000
e. 28,000
How does the founder effect influence the frequency of alleles in a population?
What will be an ideal response?