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Microbiology: Laboratory Theory & Application, Brief 3rd Edition

Microbiology: Laboratory Theory & Application, Brief (3rd Edition)

Book Edition3rd Edition
Author(s)Leboffe, Pierce
ISBN9781617314773
PublisherMorton Publishing
SubjectBiology

Chapter 4, Section 4-1, Observations and Interpretations, Exercise 1

Page 231

Here is a tip:

PEA is a selective medium and NA is a general purpose medium.

Explanation

  • Phenylethyl alcohol in PEA inhibits the growth of Gram-negative bacteria by disrupting DNA [deoxyribonucleic acid] synthesis.
  • Alcohol can easily enter Gram-negative bacterial cells because of the thin peptidoglycan layer in their cell wall. For this reason, the growth of Escherichia coli is inhibited in PEA medium, while that of Staphylococcus epidermidis and Streptococcus gallolyticus is not.
  • NA allows both Gram-positive and Gram-negative bacteria to grow as it is a general purpose medium that allows growth of a wide range of microbes.
  • Escherichia coli, Staphylococcus epidermidis, and Streptococcus gallolyticus showed good growth in NA because of the lack of selective agents for Gram-positive and Gram-negative bacteria.

Verified Answer

  • Staphylococcus epidermidis and Streptococcus gallolyticus showed good growth in phenylethyl alcohol agar [PEA] and nutrient agar [NA] media. These bacteria are not inhibited by phenylethyl alcohol present in PEA medium. Gram staining showed both of them are Gram-positive bacteria.
  • Escherichia coli showed poor growth in PEA and good growth in NA. The growth of bacteria was inhibited by the phenylethyl alcohol present in PEA media. Gram staining showed that it is a Gram-negative bacterium.
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