After Gram staining, Gram-positive bacteria appear:
Pink
Red
Purple
Colorless
The Correct Answer is C
A. Pink: Pink indicates Gram-negative bacteria, which do not retain the crystal violet stain during the decolorization step and are counterstained with safranin. Gram-positive bacteria are not pink after Gram staining.
B. Red: Red is another way to describe the color of Gram-negative bacteria after counterstaining with safranin. Gram-positive bacteria do not take up the red counterstain because they retain the primary crystal violet stain.
C. Purple: Gram-positive bacteria appear purple after Gram staining. Their thick peptidoglycan cell walls trap the crystal violet-iodine complex even after alcohol or acetone decolorization, allowing them to retain the purple color and be distinguished from Gram-negative bacteria.
D. Colorless: Colorless cells would indicate a failure to stain or a decayed sample. Gram-positive bacteria are never colorless after a proper Gram staining procedure.
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Related Questions
Correct Answer is B
Explanation
A. A thin peptidoglycan layer: A thin peptidoglycan layer is characteristic of Gram-negative bacteria, not Gram-positive bacteria. In Gram-negative cells, the thin layer is located between the inner cytoplasmic membrane and the outer membrane.
B. A thick peptidoglycan layer: Gram-positive bacteria have a thick, multilayered peptidoglycan wall that provides structural strength, maintains cell shape, and protects against osmotic pressure. This thick layer also traps the crystal violet-iodine complex during Gram staining, giving Gram-positive cells their purple color under a microscope.
C. No peptidoglycan: All bacteria except mycoplasmas have peptidoglycan in their cell walls. Gram-positive bacteria rely heavily on this thick peptidoglycan layer for structural integrity, so that they have no peptidoglycan is incorrect.
D. An outer membrane only: Gram-positive bacteria lack an outer membrane. The presence of an outer membrane is a defining feature of Gram-negative bacteria, which lies outside their thin peptidoglycan layer. Gram-positive bacteria instead have a thick peptidoglycan wall often containing teichoic acids.
Correct Answer is C
Explanation
A. Louis Pasteur: Louis Pasteur is known for his contributions to germ theory, pasteurization, and vaccines, including those for rabies and anthrax. While he advanced microbiology, he did not develop the Gram stain technique.
B. Robert Koch: Robert Koch established the germ theory of disease and developed techniques for isolating and culturing bacteria, including identifying the causative agents of tuberculosis and anthrax. However, he was not responsible for creating the Gram staining method.
C. Hans Christian Gram: Hans Christian Gram, a Danish bacteriologist, developed the Gram stain in 1884. This differential staining technique allows bacteria to be classified as Gram-positive or Gram-negative based on cell wall properties, greatly aiding in bacterial identification and diagnosis of infections.
D. Alexander Fleming: Alexander Fleming discovered penicillin, the first widely used antibiotic, in 1928. While pivotal in microbiology and medicine, he did not contribute to staining techniques such as the Gram stain.
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