Q.3 Which of the following statements is correct?
(A) Gram negative bacteria are colored purple after Gram staining
(B) Gram negative bacteria are commonly more resistant to antibiotics than Gram positive bacteria
(C) Gram negative bacteria cell wall consists of a thick layer of peptidoglycan outside the plasma membrane
(D) Cell wall of Gram negative bacteria does not contain an outer membrane
Gram Negative Bacteria: Correct Statement on Gram Staining, Antibiotic Resistance, and Cell Wall Structure
Gram-negative bacteria appear pink or red after Gram staining and possess unique cell wall features that enhance antibiotic resistance. The correct statement among the options is (B), as their structure provides a natural barrier against many drugs. This detailed analysis evaluates each option for CSIR NET Life Sciences preparation.
Option Analysis
Option (A): Gram negative bacteria are colored purple after Gram staining
Gram-negative bacteria lose the crystal violet stain during decolorization due to their thin peptidoglycan layer and outer membrane, appearing pink/red from the safranin counterstain. Gram-positive bacteria retain the purple color. This statement is incorrect.
Option (B): Gram negative bacteria are commonly more resistant to antibiotics than Gram positive bacteria
Gram-negative bacteria exhibit higher intrinsic resistance owing to their outer membrane, which blocks antibiotic entry, along with efflux pumps and periplasmic enzymes like beta-lactamases. Gram-positive bacteria, lacking this outer membrane, are generally more susceptible. This statement is correct.
Option (C): Gram negative bacteria cell wall consists of a thick layer of peptidoglycan outside the plasma membrane
Gram-negative bacteria have a thin peptidoglycan layer (7-8 nm, 10% of cell wall dry weight) between inner and outer membranes. Gram-positive bacteria feature a thick peptidoglycan layer (20-80 nm). This statement is incorrect.
Option (D): Cell wall of Gram negative bacteria does not contain an outer membrane
Gram-negative bacteria possess a distinctive asymmetric outer membrane with lipopolysaccharides (LPS) in the outer leaflet, forming a key permeability barrier. This is absent in Gram-positive bacteria. This statement is incorrect.
