Microbiology

Step 1: Reviewing Gram-negative bacteria properties.
Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane rich in lipopolysaccharides. This outer membrane forms a barrier, making them more resistant to antibiotics compared to Gram-positive bacteria.
Step 2: Analyzing the options.
(A) Incorrect — Gram-negative bacteria turn pink, not purple, after staining.
(B) Correct — Their outer membrane increases antibiotic resistance.
(C) Incorrect — They have a thin, not thick, peptidoglycan layer.
(D) Incorrect — They DO contain an outer membrane, which is a key feature.
Step 3: Conclusion.
Option (B) correctly describes Gram-negative bacteria.

Step 1: Understanding DRT.
The decimal reduction time (DRT) is the time required to reduce the bacterial population by 90%. The population decreases by a factor of 10 for every DRT. If the initial population is , and we want to reduce it to 10, we need to reduce it by five orders of magnitude (from to ).

Step 2: Calculating the time.
Each DRT reduces the population by a factor of 10. So, to reduce the population from to , it takes:

Step 3: Conclusion.
The required time to reduce the bacterial population to 10 is .

Step 1: Using the exponential growth formula.
The exponential growth formula for a population is given by: Where:
- is the final population size
- is the initial population size
- is the number of generations
We are given:
-
-
- Time = 6 hours
We need to find , the number of generations: Taking the logarithm of both sides: Step 2: Calculating generation time.
The generation time is the time per generation: Final Answer:

Step 1: Analyze the options.
- (A) Correct: Deinococcus radiodurans is known for its unique cell wall structure that includes several layers, and unlike typical Gram-negative bacteria, it lacks lipid A in the outer membrane.
- (B) Incorrect: Deinococcus radiodurans has ornithine in the peptidoglycan structure, but this does not replace diaminopimelic acid in cross-bridges.
- (C) Incorrect: Deinococcus radiodurans is a Gram-positive bacterium with a unique cell wall structure.
- (D) Correct: Deinococcus radiodurans is extremely radiation-resistant and can survive exposures up to 15000 Gy of ionizing radiation.
Step 2: Conclusion.
The correct statements are that Deinococcus radiodurans has a cell wall with several layers and lacks lipid A in its outer membrane, and it can survive extreme ionizing radiation exposure.
Final Answer:

Step 1: Recall polymyxins.
Polymyxins are cationic polypeptide antibiotics that interact with lipopolysaccharides (LPS) of Gram-negative bacterial outer membranes.
Step 2: Mechanism.
- They bind to phospholipids.
- Disrupt membrane permeability.
- Cause leakage of cellular contents and bacterial death.
Step 3: Rule out other mechanisms.
- (A) Inhibition of 30S ribosome: Done by aminoglycosides, tetracyclines.
- (B) Inhibition of peptidoglycan synthesis: Done by β-lactams, vancomycin.
- (C) Inhibition of DNA replication: Done by quinolones.
Final Answer:

Step 1: Recall peptidoglycan biosynthesis.
- Transglycosidases: Link glycan chains.
- Transpeptidases: Cross-link peptide chains.
- DD-carboxypeptidases: Modify cross-links in PG remodeling.
Step 2: Exclude non-participant.
Transaminases are involved in amino acid metabolism, not in PG cross-linking or remodeling.
Final Answer:

Step 1: Importance of gene markers.
For bacterial phylogenetic studies, highly conserved genes are required to trace evolutionary relationships across species.
Step 2: Role of 16S rRNA.
The 16S ribosomal RNA (rRNA) gene is universally present in bacteria, highly conserved, and contains both conserved and variable regions.
- Conserved regions help in universal primer design.
- Variable regions allow differentiation among bacterial species.
Step 3: Exclude other options.
- pbpG and slyD are not standard markers for evolutionary divergence.
- 18S rRNA is used in eukaryotes (not bacteria).
Step 4: Conclusion.
Hence, the 16S rRNA gene is the gold standard for measuring bacterial evolutionary divergence.
Final Answer: