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#antibiotics

27 public questions tagged with this topic.

Plant tissue culture 3

hpt gene provides resistance against:

Hygromycin B blocks translation elongation through interference with ribosomal translocation step on 80S plant ribosomes. Bacterial gene hpt from Escherichia coli encodes 341 amino acid phosphotransferase that uses ATP to phosphorylate C-4 hydroxyl of destomic acid moiety of antibiotic, creating inactive phospho derivative that cannot occupy decoding site. Expression cassette includes CaMV 35S promoter and terminator for constitutive high expression in plant cytosol. Enzyme kinetics show high affinity for hygromycin but not kanamycin or gentamicin, providing specific resistance profile. Select

Ref: Gritz & Davies Gene 1983 hpt mechanism; Herrera-Estrella Nature 1983 marker; NCBI NBK21414 phosphotransferase; PubMed 1848829 review.

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Plant tissue culture 3

Selective agent commonly used in hairy root culture is:

Effective recovery of transformed roots requires stringent chemical selection distinguishing transgenic from non-transformed background. Hygromycin B, aminoglycoside antibiotic, inhibits protein biosynthesis by stabilizing tRNA-ribosome interaction causing misreading. Plant cells are highly sensitive, with lethal concentration around 20 mg per liter for many dicots. Transformation vectors carry hpt gene under NOS promoter encoding hygromycin phosphotransferase that phosphorylates antibiotic hydroxyl group, preventing ribosome binding. Co-transformation with Ri T-DNA and binary vector results i

Ref: Beck Gene 1982 hpt hygromycin; Waldron Plant Mol Biol 1985; NCBI NBK131103 markers; Jefferson Plant Cell Rep 1987 hygromycin hairy root selection.

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Microbial Physiology

Which antibiotic is a broad-spectrum inhibitor of protein synthesis?

Protein synthesis machinery is highly conserved across domains yet bacterial 70S ribosomes differ structurally and compositionally from eukaryotic 80S ribosomes sufficiently to allow selective toxicity. Tetracycline is considered broad-spectrum because it penetrates both Gram-positive and Gram-negative envelopes via OmpF and OmpC porins and active transport, chelating Mg2+ to cross membranes as metal complex. It binds reversibly to a high-affinity pocket on the 30S small subunit, specifically helix 34 of 16S rRNA near the A-site where codon-anticodon interaction occurs. Binding sterically bloc

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 5: Tetracyclines and Broad-Spectrum Protein Synthesis Inhibition.

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Microbial Physiology

Which antibiotic is most effective against Gram-positive bacteria?

Gram-positive bacteria possess a uniquely thick, exposed mesh of peptidoglycan often 20 to 80 nm composed of many layers of glycan chains highly cross-linked, lacking an outer membrane that would otherwise act as permeability barrier. Vancomycin is a large hydrophilic glycopeptide of about 1449 Da with multiple aromatic rings and sugars, physicochemical properties that prevent efficient passage through porin channels and lipopolysaccharide outer membrane of Gram-negative bacteria but allow ready diffusion through porous peptidoglycan of Gram-positives. It binds non-covalently via five specific

Ref: Prescott's Microbiology, 11th ed., Chapter 8: Gram-Positive Cell Wall and Vancomycin Activity.

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Microbial Physiology

β-lactam antibiotics inhibit bacterial growth by:

Peptidoglycan assembly requires synthesis of soluble UDP-N-acetylmuramyl pentapeptide in cytoplasm, attachment to undecaprenyl phosphate carrier to form Lipid II, flipping to external leaflet, polymerization by transglycosylases and final cross-linking of pentapeptide stems by transpeptidases called penicillin-binding proteins possessing active-site serine. The transpeptidase recognizes the terminal D-Ala-D-Ala motif and catalyzes cleavage of the terminal D-alanine with concomitant formation of a new peptide bond between meso-diaminopimelate or lysine and D-alanine from adjacent strand, confer

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 17: Peptidoglycan Synthesis and Beta-Lactam Action.

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Microbial Physiology

Which antibiotic inhibits RNA polymerase, blocking transcription?

Bacterial transcription relies on a multisubunit DNA-dependent RNA polymerase holoenzyme core composed of two alpha, one beta encoded by rpoB, one beta-prime encoded by rpoC and one omega subunit plus sigma factor for promoter recognition. Rifampin belonging to rifamycin ansamycin class fits snugly into a deep hydrophobic pocket within the beta subunit located inside the main DNA-RNA channel but more than 12 angstroms away from the catalytic Mg2+ center that coordinates nucleotide addition. By occupying this pocket, the drug sterically obstructs the path for nascent RNA chains longer than two

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Bacterial RNA Polymerase and Rifampin Mechanism.

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Microbial Physiology

The primary mode of action of aminoglycosides is:

Aminoglycosides such as gentamicin, streptomycin and amikacin are polycationic molecules whose uptake depends on proton-motive force and electron transport, explaining reduced activity against anaerobes and biofilms. Once concentrated inside bacterial cytoplasm, they lodge with exceptionally high affinity in the decoding center formed by helix 44 of 16S rRNA within the 30S small ribosomal subunit at the A-site where aminoacyl-tRNA selection and proofreading occur during translation. The drug forces an abnormal closed conformation of rRNA, impairing proofreading and inducing misreading of mRNA

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 5: Aminoglycosides and 30S Ribosomal Subunit Inhibition.

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Microbial Physiology

Which of the following antibiotics is bactericidal?

Glycopeptide antibiotics exhibit true bactericidal killing activity rather than simple growth arrest, a distinction critical for treating severe Gram-positive infections. Vancomycin functions by recognizing and binding with high affinity to the D-alanyl-D-alanine dipeptide terminus of the peptidoglycan precursor lipid II that is exposed on the outer face of the cytoplasmic membrane after flipping from the cytoplasm. This binding creates a large steric cap that prevents transglycosylases from polymerizing the alternating N-acetylglucosamine and N-acetylmuramic acid glycan chain and simultaneous

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 5: Glycopeptide Antibiotics and Cell Wall Inhibition.

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