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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. Selection scheme uses 25 to 50 mg per liter in solid or liquid medium, causing necrosis of untransformed tissue within 10 days. Stable integration confirmed by PCR and Southern hybridization. Unlike genes conferring ampicillin resistance that function only in bacteria, hpt functions efficiently in plant cells, making it standard selectable marker for hairy root and embryogenic callus transformation experiments requiring clean antibiotic selection. Mechanism of detoxification involves ATP dependent phosphorylation preventing antibiotic binding to decoding center of ribosome, thus preserving translation fidelity. Enzyme purified shows Km of 0.1 mM for hygromycin. In plants, expression localized to cytoplasm, stable under field conditions without fitness cost. Co-transformation frequency with rol genes high due to simultaneous integration, enabling efficient selection of transformed

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 in roots expressing both rol genes and hpt, growing on hygromycin containing basal medium while non-transformed roots bleach. Kanamycin sometimes fails due to endogenous resistance in some species, making hygromycin selection more reliable in hairy root protocols for tobacco, tomato, and Withania. This system ensures high frequency recovery of genuinely transformed events for downstream metabolic studies. Killing curve determined by exposing untransformed roots to 0 to 100 mg per liter hygromycin for 14 days. Selection maintained for two subcultures to eliminate chimeric sectors. Molecular confirmation via amplification of hpt cassette confirms integration. Use of hygromycin avoids cross-resistance issues observed with kanamycin in species containing endogenous nptII-like activity, making it preferred for many dicots.

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 blocks accommodation of aminoacyl-tRNA into the A-site, preventing addition of new amino acid residues to the growing polypeptide chain without affecting peptidyl transferase center directly. Unlike more selective Gram-positive agents such as penicillin G or vancomycin that target peptidoglycan cross-linking, or bacitracin that blocks recycling of C55 lipid carrier, tetracycline inhibits essentially all bacterial translation including Gram-positives, Gram-negatives, atypicals and protozoa. Resistance arises via efflux pumps encoded by tetA and tetB and ribosomal protection proteins tetM and tetO that dislodge drug, widespread due to plasmid spread.

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 hydrogen bonds to the carbonyl and amide groups of the D-Ala-D-Ala terminus of Lipid II precursors, sequestering substrate from transglycosylase and transpeptidase enzymes. Without free substrate access, cell wall maturation halts and septal synthesis fails. The clinical consequence is potent bactericidal activity against Staphylococcus aureus including MRSA, Enterococcus faecalis and faecium, Streptococcus pneumoniae and Clostridioides difficile causing pseudomembranous colitis. Polymyxins target lipid A of lipopolysaccharide of Gram-negatives, aminoglycosides require oxygen-dependent uptake suited to Gram-negatives, while beta-lactams vary in spectrum, making vancomycin archetypal for Gram-positive coverage and reserve status.

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, conferring mechanical rigidity essential to resist turgor. Beta-lactam antibiotics including penicillins, cephalosporins, carbapenems are structural mimics of D-Ala-D-Ala dipeptide geometry. The strained four-membered beta-lactam ring acts as a suicide substrate, rapidly acylating the active-site serine of PBPs and forming a stable, long-lived acyl-enzyme intermediate that is hydrolyzed extremely slowly. Transpeptidation cannot proceed, new cross-links fail to form, and housekeeping autolysins continue to remodel old wall causing net wall degradation. Cells swell, lose shape and lyse, explaining activity preferentially against actively growing cells, Gram-positive susceptibility and resistance via beta-lactamase production or PBP2a expression.

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 to three nucleotides, preventing productive elongation after initiation. The RNA-DNA hybrid cannot extend beyond short abortive transcripts, full-length mRNA synthesis collapses and transcription aborts. Mammalian RNA polymerases I, II, III lack this conserved pocket, giving exquisite selective toxicity. Single point mutations in rpoB at cluster I, especially S531L and H526Y, alter pocket geometry and confer high-level resistance well documented in Mycobacterium tuberculosis surveillance, which underlies rifampin use as first-line antitubercular and as molecular marker of multidrug resistance.

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 codons, premature termination, frameshifting and accumulation of aberrant, misfolded membrane proteins. These defective proteins insert into inner membrane, increasing permeability, dissipating membrane potential and promoting further drug uptake, a positive feedback loop that contributes to concentration-dependent bactericidal action and post-antibiotic effect. Aminoglycosides also block formation of the 70S initiation complex comprising mRNA, formyl-methionyl-tRNA and subunits and inhibit translocation of peptidyl-tRNA from A to P site. Synergy with cell-wall agents that enhance entry underlies combination therapy for Gram-negative sepsis.

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 simultaneously blocks transpeptidases, the penicillin-binding proteins, from forming peptide cross-links between stems. Without cross-linking, the nascent peptidoglycan mesh in Gram-positive organisms with 20-80 nm thick wall remains mechanically weak and cannot withstand internal turgor pressures of 20 atmospheres, leading to activation of autolysins and osmotic lysis. Tetracycline, erythromycin and clindamycin bind reversibly to bacterial ribosomes, pausing translation temporarily and causing bacteriostatic arrest where cells resume growth after drug removal, explaining clinical preference for vancomycin in MRSA bacteremia and endocarditis where rapid killing is desired.

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

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