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#penicillin

7 public questions tagged with this topic.

Microbial Physiology

Which antibiotic disrupts bacterial cell wall synthesis?

Peptidoglycan assembly begins in cytoplasm with formation of UDP-MurNAc-pentapeptide containing terminal D-alanyl-D-alanine, followed by linkage to lipid carrier undecaprenyl phosphate, also termed bactoprenol, which flips precursor across inner membrane to periplasmic side. Once externalized, class A transglycosylases polymerize glycan chains extending the sacculus and DD-transpeptidases, members of penicillin-binding protein family such as PBP1a, PBP1b, PBP2 and PBP3, catalyze crosslink between D-alanine of one stem and diamino acid of neighboring stem, releasing terminal D-alanine and forming robust mesh. Penicillin and other beta-lactams are structural analogs of D-Ala-D-Ala dipeptide; they enter active site and acylate catalytic serine irreversibly, blocking transpeptidation and leaving newly inserted material poorly crosslinked. Growing cells with weakened wall cannot contain internal turgor and undergo lysis aided by dysregulated autolytic amidases and lytic transglycosylases. Streptomycin binds 30S ribosomal subunit causing misreading, ciprofloxacin inhibits DNA gyrase, rifampin inhibits beta subunit of bacterial RNA polymerase, distinguishing wall-active agents from other classes. This mechanistic insight is relevant for competitive examinations such as CSIR-NET and GATE, where understanding molecular detail rather than memorization enables accurate interpretation of experimental data and pathway interconnections.

Ref: Walsh, Antibiotics: Actions, Origins, Resistance, ASM Press; NCBI Bookshelf, Beta-Lactam Mechanism.

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Microbial Physiology

Which enzyme is inhibited by penicillin?

Bacterial peptidoglycan final crosslinking step catalyzed by transpeptidases penicillin binding proteins class B enzymes PBP2 for elongation, PBP3 FtsI for division. They cleave C terminal D alanine D alanine dipeptide from pentapeptide side chain MurNAc L Ala D Glu mDAP D Ala D Ala, forming acyl enzyme intermediate through active site serine nucleophile, then transfer to amino group acceptor meso diamino pimelic acid or L lysine of neighboring strand creating peptide crossbridge essential for wall rigidity. Beta lactam antibiotics penicillin contain four membered ring mimicking D Ala D Ala conformation fitting active site, acylating catalytic serine irreversibly forming stable penicilloyl enzyme unable to deacylate, blocking transpeptidation. Nascent peptidoglycan remains linear uncrosslinked degraded by endogenous autolysins lytic transglycosylases leading to osmotic lysis especially during growth when wall remodeling high. Gyrase target quinolones, RNA polymerase target rifampicin, ribosomal peptidyl transferase chloramphenicol. Thus penicillin specifically inhibits transpeptidase activity, mechanistic basis for bactericidal action and synergy with beta lactamase inhibitors clavulanate restoring efficacy against resistant strains.

Ref: Tipper & Strominger, PNAS 1965, Penicillin inhibits transpeptidase PBP crosslinking peptidoglycan.

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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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SPECIES INTERACTION

Penicillin secreted by fungi harming bacteria is an example of:

Penicillin production illustrates antibiosis and is commonly classified as amensalism when susceptible bacteria are harmed while the fungus is assumed to be unaffected. Penicillin binds penicillin-binding proteins and inhibits transpeptidation of peptidoglycan, weakening the bacterial cell wall. Actively growing cells then become vulnerable to osmotic lysis. In ecological sign notation, this simplified interaction is (0, −), unlike parasitism (+, −), mutualism (+, +), or competition (−, −). However, assigning zero effect to the fungus is an assumption. If bacterial suppression releases nutrients or space and improves fungal fitness, the same mechanism functions as interference competition rather than strict amensalism. Natural antibiotic concentrations, diffusion, microbial resistance, and community context determine the actual outcome; laboratory inhibition does not automatically establish the producer’s benefit. Penicillin also affects only bacteria with susceptible cell-wall machinery and is ineffective against organisms lacking peptidoglycan. The example is valuable because it separates the biochemical mechanism—an inhibitory metabolite—from the ecological classification, which depends on measured fitness effects for both participants.

Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 8-13

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SPECIES INTERACTION

A fungus secreting antibiotics that kill bacteria is an example of:

Antibiotic secretion by a fungus that suppresses nearby bacteria is a standard example of amensalism, specifically antibiosis, when the fungus is treated as unaffected and the bacteria are harmed. The fungal metabolite inhibits bacterial cell-wall synthesis, protein synthesis, membrane function, or another essential process, depending on the compound. Penicillin, for instance, blocks transpeptidation during peptidoglycan assembly, causing susceptible growing cells to lyse. Ecological sign notation represents this simplified outcome as (0, −): no measurable effect on the producer and a negative effect on the recipient. This differs from parasitism, where the beneficiary obtains resources through an intimate association with a host, and from mutualism or commensalism, which do not impose this negative effect. In nature, the producer may indirectly benefit through reduced competition, making some cases better interpreted as interference competition rather than strict amensalism. The textbook classification assumes no demonstrated benefit to the fungus. Recognizing that assumption is important because interaction categories describe net fitness effects under specified conditions, not immutable properties of the species.

Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 8-13

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