[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fv1JlszqwMcBQ_TgMBTKqJe784K53kliJqe7rwVrQ_Ro":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":123},[4,8,12,16,20,24,28],{"title":5,"slug":6,"path":7},"About Microbeonline.com","about-microbeonline-com","\u002Fabout-microbeonline-com\u002F",{"title":9,"slug":10,"path":11},"About Me","about-me","\u002Fabout-microbeonline-com\u002Fabout-me\u002F",{"title":13,"slug":14,"path":15},"Advertise with Us","advertise-us","\u002Fadvertise-us\u002F",{"title":17,"slug":18,"path":19},"Privacy Policy","privacy-policy","\u002Fprivacy-policy\u002F",{"title":21,"slug":22,"path":23},"Abbreviations","abbreviations","\u002Fabbreviations\u002F",{"title":25,"slug":26,"path":27},"Microbes","microbes","\u002Fmicrobes\u002F",{"title":29,"slug":30,"path":31},"Books","recommended-books","\u002Frecommended-books\u002F",{"type":33,"data":34},"blog",{"slug":35,"title":36,"description":36,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":39,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"body":43,"faq":44,"tags":45,"related":47},"tetracyclines-mode-of-action-and-mechanism-of-resistance","Tetracyclines: Mode of Action and Mechanism of Resistance",null,"Nisha Rijal","2020-07-02","2026-07-05",false,"bacteriology","![Structure of Tetracyclines - Tetracyclines](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FStructure-of-tetracyclines.png)Figure: Tetracyclines\n\nTetracyclines are a group of broad-spectrum antibiotics effective against a wide range of Gram-positive, Gram-negative bacteria, and several intracellular bacterial pathogens such as chlamydia, rickettsia, mycoplasma, etc. Tetracyclines are usually considered as **bacteriostatic antibiotics**.\n\n> The first tetracycline antibiotic, chlortetracycline and oxytetracycline were discovered in the late 1940s, and were derived naturally from Streptomyces aureofaciensandS. rimosus, respectively. Currently there are various semi-synthetic tetracyclines available.\n\nStructurally, tetracycline molecules comprise a linear fused tetracyclic nucleus (rings designated A, B, C, and D) to which a variety of functional groups are attached.\n\n## Classification of Tetracyclines\n\nConventionally, tetracyclines have three generations;\n\n1. First-generation include tetracyclines which are obtained from biosynthesis such as, chlortetracycline, oxytetracycline.\n2. Second generation tetracyclines comprise of doxycycline, minocycline, etc. obtained from semi-synthesis of tetracycline.\n3. Third generation includes tigecycline, which obtained from total synthesis.\n\n## Mode of Action of Tetracyclines\n\n![Mechanism of action of tetracyclines](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002Fmechanism-of-tetracycline.jpg)Figure: Mechanism of action of tetracyclines\n\nTetracyclines inhibit bacterial protein synthesis through reversible binding to bacterial 30S ribosomal subunits, which prevent binding of new incoming amino acids (aminoacyl-tRNA) and thus interfere with peptide growth.\n\n## Resistance Mechanism against Tetracyclines\n\nMolecular mechanisms of tetracycline resistance include 1. efflux 2. ribosomal protection 3. reduced permeability 4. ribosome [mutation](\u002Fmutation\u002F) and 5. enzymatic inactivation\n\n![Tetracyclines Mechanism of Resistance - Molecular mechanisms of tetracycline resistance.(A)Efflux, exclusion,(B)ribosome protection,(C)ribosome modification, and(D)enzymatic inactivation.](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FTetracyclines-mechanism-of-Resistance.jpg)Figure: Molecular mechanisms of tetracycline resistance.(A)Efflux, exclusion,(B)ribosome protection,(C)ribosome modification, and(D)enzymatic inactivation.\n\n#### Efflux Pumps\n\nA major way to limit access of tetracycline to ribosomes is to reduce intracellular concentrations of tetracycline by pumping the antibiotic out of the cell at a rate equal to or greater than its uptake. There are various classes of efflux pumps present in both Gram-negative and Gram-positive bacteria. Of them, TetA, is the most frequently occurring tetracycline-resistance determinant in Gram-negative bacteria. Efflux proteins exchange a proton (H+) for the tetracycline molecule against a concentration gradient and pump out the tetracycline from the cell. Most tetracycline efflux pumps confer resistance to tetracycline, but are less effective against second-generation doxycycline and minocycline, and confer little or no resistance to third-generation glycylcyclines, such as tigecycline.\n\n#### Ribosomal Protection\n\nAcquired tetracycline resistance is mediated by the production of elongation-factor G (EF-G)-like ribosomal protection proteins that interact with the ribosome in such a way that protein synthesis is unaffected by the presence of the antibiotic. The most common determinants of ribosomal protection have been those encoded by the tet(M) and tet(O) genes.\n\n#### Reduced Drug Permeability\n\nThis is achieved through morphological changes and the modification or reduced expression of porins and likely contributes to clinical tetracycline resistance.\n\n#### Ribosomal Mutations\n\nThis mechanism is less common in conferring resistance to tetracyclines, however, some resistance-conferring mutations and deletions around the tetracycline-binding site, point mutations in the 16S ribosome also confer tetracycline resistance.\n\n#### Enzyme Inactivation\n\nA group of enzymes called **tetracycline destructase** selectively oxidize tetracyclines leading to covalent destruction of the antibiotic scaffold thus destroying antimicrobial activity permanently. Newer generation of tetracyclines such as minocycline and tigecycline, which resist efflux pumps and ribosomal protection are also affected by these mechanisms of resistance.\n\n#### References and Further Readings\n\n- Askari Rizvi, S. F. (2018). [Tetracycline: Classification, Structure Activity Relationship, and Mechanism of Action as a Theranostic Agent for Infectious Lesions-A Mini-Review](https:\u002F\u002Fdoi.org\u002F10.26717\u002FBJSTR.2018.07.001475). *Biomedical Journal of Scientific & Technical Research*, *7*(2).\n- Chopra, I., & Roberts, M. (2001). [Tetracycline Antibiotics: Mode of Action, Applications, Molecular Biology, and Epidemiology of Bacterial Resistance](https:\u002F\u002Fmmbr.asm.org\u002Fcontent\u002F65\u002F2\u002F232). *Microbiology and Molecular Biology Reviews*, *65*(2), 232–260.\n- Markley, J. L., & Wencewicz, T. A. (2018). [Tetracycline-Inactivating Enzymes.Frontiers in Microbiology](https:\u002F\u002Fwww.frontiersin.org\u002Farticles\u002F10.3389\u002Ffmicb.2018.01058\u002Ffull), *9*.",[],[46],"antimicrobials-moa-amr",[48,69,77,83,89,115],{"slug":49,"title":50,"description":51,"seoTitle":37,"seoDescription":37,"author":52,"createdDate":53,"lastUpdatedDate":40,"draft":41,"category":54,"image":37,"faq":55,"tags":68},"mechanism-of-action-of-antiviral-drugs","Mechanism of Action of Antiviral Drugs: How Each Drug Class Targets the Viral Life Cycle","How acyclovir exploits viral thymidine kinase, why protease inhibitors stop HIV assembly, and which drug class blocks neuraminidase to trap flu virions.","Srijana Khanal","2022-07-26","virology",[56,59,62,65],{"question":57,"answer":58},"Why is it harder to develop antiviral drugs than antibiotics?","Viruses replicate inside host cells using much of the host's own machinery, so drugs that disrupt viral replication often damage host cells too. Successful antivirals must target viral-specific enzymes or proteins — steps the virus performs that the host cell doesn't — to achieve selective toxicity.",{"question":60,"answer":61},"Why does acyclovir work against herpes but not most other viruses?","Acyclovir requires viral thymidine kinase (TK) — an enzyme present in HSV and VZV-infected cells but not in healthy human cells — to be phosphorylated to its active form. Only herpesvirus-infected cells can activate it, giving acyclovir selective toxicity for those infections. Viruses that don't encode their own thymidine kinase (most viruses) are unaffected.",{"question":63,"answer":64},"Why do HIV patients need at least three antiviral drugs simultaneously?","HIV's RNA polymerase is error-prone, generating millions of genetic variants with each replication cycle. A single drug that inhibits one step will select for pre-existing resistant mutants in this pool. Combining three drugs from two or more drug classes means a virus would need to acquire resistance mutations to all three drugs simultaneously to survive — a near-impossible event in a single replication cycle.",{"question":66,"answer":67},"Why don't neuraminidase inhibitors work against RSV?","RSV has no neuraminidase enzyme. Neuraminidase inhibitors (oseltamivir, zanamivir) work by blocking the NA enzyme that influenza uses to cleave sialic acid bonds and release new virions from the host cell surface. Without a neuraminidase target, there is nothing for these drugs to inhibit in RSV.",[46],{"slug":70,"title":71,"description":72,"seoTitle":37,"seoDescription":37,"author":52,"createdDate":73,"lastUpdatedDate":40,"draft":41,"category":74,"image":37,"faq":75,"tags":76},"mechanism-of-action-of-antifungal-drugs","Mechanism of Action of Antifungal Drugs","Antifungal drug mechanisms — polyenes, azoles, echinocandins, allylamines, and antimetabolites explained with clinical drug-to-organism mapping, spectrum comparison table, and connection to azole resistance and treatment selection errors.","2022-07-20","mycology",[],[46],{"slug":78,"title":79,"description":79,"seoTitle":37,"seoDescription":37,"author":52,"createdDate":80,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":81,"tags":82},"mechanisms-of-action-of-antibiotics-an-overview","Mechanisms of Action of Antibiotics: An Overview","2022-07-02",[],[46],{"slug":84,"title":85,"description":85,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":86,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":87,"tags":88},"macrolides-action-resistance","Macrolides: Mode of Action, Mechanism of Resistance","2020-07-07",[],[46],{"slug":90,"title":91,"description":92,"seoTitle":37,"seoDescription":37,"author":93,"createdDate":94,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":95,"tags":114},"beta-lactam-antibiotics-mechanism-action-resistance","Beta-Lactam Resistance: Three Mechanisms, and Why Only One Can Be Outsmarted With a Combination Drug","PBP mimicry, beta-lactamase hydrolysis, altered targets, and porin loss, the three ways bacteria defeat beta-lactams, and why a clavulanate or sulbactam combination only rescues one of them.","Acharya Tankeshwar","2020-03-31",[96,99,102,105,108,111],{"question":97,"answer":98},"What are the three mechanisms of beta-lactam resistance?","Enzymatic destruction of the antibiotic by beta-lactamase, altered antibiotic targets such as modified penicillin-binding proteins, and decreased drug uptake, usually through changes in outer membrane porins in gram-negative bacteria.",{"question":100,"answer":101},"Why does a beta-lactamase inhibitor combination work against some resistant organisms but not MRSA?","Inhibitor combinations like amoxicillin\u002Fclavulanate work by disabling the beta-lactamase enzyme, rescuing the antibiotic. MRSA resistance instead works through an altered penicillin-binding protein, PBP2a, that simply does not bind the drug at all, so no enzyme inhibitor can fix it.",{"question":103,"answer":104},"Where do gram-positive and gram-negative bacteria keep their beta-lactamase enzymes?","Gram-positive bacteria secrete beta-lactamase into the surrounding environment. Gram-negative bacteria retain it within the periplasmic space, between the inner and outer membranes.",{"question":106,"answer":107},"What natural structure does the beta-lactam ring mimic?","It mimics the terminal D-Ala-D-Ala peptide sequence, the natural substrate that transpeptidase enzymes use during cell wall peptidoglycan synthesis.",{"question":109,"answer":110},"Can all beta-lactamase enzymes hydrolyze all beta-lactam antibiotics?","No. For example, staphylococcal beta-lactamase readily hydrolyzes penicillin and its derivatives but fails to hydrolyze many cephalosporins and imipenem.",{"question":112,"answer":113},"Why is decreased drug uptake a resistance mechanism mainly seen in gram-negative bacteria?","Gram-negative bacteria have an outer membrane that beta-lactams must cross through porin channels to reach their target. Changes in porin number or structure can substantially reduce drug entry, a barrier that gram-positive bacteria, lacking an outer membrane, do not have.",[46],{"slug":116,"title":117,"description":118,"seoTitle":37,"seoDescription":37,"author":93,"createdDate":119,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":120,"tags":121},"inducible-clindamycin-resistance-d-test-principle-procedure-and-interpretation","D-Test for Inducible Clindamycin Resistance: Why \"Clindamycin-Susceptible\" Isn't Always True","Erythromycin-resistant, clindamycin-susceptible on a routine report, but is it really? The D-test catches inducible resistance that standard susceptibility testing misses, and that has caused documented clindamycin treatment failures.","2013-08-02",[],[122,46],"antimicrobial-susceptibility-testing",[124,130,137,142,146,150,155,160,163,167],{"slug":125,"name":93,"description":126,"image":127,"body":128,"postCount":129},"acharya-tankeshwar","Editor-in-chief","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Ftankeshwar-acharya-author-microbeonline.jpg","***Tankeshwar Acharya, MSc (Medical Microbiology)***\n\n*Tankeshwar Acharya is an Assistant Professor in the Department of Microbiology at Patan Academy of Health Sciences (PAHS), Nepal, where he has been teaching and practicing clinical microbiology for over 14 years. He is the founder of Microbe Online, one of the leading free microbiology education resources on the web, covering bacteriology, mycology, parasitology, immunology, and clinical laboratory diagnostics written from direct experience in both the classroom and the diagnostic laboratory.*",432,{"slug":131,"name":132,"description":133,"image":134,"body":135,"postCount":136},"ashma-shrestha","Ashma Shrestha","SEO Copywriter and Science Communicator\nKathmandu, Nepal","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fashma-shrestha.png","Ashma Shrestha holds a Master of Science in Medical Microbiology from the Institute of Science and Technology (IOST), Tribhuvan University, Nepal, where she developed a strong foundation in virology, molecular biology, and diagnostic microbiology.\n\nShe now works as an SEO Copywriter at Resolution Digital, where she combines her scientific training with research-driven content strategy. She is certified in Google Analytics and Google Business Profile (GBP), and brings a data-informed approach to science communication writing content that is not only accurate but structured to reach and serve the students who need it most.\n\nAt microbeonline, Ashma contributes articles primarily in virology and molecular biology, areas she finds most compelling for their mechanistic depth and their growing clinical relevance. Her writing reflects the same standard the site is built on: factual rigor, clear explanation of the *why* behind microbiology concepts, and content that helps students move from memorization to genuine understanding.\n\nShe is passionate about making complex microbiological concepts accessible without sacrificing accuracy; a skill that sits at the intersection of her scientific training and her professional work in content and SEO.",81,{"slug":138,"name":139,"description":140,"image":37,"body":37,"postCount":141},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":143,"name":144,"description":140,"image":37,"body":37,"postCount":145},"samikshya-acharya","Samikshya Acharya",20,{"slug":147,"name":148,"description":140,"image":37,"body":37,"postCount":149},"alisha-tripathi","Alisha Tripathi",6,{"slug":151,"name":152,"description":153,"image":37,"body":37,"postCount":154},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":156,"name":157,"description":158,"image":37,"body":37,"postCount":159},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":161,"name":52,"description":140,"image":37,"body":37,"postCount":162},"srijana-khanal",18,{"slug":164,"name":165,"description":158,"image":37,"body":37,"postCount":166},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":168,"name":38,"description":140,"image":37,"body":169,"postCount":170},"nisha-rijal","**Nisha Rijal** is a microbiologist and quality assurance specialist. She served for nearly 12 years as a microbiologist at the National Public Health Laboratory (NPHL), Nepal's national reference laboratory, and continues to work as a consultant microbiologist in international public health organization. ",51]