# Phage Therapy > Phage therapy in MBBS Pharmacology: lytic versus lysogenic phages, spectrum, self-dosing at infection site, resistance, regulatory status in India. - Canonical URL: https://prepelephant.com/topics/mbbs/pharmacology/phage-therapy - Exam / course: MBBS · Subject: Pharmacology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Phage Therapy", PrepElephant, https://prepelephant.com/topics/mbbs/pharmacology/phage-therapy ## Direct answer Viruses that prey exclusively on bacteria — bacteriophages — were used against infections a century ago in the work of d'Herelle and the Georgian Eliava tradition, were buried by the antibiotic era, and have returned with the antimicrobial-resistance crisis. Only lytic phages qualify therapeutically: they inject their genome, replicate and lyse the cell, whereas lysogenic (temperate) phages integrate into the host chromosome and can carry toxin or resistance genes between bacteria — transduction — an absolute disqualifier. Their pharmacology is unlike any antibiotic: exquisite specificity requiring sensitivity testing or cocktails, self-amplification at the infection site, and synergy with antibiotics rather than competition. Evidence remains largely case-series based, so regulatory status everywhere is compassionate or investigational, India included. ## What you must remember - **Lytic over lysogenic — the first rule:** therapy uses strictly lytic phages because temperate phages integrate (lysogeny), mediate horizontal gene transfer by transduction, and can convert benign strains into pathogens (diphtheria, cholera toxin, Shiga toxin origins). - **Specificity and its cost:** receptor-level specificity (LPS, pili, capsule) means a single phage covers a fraction of clinical strains — cocktails of several phages, or pharmacy-made combinations matched to the isolate, are standard practice. - **Self-replicating pharmacokinetics:** phages amplify where the bacterial target exists ("auto-dosing") and decay when it is cleared — a dose-effect relationship antibiotics never show. - **Host interaction with immunity:** neutralising antibodies can develop against repeat doses; pre-existing antibodies limit IV use; concurrent antibiotics often improve outcomes (phage-antibiotic synergy, documented in vitro and in cases). - **Resistance is unavoidable but manageable:** bacteria evolve receptor loss or restriction-modification defence; the countermeasure is cocktail rotation, in contrast to the slower genetics of antibiotic resistance. - **Bacterial trade-off of resistance:** losing the phage receptor often costs virulence or antibiotic resistance — the outer-membrane porin changes that expel a phage can re-sensise to carbapenems, a favourite discussion point. - **Manufacturing demands:** endotoxin removal, sterile purification, and stability for liquid and lyophilised preparations; no probiotic-style laxity is permissible for parenteral products. - **Evidence and regulation:** landmark cases include intravenous phage cocktails for disseminated Mycobacterium abscessum in cystic fibrosis (published around 2019) and compassionate-use series in Pseudomonas and Acinetobacter; randomised evidence is still limited, and no phage product holds standard marketing approval in India — usage is investigational or imported under exception pathways. ## A rescue case that explains the pharmacology A young woman with cystic fibrosis and disseminated Mycobacterium abscessum, failing years of multidrug regimens, receives a three-phage intravenous cocktail engineered (in one published case, via CRISPR-assisted host-range expansion) to match her isolate; cultures clear and cutaneous lesions heal over months, with phages recoverable from blood and sputum — the self-amplification property in action, since a static drug dose could never persist at those sites. The steps behind such a case carry the exam syllabus: isolate the organism, screen a phage library for plaques that clear it, choose strictly lytic candidates (genome-sequenced to exclude toxin and lysogeny genes), purify to injectable standards, and monitor for neutralising antibodies on repeat dosing. Meanwhile, in the burns or diabetic-foot ward, topical and intra-wound phage applications face fewer regulatory hurdles than intravenous — local use with direct biofilm contact, which phage depolymerases digest better than most antibiotics penetrate. ## Where students slip The examination trap is phrasing — "phages are viruses that kill bacteria, therefore broadly like antibiotics": they are narrower, narrower than the narrowest antibiotic, and that is simultaneously their promise (spare the microbiome) and their logistical burden (matching, cocktails). Second, students forget transduction as the reason lysogenic phages are banned from therapy: the very mechanism that made diphtheria toxin is not something to administer. Third, overclaiming maturity: describing phage therapy as "established" when its evidence base remains case series and early trials is the error; the accurate register is "promising, individualised, investigational". ## Frequently asked questions ### Why must therapeutic phages be strictly lytic rather than lysogenic? Lytic phages reproduce and lyse the target bacterium; lysogenic phages integrate into the genome and can transfer toxin or resistance genes by transduction, risking conversion of commensals into pathogens. ### How does phage self-replication change the dose-effect relationship? Phages multiply at the infection site while susceptible bacteria persist and decline when the target is gone — an auto-dosing dynamic impossible with chemical antibiotics. ### How do bacteria become phage-resistant, and what is the trade-off? Mostly by losing or altering the surface receptor the phage binds; that change frequently impairs virulence or restores antibiotic susceptibility, which cocktails and phage-antibiotic combinations exploit. ### What is the current regulatory status of phage therapy in India? No phage product holds standard marketing approval; use remains investigational or compassionate, while magistral (pharmacy-prepared, patient-matched) formulations operate in some European models. ### Why do phages work against biofilms better than many antibiotics? Phage depolymerases digest the extracellular polysaccharide matrix, and phages replicate within biofilm-embedded cells, penetrating where antibiotic diffusion fails.