Notes on Beyond antibiotics: non-antibiotic strategies against bacterial resistance

Introduction

• Topic: Beyond antibiotics—multifaceted approaches to combat bacterial resistance in the modern era; narrative review compiling primary research and comprehensive reviews.
• Context: Antibiotics revolutionized medicine, but rapid emergence of antibiotic resistance (ABR) is a global health crisis. Key drivers include improper antibiotic use in livestock and human activities; environmental/public health implications.
• Scope of review: Mechanisms of resistance, role of livestock antibiotic use, and evaluation of non-antibiotic therapies (bacteriophages, probiotics/postbiotics/synbiotics, fecal microbiota transplantation, nanoparticles, antimicrobial peptides, antibodies, traditional medicines, toxin–antitoxin systems).
• Publication notes: Frontiers in Cellular and Infection Microbiology; open access; WHO/CDC references cited; Figures and Tables referenced (Figure 1; Tables 1–3).
• Notable quantitative context:
  • Global deaths from antibiotic-resistant infections projected to reach 7 imes 10^5 annually (700,000) with potential rise to 1 imes 10^7 (10 million) by 2050.
  • In the US, ~3.5 imes 10^4 deaths/year attributed to antibiotic-resistant infections; ~2.8 imes 10^6 people affected annually; ~$4.6$ billion in costs to address resistant infections.
  • WHO priority pathogens are categorized into three tiers: critical, high, and medium.
• Key findings: Non-antibiotic strategies show promise but face safety, cost-effectiveness, regulatory, and scalability challenges; integration with conventional antibiotics and robust clinical validation are required.

2 Review methodology

• Structured literature search in PubMed, Scopus, Web of Science (1958–2025).
• Keywords included: antibiotic resistance, alternative therapies, bacteriophages, probiotics, FMT, antimicrobial peptides, antibodies, nanoparticles, traditional medicines, toxin–antitoxin systems.
• Inclusion: Studies addressing resistance mechanisms, non-antibiotic therapies, experimental/clinical data.
• Exclusion: Irrelevant, non-English, or duplicates.
• Data synthesis focused on mechanisms of resistance, therapeutic efficacy, and clinical applicability of non-antibiotic approaches.

3 The current state of antibiotic resistance

• ABR trend: rising resistance with development of MDR (multidrug-resistant) and XDR (extensively drug-resistant) phenotypes; last-resort drugs like tigecycline and colistin facing reduced efficacy.
• WHO priority pathogens list (2024) categorized into critical, high, and medium tiers; non-traditional antibacterials (including phages and antibodies) are increasingly considered in policy and practice.
• India’s antimicrobial resistance surveillance (ICMR) highlights declining sensitivities to β-lactams, β-lactamase inhibitors, cephalosporins, monobactams, and carbapenems; notable pathogens in COVID-19 contexts include K. pneumoniae, A. baumannii, and E. coli.
• US CDC threat classification: 18 AB pathogens/fungi separated into urgent, serious, and concerning categories.
• Contextual drivers:
  • ABR can arise even before antibiotics were discovered (environmental resistome).
  • Resistance genes often reside on plasmids or transposons, enabling horizontal transfer.
• Illustrative figures/tables (as described in the review):
  • Figure 1: WHO bacterial priority pathogens list (2024).
  • Tables 1–2: In vitro and in vivo phage therapy studies against MDR/XDR pathogens (A. baumannii, K. pneumoniae, P. aeruginosa, MRSA, VRE, etc.).

4 Mechanisms of antibiotic resistance in bacteria

• Core idea: Bacteria evolve resistance via enzyme-mediated inactivation, target modification, decreased intracellular drug accumulation, and metabolic-state changes; often involve mobile genetic elements.
• Figure 2 (conceptual): Primary mechanisms of resistance (enzymatic inactivation, target alteration, efflux, reduced permeability, biofilms, metabolic state changes).

4.1 Modification and destruction of the antibiotic

• Enzymatic inactivation is a major resistance pathway; many enzymes alter or destroy different antibiotic classes (β-lactams, carbapenems, fluoroquinolones, aminoglycosides, tetracyclines, macrolides).
• β-lactamases are key enzymes; two main types: serine β-lactamases and metallo-β-lactamases (MBLs).
• Extended-Spectrum β-lactamases (ESBLs) hydrolyze many β-lactams (e.g., cefotaxime, ceftazidime, aztreonam).
• Metallo-β-lactamases (MBLs) require metal ions (Zn) and are not inhibited by classic lactamase inhibitors; broad substrate range.
• NDМ-1 (New Delhi metallo-β‑lactamase-1) inactivates carbapenems and many β-lactams; plasmid-borne and horizontally transferable; co-carriage with other ESBLs/carbapenemases increases resistance.
• Consequences: Bacteria producing NDМ-1 are often resistant to most β-lactams and other antibiotic classes, with limited options other than last-resort drugs (tigecycline, polymyxins).
• Aminoglycoside-modifying enzymes (AAC, APH, ANT) alter drug structure, reducing ribosomal binding; plasmid/transposon-borne genes; APMA (acetyltransferase) inactivates apramycin. ESBLs co-occurrence with aminoglycoside-modifying enzymes raises MDR risk.
• Other modifying enzymes: lnu (lincosamide nucleotidyltransferase) confers resistance to lincosamides; macrolide-modifying enzymes (e.g., erythromycin esterase, macrolide phosphotransferases) degrade macrolides.
• Summary examples:
  • β-lactamase-driven inactivation leads to broad β-lactam resistance; NDМ-1 expands carbapenem resistance.
  • Aminoglycoside resistance via acetylation/phosphorylation/nucleotidylation enzymes is often plasmid-borne and co-occurs with ESBLs.
• Targeted enzymatic resistance vs. direct drug inactivation: both contribute to multidrug resistance.

4.2 Changes in target sites

• Mutations or enzymatic alterations at antibiotic targets reduce binding and efficacy.
• Rifampicin resistance: a single mutation in rpoB (RNA polymerase β-subunit) can confer resistance; multiple mutations increase resistance level.
• Fluoroquinolone resistance: mutations in DNA gyrase (gyrA/gyrB) and topoisomerase IV (parC/parE) reduce drug efficacy; multilocus mutations often needed for high-level resistance.
• β-lactam resistance via PBPs: mutations in PBPs (e.g., PBP5 in Enterococcus faecium) and alternative cross-linking enzymes (L,D-transpeptidase YcbB) enable bypass of usual targets, contributing to β-lactam resistance.
• Macrolide resistance: erm-mediated ribosomal methylation (erm(A), erm(B), erm(C)) prevents macrolide binding at the 50S subunit.
• Colistin resistance: mcr genes encode phosphoethanolamine transferases that modify lipid A, reducing colistin binding to LPS.
• CFR (chloramphenicol/florfenicol resistance) via plasmid-encoded methyltransferases that modify 23S rRNA (A2503 site), conferring linezolid resistance in some contexts.
• Target bypass strategies: PBP2a (mecA) in S. aureus confers methicillin resistance; alternative cross-linking enzymes (YcbB) bypass PBPs in E. coli.
• Overall: resistance can arise via single-point mutations or via combination thereof, often augmented by mobile elements carrying multiple resistance determinants.

4.3 Decrease in intracellular accumulation of antibiotics

• Reduced drug entry and increased efflux drive resistance.
• Permeability barriers: Gram-negative outer membrane with porins (e.g., OmpC, OmpF in E. coli; OmpD in Salmonella; OmpK35/36 in K. pneumoniae; OprD in P. aeruginosa) limit uptake of hydrophilic antibiotics; loss or downregulation of porins decreases intracellular drug concentrations.
• Efflux pumps: active extrusion of antibiotics via five major superfamilies—ABC, MFS, RND, SMR, MATE.
• RND pumps are especially important in Gram-negatives; AcrAB-TolC (AcrB inner membrane transporter, AcrA periplasmic adaptor, TolC outer channel) exports diverse substrates; overexpression correlates with MDR.
• Specific examples: AdeABC (A. baumannii) linked to aminoglycoside resistance and interaction with carbapenemases/OMPs; MexE-MexF-OprN (P. aeruginosa) exports carbapenems, chloramphenicol, and fluoroquinolones.
• Biofilms as a combined barrier: physical diffusion hindrance plus enzymatic degradation (e.g., β-lactamases) within biofilms; biofilms contribute to reduced antibiotic penetration and tolerance.
• Interplay: porin loss/downregulation often accompanies efflux pump upregulation and biofilm formation, compounding resistance.

4.4 Change the metabolic state of bacteria

• Metabolic state affects antibiotic efficacy; bactericidal antibiotics require active metabolism and respiration.
• Reduced metabolic activity (quiescence/persister states) correlates with tolerance to several antibiotics; higher metabolic activity increases drug susceptibility.
• Central carbon metabolism and energy pathways influence resistance; studies show links between metabolic gene mutations and resistance phenotypes.
• TCA cycle and respiration: lower basal respiration can reduce antibiotic lethality; some mutations that boost respiration increase bactericidal antibiotic efficacy.
• Metabolomics: shifts in glucose and amino acid metabolism can perturb central metabolism and electron transport, modulating antibiotic sensitivity.
• Exogenous glucose or alanine can restore antibiotic susceptibility in some resistant strains, underscoring metabolic state as a lever for therapy.
• Overall: bacterial physiology and metabolism are integral to antibiotic sensitivity, suggesting combination strategies that modulate metabolism could enhance efficacy.

5 One of the primary sources of antibiotic resistance: livestock

• Key point: antibiotic use in animals (growth promotion, disease prevention, production) contributes to resistance reservoirs that can transfer to humans and the environment.
• Examples and regulatory actions:
  • EU banned avoparcin (a glycopeptide growth promoter) after observing vancomycin-resistant Enterococci (VRE) spread via animal products; resistance declined in humans and animals thereafter.
  • Fluoroquinolones and other antibiotics used in livestock select for resistant Salmonella, E. coli, and Campylobacter, complicating human infections.
  • Denmark’s Integrated Antimicrobial Resistance Monitoring and Research Program (started 1995) as a pioneering surveillance effort.
  • DISARM (Disseminating Innovative Solutions for Antibiotic Resistance Management) involves multiple European countries to address ABR.
• Transmission pathways: resistant bacteria/genes enter the environment via biowaste, meat/milk contamination, direct contact with animals/workers; global trade and travel spread resistance.
• Natural resistome: resistance genes exist well before antibiotic use in agriculture; metallo-β-lactamases (NDM family) have ancient origins with broad consequences for resistance.
• Non-clinical reservoirs (animals/environment) complicate containment; resistance genes from animals can transfer to human pathogens.
• Alternative livestock strategies: replacing preventative in-feed antibiotics with feed additives (organic/inorganic acids, immunoglobulins, omega-3, prebiotics, probiotics, zinc oxide, cysteine, threonine, herbs/spices) to mitigate health problems and maintain production; some studies show modest health/productivity trade-offs when removing prophylactic antibiotics.
• Summary: livestock practices are a major driver of ABR due to selection pressure and gene flow; regulatory actions and stewardship in animal husbandry are critical to curb resistance spread.

6 Non-antibiotic therapies

• Rationale: Non-antibiotic approaches aim to control infections and mitigate ABR while preserving antimicrobial effectiveness; focus on strategies with specific targets, reduced collateral damage to microbiota, or novel mechanisms.
• General considerations: safety, cost-effectiveness, regulatory hurdles, and large-scale implementation remain critical challenges; many strategies show strong in vitro or animal model results but require robust clinical validation.

6.1 Bacteriophages (phages)

• What they are: viruses that infect and lyse specific bacteria; highly selective, often sparing host microbiota.
• Historical context: phage therapy explored for centuries; early success followed by antibiotic dominance; renewed interest due to ABR crisis.
• Rationale for advantage: host specificity minimizes collateral damage; effective against MDR/XDR bacteria in many studies; potential synergy with antibiotics (PAS).
• In vitro/in vivo evidence (highlights from Tables 1–2):
  • MDR A. baumannii: phage cocktails (e.g., Psu1, Psu2, Psu3) show efficient lysis; endolysin ElyA1 active against A. baumannii and K. pneumoniae isolates with reductions of ≥ two log10 CFU in some strains; combination therapies with endolysins enhanced efficacy against MDR pathogens; phage cocktails and endolysins can reduce bacterial load in vivo.
  • Colistin-resistant A. baumannii and XDR A. baumannii: phage therapy demonstrated significant bacterial reductions and improved survival in Galleria mellonella or mouse models; mono-phage or phage cocktails used.
  • MDR P. aeruginosa: phage therapies (individual or cocktails) show antibacterial activity; some studies demonstrate restored antibiotic susceptibility when combined with antibiotics.
  • MRSA and other Gram-positives: phage S13¢ showed significant protection in mouse MRSA pneumonia; phage JD007 reduced dermal abscess formation in mice.
• Phage-derived enzymes and alternatives: endolysins (e.g., ElyA1) and phage-derived enzymes can act on Gram-positive and Gram-negative cells; PA-PP (phage-encoded serine protease) targets outer membrane proteins.
• Delivery and engineering options:
  • Engineered phages to enhance antibacterial capacity;
  • Phage encapsulation (e.g., PEGylation) to improve pharmacokinetics and reduce immune clearance;
  • Use of phage components (enzymes) rather than whole phages to reduce replication risk;
  • Phage cocktails (2–10 phages) to broaden host range; note risk of resistance if poorly matched.
• Phage–antibiotic synergy (PAS): sub-inhibitory antibiotics can enhance phage activity, enabling better bacterial clearance.
• Regulatory challenges: regulatory pathways for phage therapy are evolving; IP issues differ from conventional drugs; publicly funded phage libraries proposed to improve access; streamlined FDA pathways and clear safety/efficacy data are needed.
• Limitations: phage resistance can emerge; narrow host ranges require personalized or cocktail approaches; long-term safety and immune responses need more study.

6.2 Probiotics, postbiotics, and synbiotics

• Definitions (WHO/related):
  • Probiotics: live microorganisms conferring health benefits when consumed in adequate amounts.
  • Prebiotics: substrates that feed beneficial microbes.
  • Synbiotics: combination of probiotics and prebiotics.
  • Postbiotics: non-viable microbial products with health benefits.
• Rationale for antibiofilm/antagonism effects: probiotics can impede pathogen growth, quorum sensing, and biofilm formation; they produce antimicrobial compounds (bacteriocins, enzymes, exopolysaccharides).
• Common probiotic taxa: Bifidobacterium spp., Lactobacillus spp., Enterococcus spp. (non-pathogenic strains), non-pathogenic E. coli strains, Bacillus subtilis, Saccharomyces boulardii, and emerging candidates like Akkermansia muciniphila.
• Evidence and applications:
  • H. pylori eradication: probiotics added to standard triple/quadruple therapies improve cure rates and may block urease-mediated adherence; randomized trial shows 92% cure with probiotics + antibiotics vs 86.8% with antibiotics alone in H. pylori treatment.
  • Anti-biofilm/quorum sensing modulation: L. plantarum supernatants inhibit K. pneumoniae biofilms; Bifidobacterium longum 5(1A) reduces infection and modulates neutrophil recruitment and cytokines in murine models.
  • VRE and other colonization: some lactobacilli strains influence decolonization dynamics; lactobacilli may reduce VRE colonization in certain settings.
  • UTIs and vaginal microbiota: Lactobacillus vaginalis/Cr. can reduce recurrent UTI episodes; probiotic colonization can prevent pathogen establishment.
• Mechanisms: competitive exclusion; production of bacteriocins; immune modulation; quorum-sensing interference; alteration of gut microbiota to reduce pathogen niches.
• Considerations/limits: strain-specific effects; need for standardization of formulations; variable clinical efficacy; safety considerations in immunocompromised individuals.

6.3 Fecal microbiota transplantation (FMT)

• Concept: transfer of donor fecal microbiota to restore gut microbial diversity and function in the recipient.
• Historical roots: ancient China; modern clinical use revived for C. difficile infection (CDI) and other GI disorders.
• Efficacy in CDI and beyond:
  • CDI: randomised trials show high cure rates; donor engraftment correlates with reduction in antimicrobial resistance genes (ARGs).
  • ARG decolonization: post-FMT analyses show reduced carriage of ARGs in responders vs non-responders; AUT/follow-up studies indicate long-lasting ARG reductions in some cases.
  • RBX2660 (REBYO-TM): microbiota restoration therapy reduces Enterobacterales resistant to antibiotics; donor engraftment correlates with ARG reduction; RBX2660 licensed for CDI treatment as REBYOTM (late 2022).
  • Autologous FMT (aFMT) can restore gut microbiota more quickly than probiotics in antibiotic-treated patients; higher clinical cure rates in CDI cases with donor FMT vs aFMT.
• Pediatric and special populations: successful CDI dec dd; cases of decolonization in pediatric and immunocompromised patients reported; some recolonization can occur within weeks to months depending on host/microbiota factors.
• Regulatory and safety considerations: ongoing trials (clinicaltrials.gov) assess decolonization efficacy in larger pediatric cohorts; regulatory pathways evolving for donor screening, standardization, and safety monitoring.

6.4 Nanoparticles (NPs)

• Rationale: nanotechnology offers novel antibacterial strategies and delivery platforms; metals and metal oxide NPs can have intrinsic antibacterial activity and can serve as carriers for antibiotics or biologics.
• Mechanisms of action:
  • Ion/ROS generation leading to oxidative stress;
  • Membrane disruption and protein/DNA damage;
  • Interaction with microbial surfaces via electrostatic forces; improved permeability through biofilms.
• Common NP types:
  • Silver (Ag) and copper oxide (CuO) hybrids; zinc oxide (ZnO) NPs; cerium oxide (CeO2) NPs; iron oxide (IONPs).
• Material-specific notes:
  • CeO2 NPs: disrupt DNA, alter gene expression; antibacterial against both Gram-positive and Gram-negative strains; biofilm inhibition potential.
  • Ag NPs: induce oxidative stress and disturb antioxidant defenses; impact on E. coli and S. aureus gene expression; potential for resistance development.
  • IONPs: broad-spectrum activity via ROS and electrostatic interactions; effectiveness varies with bacterial cell wall structure (Gram-positive generally more susceptible than Gram-negative).
• Delivery considerations: NPs as carriers for antibiotics or other therapeutics; mucosal targeting via chitosan-based systems (e.g., H. pylori gastric colonization).
• Plant-based NP synthesis: nano-hybrids using plant extracts (e.g., Murraya koenigii) to produce antibacterial NP formulations; MIC-like assessments via diffusion and broth dilution.
• Challenges and limitations:
  • Scale-up and manufacturing: translation from lab to industry requires robust, reproducible synthesis and standardized quality control.
  • Toxicity and safety: size, shape, surface chemistry influence cytotoxicity; potential for oxidative stress or immune activation; placental transfer and hormonal disruption concerns in animal models;
  • Regulatory landscape: EU/US regulatory frameworks treat nanomaterials as chemicals; risk assessment must consider exposure routes and product category (cosmetics, foods, medical devices).
• Regulation and governance: need for global standards; EU REACH-like considerations; FDA oversight with product-focused regulation; multiple agencies (EFSA, EMA, JRC-ICH) provide guidance.

6.5 Antimicrobial peptides (AMPs)

• Overview: small, low-molecular-weight peptides with broad antimicrobial activity; two main classes: membrane-acting and non-membrane-acting peptides.
• Advantages: rapid killing; low rates of resistance development; broad spectrum; low toxicity relative to many small-molecule antibiotics; potential to combine with other therapies with reduced drug interactions.
• Notable AMPs discussed:
  • Cbf-K16: broad-spectrum, potent against MRSA with low cytotoxicity.
  • Ib-AMP4 (plant-derived): potent against MRSA; disrupts MRSA biofilms; demonstrated efficacy in vitro and in vivo (mouse model).
  • Melittin: bee venom peptide with strong antimicrobial activity; currently in Phase 1 trials; also reported antiviral activity; significant cytotoxicity concerns at high concentrations; hydrophobicity must be balanced to enhance selectivity.
  • Daptomycin: a cyclic lipopeptide; disrupts membrane potential; effective against MDR Gram-positive bacteria; pharmacokinetics suggest limited CSF penetration, relevant for meningitis.
  • General considerations: AMPs show promise but face stability and delivery challenges in physiological conditions (salt sensitivity, pH sensitivity, protease degradation).
• Limitations and challenges:
  • Mechanism of action not fully understood for many AMPs; toxicity concerns and potential off-target effects;
  • Stability: proteolytic degradation, serum component interactions, salt sensitivity, pH variations;
  • Cost: synthesis and purification of longer peptides can be expensive; typically, longer peptides require >7–8 amino acids for amphiphilicity and >22 for optimal α-helix activity in membranes; shorter peptides may be less effective.
  • Delivery strategies: encapsulation in liposomes or nanoparticles to improve stability and targeting; balancing hydrophobic/hydrophilic residues to reduce mammalian toxicity.
• Development strategies: sequence modification, D-enantiomer substitution, truncation, dimerization, hybridization, and NP-based delivery platforms to improve stability and targeting.

6.6 Antibodies as non-traditional antibacterial agents

• Concept: harness specific immune-targeted antibodies to combat bacterial pathogens; monoclonal antibodies (mAbs) offer high specificity, long half-life (IgG ~21 days), and favorable safety profiles.
• Examples and mechanisms:
  • DNABII-targeting antibody: monoclonal antibody disrupts biofilms by targeting DNABII proteins essential for biofilm integrity; potential to enhance antibiotic efficacy when used with antibiotics.
  • S. aureus-targeted antibody–antibiotic conjugate (AstraZeneca Roche developments): conjugates bring antibiotics to the bacterial surface to enhance local efficacy.
  • P. aeruginosa-targeted bispecific antibody MEDI3902: targets a surface polysaccharide to impair biofilm formation.
  • SUVRATOXUMAB (suvratoxumab): anti-a-toxin mAb evaluated in phase 2 for ventilator-associated pneumonia due to S. aureus; shows reduced toxin-mediated damage.
• Advantages: high specificity, reduced disruption to commensals, potential for synergy with antibiotics; lower likelihood of driving broad resistance.
• Challenges: high production costs; potential for immune reactions; ensuring consistent efficacy across diverse strains; scalability and regulatory pathways for biologics.

6.7 Traditional medicines

• Rationale: plant-derived compounds can inhibit biofilm formation and virulence pathways, offering complementary strategies to antibiotics.
• Examples and mechanisms:
  • Andrographolide (AG) from Andrographis paniculata: inhibits LasI/LasR quorum-sensing regulators in P. aeruginosa; analogs can modulate RsmA, reducing pathogenicity; can inhibit S. aureus biofilm via SarA pathway.
  • Houttuynia cordata: ethanolic extract inhibits biofilm formation in S. aureus (including MRSA) through anti-inflammatory and anti-biofilm effects; may deplete DNA required for biofilm assembly.
  • Polygonum cuspidatum: active compounds target S. mutans in dental plaque; inhibits biofilm formation and acid production; more activity against S. aureus than P. aeruginosa in some studies.
  • Naringin (grapefruit-derived flavonoid): enhances ciprofloxacin/tetracycline efficacy against P. aeruginosa biofilms, indicating potential for combination therapy.
• Challenges: complex phytochemical mixtures; standardization and identification of active components; potential side effects and herb–drug interactions; variability in formulations.

6.8 Toxin–antitoxin (TA) system

• Concept: TA systems are compact chromosomal/plasmid-encoded modules with a toxin and an antitoxin; toxins can disrupt essential processes to slow or halt growth, aiding persistence under stress.
• Types: seven types; type II (protein toxins) is the most studied; regulation occurs via toxin–antitoxin complexes.
• Roles in bacterial physiology:
  • Persistence and dormancy: TA can contribute to entry into and maintenance of persister states; toxin activation can drive dormancy and antibiotic tolerance.
  • Stress response and virulence regulation: TA loci modulate drug resistance genes and pathogenicity in certain species (e.g., Streptococcus suis).
  • Mobilome distribution: TA systems are abundant in both Gram-positive and Gram-negative pathogens and can reside on chromosomes or plasmids.
• Therapeutic angle: strategies to selectively activate TA toxins (or inhibit antitoxins) could drive bacterial self-destruction; conversely, understanding TA networks could help prevent unintended persistence.
• Challenges: universal activation strategies across diverse strains; risk of unintended consequences on host microbiota; complex regulation and potential compensatory pathways in bacteria.

6.9 Summary of non-antibiotic therapies (Table 3 overview)

• Bacteriophages: advantages—high specificity, effectiveness against MDR bacteria; limitations—phage resistance, immune reactions, regulatory hurdles, production scalability; implementation challenges—regulated pathways, access to phage libraries.
• Probiotics/Postbiotics/Synbiotics: advantages—restores gut microbiota, reduces pathogen colonization, enhances immune response; limitations—variable efficacy, strain specificity; implementation—standardization, large-scale validation.
• FMT: advantages—restores gut microbiota balance, effective against CDI; limitations—risk of transmission, donor variability; implementation—regulatory oversight, standardized protocols.
• Nanoparticles: advantages—broad-spectrum, anti-biofilm potential, delivery platforms; limitations—toxicity, stability, scale-up; implementation—regulatory compliance, safety assessments.
• AMPs: advantages—low resistance potential, broad activity; limitations—stability, high production costs, potential cytotoxicity; implementation—delivery strategies, stability optimization.
• Antibodies: advantages—high specificity, reduced microbiome disruption; limitations—cost, immune reactions, production scalability; implementation—clinical development and regulatory pathways.
• Traditional medicines: advantages—synergistic effects, wide availability; limitations—scientific validation, formulation standardization; implementation—identification of active components, standardization.
• TA systems: advantages—potential to induce self-destruction; limitations—activation strategies are context-dependent and complex; implementation—developing reliable activators and broad applicability.

7 Conclusion

• Summary: While antibiotics remain foundational, ABR is escalating to a level where conventional therapies may become inadequate. The golden era of antibiotics is cited as over; new strategies—especially non-antibiotic therapies—offer hope to control infections and mitigate ABR.
• Key drivers and context: ABR originates from human activity and agricultural practices; biofilms play a critical role in chronic infections; environmental resistomes predate clinical antibiotics.
• Strategic outlook: non-antibiotic therapies are under active investigation, with phages receiving particular attention for their specificity and potential for synergy with antibiotics. However, robust clinical trials, standardized guidelines, and supportive regulatory/policy frameworks are essential for translation to routine clinical use.
• Future directions: interdisciplinary work in genomics, bioinformatics, and synthetic biology can optimize non-antibiotic therapies; integration with existing antibiotic regimens could shorten treatment durations and reduce resistance emergence; continued policy support and public health education are necessary.

Table 3 (summary of non-antibiotic therapies): key takeaways

• Bacteriophages: High specificity; effective against MDR pathogens; Limitations include phage resistance and regulatory production hurdles; Implementation requires regulatory clarity and scalable phage libraries.
• Probiotics/Postbiotics/Synbiotics: Restore gut microbiota; reduce pathogen colonization; Limitations include variable efficacy and standardization needs; Implementation requires formulation standardization and large-scale validation.
• FMT: Restores microbial balance; effective for CDI; Risks include donor variability and pathogen transmission; Implementation requires regulatory oversight and standardized protocols.
• Nanoparticles: Broad antibacterial activity; can aid drug delivery; Limitations include potential toxicity, stability, and regulatory compliance; Implementation requires safety/efficacy data and scalable manufacturing.
• AMPs: Broad spectrum; low resistance potential; Limitations include stability/cytotoxicity and production costs; Implementation requires advanced delivery systems and stability optimization.
• Antibodies: High specificity; adaptable to targets; Limitations include high cost and potential immune reactions; Implementation requires scalable production and regulatory approval.
• Traditional medicines: Potential synergistic effects with antibiotics; Limitations include standardization and validation; Implementation requires identification of active components and validation.
• TA systems: Potential to trigger bacterial self-destruction; Limitations include activation control and strain variability; Implementation requires reliable activators and broader applicability.

Connections to foundational principles and real-world relevance

• Pharmacology and microbiology foundations: understanding resistance mechanisms (enzymes, targets, permeability, efflux, metabolism) is essential to design effective non-antibiotic therapies.
• Public health relevance: ABR is a systemic problem linking human health, animal agriculture, and environment; strategies like probiotics and FMT intersect with gut microbiome science and infection control.
• Regulatory science and ethics: phage therapy, nanoparticles, and biological-based therapies demand clear regulatory pathways, IP considerations, and safe, ethical deployment.
• Real-world relevance: strategies such as phage cocktails, approved mAb conjugates, and FMT have progressed to clinical contexts, including CDI and hospital-acquired infections, illustrating translational potential.

Equations and numerical references (LaTeX rendering)

• Global ABR deaths: ext{ABR deaths}
  ightarrow ext{projected} egin{cases} 7 imes 10^5 ext{ per year} \ 1 imes 10^7 ext{ by } 2050 ext{ (10 million)} \ ext{(contextual risk)} \ ext{WHO/ICMR/CDC data referenced in review} \ ext{(examples)} \end{cases}
• B. prioritize pathogens tiers: three tiers:
  • Critical, High, Medium (as per WHO 2024 priority pathogens list).
• CDI and ARG trends:
  • Responders vs non-responders in FMT ARG carriage: down-regulation observed in responders for long-term follow-up (minimum 1 year).
• Examples of phage therapy outcomes (in Tables 1–2): e.g.,
  • Endolysin ElyA1 reduced bacterial load by at least 2\log_{10}\text{ CFU} in tested strains.
• Probiotic cure rate in H. pylori trial: 92\% vs 86.8\% in control.
• Figurative numbers: e.g., Galleria mellonella larvae survival improved with phage treatment by substantial margins (percent values vary by study).

Key terms to remember for the exam

• ABR, MDR, XDR, ESBL, MBL, NDM-1, PBP2a, mcr, CFR, porins, efflux pumps (ABC, MFS, RND, SMR, MATE), biofilm, ppGpp, TCA cycle, PAS, TA systems, FMT, AMPs, phages, endolysins, DNABII, MEDI3902, suvratoxumab, RBX2660/REBYOTA, NPs.
• Non-antibiotic therapies: phages and phage components; probiotics/postbiotics/synbiotics; FMT; nanoparticles; antimicrobial peptides; antibodies; traditional medicines; toxin–antitoxin systems.
• Regulatory/regulatory challenges: FDA/EMA/ECHA guidance, public phage libraries, IP/ownership issues, safety/efficacy data, standardized production and quality control.

Author notes and references (high-level)

• Core themes and data are drawn from a comprehensive 2025 Frontiers in Cellular and Infection Microbiology review by Yarahmadi et al. on non-antibiotic therapies for ABR, including WHO/CDC data, phage therapy studies (in vitro and in vivo), probiotic and FMT research, nanoparticle toxicology and regulation, AMPs, TA systems, and traditional medicines.
• Figures: WHO bacterial priority pathogens list (Figure 1).
• Tables: in vitro and in vivo phage therapy studies (Tables 1–2); Table 3 summarizes non-antibiotic therapies with advantages/limitations/implementation.
• Representative numeric items cited in the review (for quick recall): 7\times 10^5 projected ABR deaths/year globally by mid-century, 1\times 10^7 by 2050; 3.5\times 10^4 US ABR deaths/year; 2.8\times 10^6 US ABR cases/year; phage-therapy efficacy examples include reductions of up to \ge 2\log_{10} CFU in key MDR pathogens; specific therapy cure rates and outcomes vary by model and pathogen.