Residency · Residency · Preventive Medicine

Antimicrobial Resistance: A Public Health Crisis

Overview

Antimicrobial resistance (AMR) occurs when microorganisms evolve to withstand drugs designed to kill or inhibit them. AMR is a leading global health threat: estimated 4.95 million deaths associated with bacterial AMR in 2019 (1.27 million directly attributable) Driven by overuse and misuse of antibiotics in human healthcare, agriculture, and animal husbandry. Without effective antibiotics, routine surgeries, cancer chemotherapy, organ transplants, and neonatal care become high-risk. Requires a One Health approach spanning human health, animal health, and the environment.

Mechanisms of Resistance

Genetic Basis

Intrinsic resistance: naturally occurring in certain organisms (e.g., Pseudomonas inherent resistance to many antibiotics) Acquired resistance: through mutation or horizontal gene transfer. Conjugation (plasmid transfer between bacteria -- most clinically significant) Transduction (bacteriophage-mediated gene transfer) Transformation (uptake of free DNA from the environment) Mobile genetic elements: plasmids, transposons, integrons facilitate rapid spread of resistance genes.

Biochemical Mechanisms

Enzymatic inactivation: beta-lactamases (ESBLs, carbapenemases/KPC, NDM-1) Target modification: altered penicillin-binding proteins (MRSA), ribosomal methylation (macrolide resistance) Efflux pumps: active extrusion of drug from bacterial cell (multidrug resistance) Decreased permeability: porin mutations reducing drug entry (Gram-negative bacteria) Target bypass: alternative metabolic pathways (vancomycin resistance in VRE)

Key Resistant Organisms

ESKAPE Pathogens

E: Enterococcus faecium (VRE) S: Staphylococcus aureus (MRSA) K: Klebsiella pneumoniae (ESBL, CRE) A: Acinetobacter baumannii (MDR) P: Pseudomonas aeruginosa (MDR) E: Enterobacter species (ESBL, AmpC, CRE)

CDC Threat Categories (2019 AR Threats Report)

Urgent threats: Carbapenem-resistant Acinetobacter, Candida auris, Clostridioides difficile, Carbapenem-resistant Enterobacterales (CRE), Drug-resistant Neisseria gonorrhoeae. Serious threats: ESBL-producing Enterobacterales, VRE, MDR Pseudomonas, Drug-resistant Campylobacter, MRSA, Drug-resistant Streptococcus pneumoniae, Drug-resistant TB. Concerning threats: various organisms with emerging resistance patterns.

Drug-Resistant Tuberculosis

MDR-TB: resistant to isoniazid AND rifampin. XDR-TB: MDR-TB plus resistance to fluoroquinolone AND at least one injectable agent (or bedaquiline/linezolid per updated definition) Pre-XDR-TB: MDR-TB plus resistance to fluoroquinolone. New regimens (BPaL: bedaquiline, pretomanid, linezolid) have improved treatment of drug-resistant TB.

Drug-Resistant Gonorrhea

Neisseria gonorrhoeae has developed resistance to nearly every antibiotic class used for treatment. Current recommended regimen: ceftriaxone 500 mg IM (CDC, 2021) Emerging ceftriaxone resistance is an urgent threat. Dual therapy with azithromycin removed from guidelines due to increasing macrolide resistance.

Drivers of Resistance

Human Healthcare

Approximately 30% of outpatient antibiotic prescriptions in the U.S. are unnecessary. Inappropriate prescribing: antibiotics for viral infections (URI, bronchitis, viral pharyngitis) Broad-spectrum use when narrow-spectrum would suffice. Subtherapeutic dosing and incomplete courses (though "complete the full course" dogma is being questioned) Healthcare-associated infections with resistant organisms.

Agriculture and Animal Husbandry

~65% of medically important antibiotics in the U.S. (by weight) are sold for use in food-producing animals. Growth promotion: sub-therapeutic antibiotics used to accelerate animal growth (banned in EU since 2006; FDA Guidance 213 phased out growth promotion claims in 2017) Prophylactic use in animal agriculture continues. Resistant bacteria and resistance genes transfer from animals to humans through food, direct contact, and the environment.

Environmental

Pharmaceutical manufacturing waste, hospital effluent, agricultural runoff. Wastewater treatment plants as hotspots for resistance gene exchange. Environmental reservoirs of resistance genes in soil and water.

Antibiotic Stewardship Programs (ASPs)

Core Elements (CDC)

Leadership commitment: dedicated resources and support. Accountability: physician leader responsible for program outcomes. Drug expertise: pharmacist leadership. Action: implementing stewardship interventions. Tracking: monitoring antibiotic prescribing and resistance patterns. Reporting: sharing data with clinicians (antibiogram, prescribing feedback) Education: ongoing training for clinicians.

Stewardship Strategies

Prospective audit and feedback: review of antibiotic orders with recommendations to prescribers. Formulary restriction and pre-authorization: requiring approval for specific broad-spectrum agents. Antibiotic time-outs: reassessing antibiotic necessity at 48-72 hours when culture results available. IV-to-oral conversion: switching to oral when clinically appropriate. Dose optimization: using pharmacokinetic/pharmacodynamic principles. De-escalation: narrowing spectrum based on culture and sensitivity data. Outpatient stewardship: clinical decision support, delayed prescriptions, patient education.

Evidence for Stewardship

Hospital ASPs reduce antibiotic use by 11-38%. Associated with decreased C. difficile infections, shorter hospital stays, reduced costs. No evidence of increased mortality when implemented properly. CMS requires ASPs for hospital participation in Medicare (Conditions of Participation)

Surveillance

National Systems

NHSN (National Healthcare Safety Network): tracks healthcare-associated infections and resistance. NARMS (National Antimicrobial Resistance Monitoring System): tracks resistance in foodborne bacteria (human, retail meat, animal) AR Lab Network: CDC network of state and local public health laboratories for resistance testing. Antibiogram: facility-level summary of antimicrobial susceptibility patterns, updated annually.

Global Systems

GLASS (Global Antimicrobial Resistance and Use Surveillance System): WHO initiative. EARS-Net: European AMR surveillance. Fleming Fund: supporting AMR surveillance in low-income countries.

Novel Approaches and Pipeline

New Antibiotics

Pipeline is thin: few new antibiotic classes in development (most are modifications of existing classes) Market failure: antibiotics are unprofitable because they are used short-term and stewardship limits use. GAIN Act (2012): incentivizes development with extended exclusivity for Qualified Infectious Disease Products (QIDPs) PASTEUR Act (proposed): subscription-based payment model ("Netflix model") to delink revenue from volume.

Alternative Approaches

Phage therapy: bacteriophages targeting specific bacteria. Antimicrobial peptides. Anti-virulence strategies: targeting bacterial virulence factors rather than growth. Microbiome-based therapies: fecal microbiota transplantation for recurrent C. difficile. Rapid diagnostics: point-of-care tests to distinguish bacterial from viral infections and guide narrow-spectrum therapy. Vaccines: preventing infections reduces antibiotic use (pneumococcal vaccine reduced resistant pneumococcal disease)

Global Action Plans

WHO Global Action Plan on AMR (2015): five objectives -- awareness, surveillance, sanitation, optimized antibiotic use, sustainable investment in new drugs. U.S. National Action Plan for Combating Antibiotic-Resistant Bacteria (2015, updated 2020) One Health approach: coordinating human health (CDC/HHS), animal health (USDA), and environmental (EPA) sectors.

<image>A diagram showing the cycle of antimicrobial resistance development and spread. Starting with antibiotic use in humans and animals, arrows show selective pressure leading to resistant organism emergence, horizontal gene transfer spreading resistance genes, environmental contamination through wastewater and agriculture, and transmission back to humans through food, water, direct contact, and healthcare settings. A central cycle shows: antibiotic use -> selective pressure -> resistance emergence -> treatment failure -> increased morbidity/mortality -> need for new antibiotics. One Health concept is highlighted with human, animal, and environmental sectors interconnected. AMR education illustration.</image>

<image>An infographic showing the CDC's core elements of antibiotic stewardship arranged as pillars supporting improved patient outcomes: Leadership Commitment, Accountability, Drug Expertise, Action, Tracking, Reporting, and Education. Below the pillars, key stewardship interventions are listed: prospective audit and feedback, formulary restriction, antibiotic time-outs, IV-to-oral conversion, de-escalation, and outpatient stewardship. Antimicrobial stewardship education diagram.</image>

Clinical Pearls

Approximately 30% of outpatient antibiotic prescriptions are unnecessary -- this is the single most modifiable driver of resistance in human healthcare. The antibiotic pipeline is failing because of market economics, not science -- novel payment models (subscription-based) are needed to sustain antibiotic development. C. difficile infection is the most common immediate consequence of inappropriate antibiotic use and itself causes ~12,800 deaths annually in the U.S. Stewardship is not about withholding antibiotics -- it is about using the right drug, at the right dose, for the right duration, for the right indication. Vaccination is an underrecognized tool against AMR: preventing infections reduces the need for antibiotics. For boards: know the ESKAPE pathogens, the CDC core elements of stewardship, the mechanisms of resistance (enzymatic, efflux, target modification), and the One Health concept.

References

• Murray CJL, et al. Global burden of bacterial antimicrobial resistance in 2019: a systematic analysis. Lancet. 2022;399(10325):629-655.
• CDC. Antibiotic Resistance Threats in the United States, 2019. Atlanta: CDC; 2019.
• Barlam TF, et al. Implementing an antibiotic stewardship program: guidelines by IDSA and SHEA. Clin Infect Dis. 2016;62(10):e51-e77.
• WHO. Global Action Plan on Antimicrobial Resistance. WHO; 2015.
• Laxminarayan R, et al. Antibiotic resistance: the need for global solutions. Lancet Infect Dis. 2013;13(12):1057-1098.

Read this lecture as Markdown