What do the acronyms AMR (Antimicrobial Resistance), ESBL (Extended-Spectrum Beta-Lactamase), and MDR (Multi-Drug Resistance) mean?

Understanding Antimicrobial Resistance Terminology

AMR stands for Antimicrobial Resistance, ESBL stands for Extended-Spectrum Beta-Lactamase, and MDR stands for Multi-Drug Resistant organisms—these are critical terms that define the global threat of bacteria that have developed resistance to multiple classes of antibiotics, leading to increased morbidity, mortality, and healthcare costs. 1

Antimicrobial Resistance (AMR)

AMR refers to the phenomenon where microorganisms develop the ability to survive exposure to antimicrobial agents that would normally kill them or inhibit their growth. 1

• AMR has been recognized by the World Health Organization as one of the leading threats to human health and a worldwide priority requiring urgent multisectoral action 1
• In 2019, AMR was the attributable cause of death for approximately 1.27 million people worldwide, with estimates suggesting as many as ten million people could die annually from AMR in coming decades 1
• The WHO Global Action Plan on AMR (2015) sets out five strategic objectives: improve awareness and understanding of AMR, strengthen knowledge through surveillance and research, reduce incidence of infection, optimize use of antimicrobial medicines, and ensure sustainable investment in countering AMR 1
• Misuse and overuse of antibiotics are the main drivers of the emergence and spread of AMR 1

Extended-Spectrum Beta-Lactamase (ESBL)

ESBL refers to enzymes produced by certain bacteria (primarily Enterobacteriaceae like E. coli and Klebsiella pneumoniae) that confer resistance to third-generation cephalosporins and other beta-lactam antibiotics. 1

• ESBL-producing organisms are included in the WHO list of AMR pathogens of concern, specifically E. coli and K. pneumoniae with resistance to third-generation cephalosporins 1
• These enzymes represent one of the antibacterial drug-destroying mechanisms that bacteria use to develop resistance 2
• ESBL-producing isolates may confer resistance to other antibiotics like tigecycline via additional resistance mechanisms beyond the beta-lactamase production itself 3
• More than one-half of E. coli and more than one-third of K. pneumoniae isolates in the EU/EEA were resistant to at least one antimicrobial group, highlighting the widespread nature of ESBL resistance 1

Multi-Drug Resistant (MDR) Organisms

MDR organisms are bacteria that demonstrate non-susceptibility to at least one agent in three or more antimicrobial categories that the organism would typically be susceptible to. 1

• MDROs include both Gram-positive bacteria (such as methicillin-resistant Staphylococcus aureus or MRSA) and Gram-negative bacilli (such as carbapenem-resistant Pseudomonas aeruginosa, carbapenem-resistant Acinetobacter baumannii, and carbapenem-resistant Enterobacterales) 1
• Healthcare-associated infections caused by MDROs are a leading cause of morbidity and mortality as well as increased healthcare costs worldwide 1
• The European Antimicrobial Resistance Surveillance Network (EARS-Net) reports alarming increases in carbapenem resistance: K. pneumoniae (7.9% of isolates), P. aeruginosa (16.5% of isolates), and A. baumannii (>30% of isolates) 1, 4
• MDRO infections represent a clinical challenge because of limited treatment options, often including only last-resort antibiotics that are generally associated with high toxicity or poor efficacy 1, 4

Clinical Implications

Understanding these acronyms is essential because awareness alone does not translate into appropriate prescribing practices—contextual factors such as improved infrastructure, information, and regulation are necessary to reduce reliance on antibiotics. 5

• High awareness of AMR among healthcare professionals does not automatically lead to reduced antibiotic prescribing; rather, it often links to the ready use of next-line antibiotics 5
• Resistance in MDR organisms like Acinetobacter is associated with multi-drug resistant efflux pumps, and resistance can develop during the course of standard treatment, requiring more frequent monitoring for relapse 3
• Tigecycline resistance in ESBL-producing isolates may occur via mechanisms other than beta-lactamase production, and some resistance mechanisms (like carbapenemases and metallo-β-lactamases) can render infections virtually untreatable 3, 2, 6