Pseudomonas aeruginosa and Other Multidrug-Resistant Gram-Negatives

Pseudomonas aeruginosa is a versatile, environmentally hardy Gram-negative bacterium that combines intrinsic resistance to many antibiotics with a capacity to acquire further resistance, making multidrug-resistant strains a frequent cause of healthcare-associated infection. Together with other non-fermenting Gram-negatives such as Acinetobacter baumannii, it represents a group of organisms whose layered resistance mechanisms place them among the highest-priority resistant pathogens.

Definition

Multidrug-resistant Pseudomonas aeruginosa and related non-fermenting Gram-negative bacilli are organisms non-susceptible to agents across several antimicrobial categories through combinations of intrinsic resistance, regulated efflux and reduced permeability, and acquired enzymes such as beta-lactamases and carbapenemases.

Scope

The entry covers the intrinsic and acquired mechanisms that make Pseudomonas aeruginosa and related non-fermenting Gram-negatives broadly resistant, including efflux, porin changes, and beta-lactamases, and their epidemiologic significance. It describes the organisms and resistance for reference and does not provide treatment or dosing guidance.

Core questions

• Why is Pseudomonas aeruginosa intrinsically resistant to many antibiotics?
• How do efflux pumps and porin changes contribute to multidrug resistance in non-fermenting Gram-negatives?
• What makes Pseudomonas aeruginosa and Acinetobacter baumannii prominent healthcare-associated pathogens?

Key concepts

• Intrinsic resistance
• Efflux pumps (for example MexAB-OprM)
• Outer-membrane porin loss
• Acquired beta-lactamases and carbapenemases
• Fluoroquinolone target mutations (gyrA, parC)
• Biofilm formation
• Non-fermenting Gram-negative bacilli

Mechanisms

Pseudomonas aeruginosa is intrinsically resistant to many agents because of low outer-membrane permeability combined with constitutive and inducible efflux systems and a chromosomal AmpC beta-lactamase. On this baseline it can acquire additional resistance: target mutations in DNA gyrase and topoisomerase (gyrA, parC) reduce fluoroquinolone activity, up-regulated efflux and porin loss lower intracellular drug levels, and acquired beta-lactamases or carbapenemases extend resistance across beta-lactams. Related non-fermenters such as Acinetobacter baumannii deploy analogous layered mechanisms, and biofilm formation further limits drug effect; the cumulative result is the multidrug-resistant phenotype.

Clinical relevance

These organisms cause ventilator-associated pneumonia, bloodstream and urinary infections, wound and burn infections, and device-related infection, predominantly in hospitalized and immunocompromised patients, and their layered resistance can leave few active agents. The entry describes the resistance biology for educational reference and is not a basis for selecting therapy for an individual patient.

Epidemiology

Multidrug-resistant Pseudomonas aeruginosa and carbapenem-resistant Acinetobacter baumannii are leading non-fermenting Gram-negative causes of healthcare-associated infection worldwide and are ranked among critical or high priority on international pathogen lists because of limited treatment options and their persistence in the hospital environment.

History

Recognition of Pseudomonas aeruginosa as a major opportunistic and healthcare-associated pathogen grew through the late twentieth century alongside intensive care, indwelling devices, and immunosuppressive therapy, and increasing carbapenem and multidrug resistance in it and in Acinetobacter baumannii led to their inclusion among the highest-priority resistant organisms in the twenty-first century.

Debates

How should susceptibility be interpreted given inducible and combination resistance?

Because resistance in these organisms can be inducible and arise from several simultaneous mechanisms, in vitro susceptibility may not fully predict behaviour, complicating interpretation and surveillance.

Seminal works

• peleg-2010
• tacconelli-2018

Frequently asked questions

Why is Pseudomonas aeruginosa hard to treat even before acquiring new resistance?

It has intrinsic resistance from a relatively impermeable outer membrane, active efflux systems, and a chromosomal beta-lactamase, so many antibiotics are weakly active against it from the outset, before any additional resistance is acquired.

What are non-fermenting Gram-negative bacilli?

They are Gram-negative bacteria, such as Pseudomonas aeruginosa and Acinetobacter baumannii, that do not ferment glucose and are notable for environmental hardiness and a tendency toward multidrug resistance in healthcare settings.