Multidrug-resistant gram-negative bacterial infections

1. Introduction

Antimicrobial resistance is becoming one of the greatest global health crises of the twenty-first century, accounting for approximately 4.71 million deaths in 2021. Among the major contributors are multidrug-resistant Gram-negative bacteria.

Factors contributing to the increasing prevalence of multidrug-resistant Gram-negative bacteria include:

• Overuse and inappropriate use of antibiotics in humans and in animal husbandry;
• Inadequate access to clean water, sanitation, and infection prevention and control;
• The high genetic adaptability of bacteria, particularly through plasmid-mediated horizontal transfer of resistance genes.

At present, antimicrobial resistance is becoming increasingly severe, with many Gram-negative bacterial strains now resistant to carbapenems, polymyxins, and even newer β-lactam combination agents. This trend raises the risk of the emergence of pan-resistant organisms and poses a major challenge to both treatment and infection control.

Mechanisms of antibiotic resistance in Gram-negative bacteria include:

• Modification or destruction of antibiotics, for example, hydrolysis of β-lactam antibiotics by β-lactamase enzymes;
• Reduced antibiotic entry into the cell, due to loss of outer membrane porins;
• Alteration of the antibiotic target site, for example, ribosomal modification preventing drug binding;
• Enhanced efflux of antibiotics out of the cell, mediated by overactive transmembrane efflux pumps.

2. Selected Multidrug-Resistant Gram-Negative Bacteria

2.1. Enterobacterales

The order Enterobacterales includes many common Gram-negative pathogens such as Escherichia coli, Klebsiella spp., Proteus spp., Enterobacter spp., and Serratia marcescens. These organisms commonly colonize the gastrointestinal tract and may cause urinary tract infections, intra-abdominal infections, and bloodstream infections. Enterobacter spp. and S. marcescens are particularly associated with the hospital environment.

Major resistance mechanisms

Extended-spectrum β-lactamases (ESBLs): ESBLs are serine β-lactamases that hydrolyze the β-lactam ring. They are often encoded on mobile plasmids, facilitating rapid dissemination among bacteria. ESBL-producing Enterobacterales (ESBL-E) have become endemic pathogens worldwide. The prevalence of ESBLs in Enterobacterales varies considerably by geographic region:

• 5–25% in Western Europe, and >50% in Southern and Eastern Europe;
• 30% in Latin America and 11–13% in the United States;
• 30–80% in Asia and 10–15% in Australia and New Zealand.

AmpC β-lactamases: These are found in Enterobacter spp., S. marcescens, Citrobacter freundii, Morganella morganii, and Providencia spp. These organisms are resistant to third-generation cephalosporins but usually remain susceptible to cefepime and carbapenems. They are not inhibited by clavulanic acid. The ampC gene is often chromosomally encoded and may be induced during antibiotic exposure, leading to secondary resistance.

Carbapenemases: These enzymes hydrolyze a broad range of β-lactam antibiotics, including carbapenems, resulting in carbapenem resistance.

Carbapenem resistance in Enterobacterales may also result from non-carbapenemase mechanisms such as:

• ESBL hyperproduction,
• Porin loss,
• Upregulated efflux pumps.

2.2. Pseudomonas aeruginosa

Pseudomonas aeruginosa is a common cause of healthcare-associated infection and has a remarkable capacity to rapidly develop complex multidrug resistance.

Resistance mechanisms

• Efflux pumps such as MexAB-OprM actively expel antibiotics from the bacterial cell;
• Porin channel mutations reduce membrane permeability and limit antibiotic entry;
• Intrinsic cephalosporinase production.

Resistance patterns

• Carbapenem resistance rate in P. aeruginosa: approximately 10–20%;
• Multidrug resistance rate: approximately 5–30%, depending on geographic region and infection type.

2.3. Acinetobacter baumannii

Acinetobacter baumannii is one of the most dangerous Gram-negative hospital pathogens and, similar to P. aeruginosa, exhibits a high degree of antimicrobial resistance.

Resistance mechanisms

• Porin mutations reducing membrane permeability to antibiotics;
• Production of multiple resistance enzymes, including β-lactamases and aminoglycoside-modifying enzymes;
• Efflux pumps that eliminate antibiotics from the cell.

Resistance patterns
The global carbapenem resistance rate of A. baumannii exceeds 30%. Particularly high rates have been reported in:

• Southern and Eastern Europe: >50%;
• Some parts of Asia: >80%.

3. Antibiotics for the Treatment of Multidrug-Resistant Gram-Negative Bacteria

Over the past decade, many new antibiotics have been developed and introduced for the treatment of multidrug-resistant Gram-negative infections.

Antibiotic selection should be based on four major clinical considerations:

• Site of infection (urinary tract, lungs, bloodstream, etc.);
• Severity of infection;
• Causative pathogen (Enterobacterales, Pseudomonas, Acinetobacter);
• Relevant resistance mechanism (ESBL, AmpC, KPC, etc.).

Conventional antibiotics

• Trimethoprim-sulfamethoxazole, quinolones, and nitrofurantoin remain effective for some urinary tract infections when susceptibility has been confirmed.
• β-lactams remain the mainstay of therapy:
  • Carbapenems: recommended for Enterobacterales infections resistant to third-generation cephalosporins;
  • Cefepime: may be used for Enterobacter spp. producing AmpC when the level of resistance is moderate;
  • Aminoglycosides: an alternative option in urinary tract infections, although their use is limited by nephrotoxicity and ototoxicity;
  • Tetracyclines: used in combination regimens for A. baumannii infection or carbapenem-resistant Enterobacterales (CRE), but not effective against Pseudomonas;
  • Fosfomycin: oral formulation used for cystitis caused by ESBL-producing E. coli or CRE;
  • Sulbactam (ampicillin-sulbactam): has specific activity against A. baumannii and is often used at high doses in combination with other antibiotics.

New antibiotics for multidrug-resistant bacteria

• Ceftolozane-tazobactam: highly active against carbapenem-resistant P. aeruginosa;
• Ceftazidime-avibactam, meropenem-vaborbactam, imipenem-relebactam: first-line options for CRE;
• Cefepime-enmetazobactam: good activity against ESBL and AmpC producers;
• Sulbactam-durlobactam: specifically developed for carbapenem-resistant A. baumannii, with less nephrotoxicity than colistin;
• Cefiderocol: a siderophore cephalosporin that binds iron and enters bacterial cells through iron-transport channels; it shows high activity, including 97% activity against CRE and 97% against P. aeruginosa;
• Eravacycline: a novel tetracycline with activity against ESBL producers and CRE and limited impact from common resistance mechanisms.

4. Non-Antibiotic Therapeutic Approaches for Multidrug-Resistant Gram-Negative Bacteria

4.1. Phage therapy

This approach uses naturally occurring viruses, known as bacteriophages or phages, to kill bacteria. Phage therapy is highly specific, does not disrupt the beneficial microbiota, and does not promote new resistance in the same way as antibiotics. When used in combination, it may enhance antibiotic susceptibility and generate a synergistic effect.

Ongoing clinical trials include NCT05453578, NCT05498363, and NCT04596319. Phage-derived peptides such as endolysins are also being developed as recombinant proteins to disrupt bacterial cell structures, particularly in A. baumannii.

4.2. Microbiome-based therapy

Fecal microbiota transplantation is being investigated as a strategy to decolonize the intestinal tract before resistant organisms cause overt infection, with a focus on ESBL-producing Enterobacterales and CRE. Results have been variable. The first randomized trial did not demonstrate clear efficacy, partly because of small sample size, but further studies are ongoing.

4.3. Anti-virulence therapy

Rather than killing bacteria directly, this approach targets bacterial virulence factors to reduce pathogenicity. Research strategies include:

• Preventing bacterial adhesion and biofilm formation;
• Inhibiting bacterial toxins;
• Disrupting specialized secretion systems;
• Modulating virulence gene expression.

This strategy has considerable potential because it exerts less selective pressure for resistance, although large-scale clinical trials in Gram-negative bacteria are still lacking.

4.4. Immunotherapy

Immunotherapeutic approaches aim to activate or modulate host immune responses against bacteria, including:

• Monoclonal antibodies targeting virulence factors;
• Antibody-drug conjugates with dual mechanisms of action;
• Vaccines, currently under investigation for E. coli and K. pneumoniae, while vaccines targeting P. aeruginosa and A. baumannii remain at the preclinical stage.

Other immunomodulatory strategies, including checkpoint inhibitors, cytokine therapy, and cell-based therapies, are also under development.

4.5. Antibiotic adjuvants

These are compounds that enhance antibiotic efficacy by overcoming resistance mechanisms:

• Efflux pump inhibitors prevent bacteria from expelling antibiotics;
• Membrane permeabilizers facilitate antibiotic penetration through the Gram-negative outer membrane.

Some molecules, such as NV716, have dual activity as both efflux pump inhibitors and membrane permeabilizers and are currently in preclinical development.

REFERENCES

Macesic N, Uhlemann AC, Peleg AY. Multidrug-resistant Gram-negative bacterial infections. Lancet. 2025 Jan 18;405(10474):257-272. doi:10.1016/S0140-6736(24)02081-6. PMID: 39826970.

MSc. Kim Ngoc Son
MSc. Nguyen Hieu Minh