Metallo-beta-lactamases (MBLs)- Introduction, List of Bacteria, Pathogenicity, Symptoms, Lab Diagnosis, Treatment, Prevention, and Keynotes

Introduction of Metallo-beta-lactamases (MBLs)

Metallo-beta-lactamases (MBLs) are a group of enzymes produced by certain bacteria that impart resistance to a wide range of beta-lactam antibiotics. These enzymes play a crucial role in the global health challenge posed by antibiotic-resistant bacterial infections.

Origin and Mechanism:

  • Enzymatic Action: MBLs break down beta-lactam antibiotics by hydrolyzing their beta-lactam ring, which is central to their antimicrobial activity. This hydrolysis renders the antibiotics ineffective.
  • Metal Dependency: As their name suggests, MBLs require metal ions for their enzymatic activity, typically zinc ions.
  • Broad Resistance: MBLs can degrade a wide variety of beta-lactam antibiotics, including penicillins, cephalosporins, and notably, carbapenems. Carbapenems are often regarded as the last line of defense against multi-drug resistant bacterial infections.
  • Resistance to Inhibitors: Unlike other beta-lactamases, MBLs are not inhibited by commercially available beta-lactamase inhibitors such as clavulanic acid.

Clinical Implications:

  • Bacterial Strains: MBLs are produced by a range of bacteria, but their clinical significance is most pronounced in Gram-negative bacteria such as Pseudomonas aeruginosa, Acinetobacter baumannii, and certain members of the Enterobacteriaceae family.
  • Treatment Challenges: The presence of MBLs in bacterial pathogens can severely limit treatment options. Often, only a few antibiotics remain effective, and these may have undesirable side effects or be less efficacious.
  • Spread: Initially identified in the 1990s, the prevalence of MBL-producing bacteria has been rising globally, especially in healthcare settings. Some MBL variants, like the NDM (New Delhi Metallo-beta-lactamase), have drawn international concern due to their rapid spread and association with international travel and medical tourism.

Detection and Control:

  • Detection: Detecting MBL production in clinical isolates can be challenging. Phenotypic methods, such as the imipenem-EDTA synergy test, and genotypic methods like PCR for specific MBL genes, are employed.
  • Prevention: Rigorous infection control measures, antibiotic stewardship, surveillance, and rapid identification of MBL-producing organisms are critical to control their spread.

List of Bacteria

Metallo-beta-lactamases (MBLs) are produced by a variety of bacteria, especially among Gram-negative pathogens. The presence of MBLs confers resistance to a wide spectrum of beta-lactam antibiotics, including the carbapenems, which are often considered the last line of defense against many bacterial infections. Here’s a list of some bacteria that are known to produce MBLs:

  1. Pseudomonas aeruginosa: A notorious pathogen often associated with hospital-acquired infections, particularly in intensive care units.
  2. Acinetobacter species, especially:
    • Acinetobacter baumannii: Frequently linked to various nosocomial infections, including pneumonia, bloodstream infections, and wound infections.
  3. Enterobacteriaceae: This is a large family of Gram-negative bacteria, some members of which have been found to produce MBLs. This includes:
    • Klebsiella pneumoniae: Known to cause pneumonia, bloodstream infections, and other conditions.
    • Escherichia coli (E. coli): While E. coli is often a benign inhabitant of the human gut, pathogenic strains can cause a range of infections.
    • Enterobacter species: Such as Enterobacter cloacae and Enterobacter aerogenes.
    • Salmonella species: Some strains have been reported to produce MBLs.
    • Proteus species: Including Proteus mirabilis.
  4. Stenotrophomonas maltophilia: Though less common, this bacterium is another known MBL producer and can cause various infections, especially in immunocompromised patients.
  5. Bacteroides fragilis: Some strains of this anaerobic bacterium, typically associated with intra-abdominal infections, have been reported to produce MBLs.

It’s important to note the following:

  • Variants: Different MBL variants have been identified, such as IMP, VIM, SIM, GIM, SPM, and notably NDM (New Delhi Metallo-beta-lactamase). The NDM variant, first identified in India, has garnered significant international attention due to its rapid global spread and association with travel and medical tourism.
  • Gene Mobility: Genes encoding MBLs are often located on mobile genetic elements, such as plasmids, which can be transferred between different bacterial species, contributing to the spread of resistance.
  • Global Distribution: The distribution of specific MBLs can vary by region. Continuous surveillance and molecular characterization are essential to track the emergence and spread of these resistance mechanisms.

Pathogenicity of Metallo-beta-lactamases (MBLs) Producers

The pathogenicity of bacteria producing Metallo-beta-lactamases (MBLs) pertains to their ability to cause disease in humans or animals. The presence of MBLs in these bacteria does not directly contribute to their virulence or disease-causing ability; rather, MBLs confer resistance to a broad spectrum of beta-lactam antibiotics, complicating treatment. The pathogenic potential of MBL-producing bacteria arises from a combination of their intrinsic virulence factors and the challenge posed by their antibiotic resistance.

Here’s a closer look at the pathogenicity of MBL-producing bacteria:

  1. Treatment Challenges:
    • Resistance to Last-Resort Antibiotics: MBLs confer resistance to carbapenems, which are often seen as the drugs of last resort for multi-drug resistant infections.
    • Multidrug Resistance: Many MBL-producing bacteria are also resistant to other antibiotic classes, further narrowing treatment options and increasing mortality and morbidity rates.
  2. Common Pathogens:
    • Pseudomonas aeruginosa: This bacterium can cause a range of severe infections, particularly in hospitalized patients or those with compromised immune systems. Common infections include pneumonia, bloodstream infections, urinary tract infections, and wound infections.
    • Acinetobacter baumannii: Especially notorious in healthcare settings, it can cause ventilator-associated pneumonia, bloodstream infections, and wound infections. This bacterium can survive on surfaces for extended periods, facilitating its spread.
    • Enterobacteriaceae: This family includes bacteria like Klebsiella pneumoniae and E. coli, which can cause infections ranging from urinary tract infections to sepsis.
  3. Virulence Factors: Many MBL-producing bacteria have additional virulence factors. For example:
    • Pseudomonas aeruginosa has flagella aiding mobility, pili facilitating attachment, and a range of toxins causing tissue damage.
    • Acinetobacter baumannii can form biofilms that contribute to its persistence and resistance to antibiotics and disinfectants.
  4. Healthcare-Associated Infections: Many MBL-producing bacteria are primarily associated with healthcare settings, especially intensive care units. They can cause outbreaks, challenging to control due to limited treatment options.
  5. Community-Acquired Infections: While many MBL-producers are associated with hospital settings, there have been increasing reports of community-acquired infections, particularly with certain strains of MBL-producing Enterobacteriaceae.
  6. Global Spread: The international movement of people, including medical tourism, has contributed to the spread of certain MBL-producing bacteria, notably the NDM (New Delhi Metallo-beta-lactamase) variant.

Symptoms of Metallo-beta-lactamases (MBLs)

The symptoms of infections caused by Metallo-beta-lactamase (MBL) producing bacteria depend on the type and site of the infection, rather than the MBL production itself. The concern with MBL-producing bacteria is their resistance to many commonly used antibiotics, which can complicate and prolong treatment. Here are the common types of infections caused by MBL-producing bacteria and their associated symptoms:

  1. Urinary Tract Infections (UTIs):
    • Burning sensation during urination
    • Frequent urination
    • Cloudy, dark, or bloody urine
    • Lower abdominal pain or discomfort
    • Feeling of incomplete bladder emptying
    • Fever or chills (indicative of upper UTIs or pyelonephritis)
  2. Bloodstream Infections (Sepsis or Bacteremia):
    • High or low body temperature
    • Rapid heart rate or palpitations
    • Rapid or shallow breathing
    • Confusion or disorientation
    • Reduced urine output
    • Nausea or vomiting
  3. Pneumonia:
    • Fever and chills
    • Persistent cough, which may produce phlegm
    • Shortness of breath or difficulty breathing
    • Sharp or stabbing chest pain that worsens with deep breathing or coughing
    • Rapid heartbeat
  4. Skin and Soft Tissue Infections:
    • Redness, warmth, and swelling of the affected area
    • Pain or tenderness at the site
    • Drainage or pus from the area
    • Fever
  5. Intra-abdominal Infections:
    • Abdominal pain or tenderness
    • Bloating or a feeling of fullness
    • Nausea or vomiting
    • Diarrhea or constipation
    • Fever and chills
  6. Meningitis (though less common with most MBL producers):
    • Severe headache
    • Stiff neck
    • Sensitivity to light
    • Fever and chills
    • Nausea and vomiting
    • Sleepiness or difficulty waking up
  7. Wound Infections (especially in surgical sites or trauma-induced wounds):
    • Increasing redness and swelling around the wound
    • Warmth at the wound site
    • Increasing pain
    • Pus or liquid discharge from the wound
    • Fever
  8. Central Line-Associated Bloodstream Infections:
    • Redness or tenderness at or near the site of the central line
    • Chills or fever
    • Any signs of sepsis as mentioned above

Lab Diagnosis of Metallo-beta-lactamases (MBLs)

The laboratory diagnosis of Metallo-beta-lactamase (MBL) producing bacteria is vital for appropriate antimicrobial therapy. Diagnosing MBL production involves phenotypic (functional) tests and genotypic (molecular) methods. Here’s an overview:

Phenotypic Tests:

  1. Disk Diffusion Methods:
    • Imipenem-EDTA Double-Disk Synergy Test: This is a common method. A bacterial suspension is streaked on an agar plate, and two disks (one with imipenem and another with EDTA, a metal ion chelator) are placed on the agar. If the zone of inhibition around the imipenem disk increases towards the EDTA disk, it suggests MBL production.
    • Combined Disk Test: Similar to the above, but uses disks containing imipenem alone and imipenem combined with EDTA. An increase in the zone diameter for the combination disk compared to the imipenem alone disk indicates MBL production.
  2. E-test: The E-test MBL strip contains a gradient of imipenem and imipenem with EDTA. A significant reduction in the minimum inhibitory concentration (MIC) in the presence of EDTA indicates MBL production.

Genotypic Methods:

  1. Polymerase Chain Reaction (PCR): PCR can detect specific genes responsible for MBL production, such as bla_NDM, bla_VIM, bla_IMP, etc. This method identifies the presence of MBL genes in the bacterial genome.
  2. DNA Sequencing: After PCR amplification, sequencing can identify specific MBL types and variants.
  3. Real-time PCR: This allows for faster detection and, in some cases, quantification of specific MBL genes.
  4. Whole Genome Sequencing (WGS): Offers a comprehensive look into all resistance mechanisms in a bacterial isolate, including MBL genes.

Confirmatory Tests:

  • When conducting phenotypic tests for MBLs, it’s essential to use control strains, both MBL-positive and MBL-negative, to ensure the accuracy of the results and differentiate true MBL production from other resistance mechanisms.


  • Guidelines provided by organizations such as the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) offer standards for the interpretation of tests for MBLs.


  • Phenotypic tests can sometimes give false-positive or false-negative results due to the presence of other resistance mechanisms or the variable expression of MBLs.
  • Not all laboratories, especially in resource-limited settings, are equipped to conduct molecular tests, which provide definitive evidence of MBL genes.

Treatment of Metallo-beta-lactamases (MBLs) Producers

The treatment of infections caused by Metallo-beta-lactamase (MBL) producing bacteria is challenging due to their resistance to a broad spectrum of beta-lactam antibiotics, including the carbapenems. Here are the general approaches and specific agents often considered for treating these infections:

  1. Antibiotic Susceptibility Testing: This is paramount for guiding therapy. Even among MBL-producing strains, there might be variability in resistance profiles, and some agents may remain effective.
  2. Antibiotic Options:
    • Colistin (Polymyxin E): Frequently active against MBL-producing bacteria, colistin is often considered a drug of last resort because of concerns about nephrotoxicity and other adverse effects.
    • Tigecycline: A glycylcycline antibiotic, tigecycline has activity against many MDR Gram-negative bacteria, including some MBL producers. However, its use is limited by its pharmacokinetics and concerns about increased mortality in some situations.
    • Aminoglycosides: Agents such as gentamicin, amikacin, or tobramycin may remain effective against some MBL-producing strains. Their use is often limited by potential nephrotoxicity and ototoxicity.
    • Fosfomycin: While primarily used for uncomplicated UTIs caused by MDR E. coli, it might be effective against some MBL-producing bacteria.
    • Cefiderocol: A siderophore cephalosporin with activity against a broad range of Gram-negative bacteria, including some MBL producers. It uses iron transport mechanisms to enter bacterial cells, where it exerts its antibiotic effect.
    • Aztreonam: A monobactam antibiotic that might remain effective against some MBL-producing Enterobacteriaceae, especially when combined with other agents.
  3. Combination Therapy:
    • Given the limited options and concerns about resistance development, combination therapy with two or more antibiotics is often considered. This might include combinations like colistin with meropenem or aztreonam with ceftazidime-avibactam.
    • Research on effective combinations is ongoing, and more clinical data is needed.
  4. New Agents and Approaches:
    • The rise of MBL-producing bacteria has spurred research into new antimicrobial agents and strategies. Some experimental agents are in various stages of development.
  5. Supportive Care:
    • Management of infections with MBL-producers also involves robust supportive care, especially if the infection is severe. This might include fluid resuscitation, vasopressors, or organ support like mechanical ventilation.
  6. Infection Control:
    • Preventing the spread of MBL-producing bacteria, especially in healthcare settings, is crucial. This includes isolation precautions for affected patients, stringent hand hygiene, and thorough environmental cleaning.
  7. Antibiotic Stewardship:
    • The judicious use of antibiotics is critical in preventing the emergence and spread of resistant organisms, including MBL producers.

Prevention of Metallo-beta-lactamases (MBLs) producing Bacteria

Preventing the spread and emergence of Metallo-beta-lactamase (MBL) producing bacteria is of utmost importance given their resistance to a broad spectrum of antibiotics, including carbapenems. Here are several strategies and measures that can be employed:

  1. Infection Control in Healthcare Settings:
    • Hand Hygiene: Proper hand hygiene, particularly before and after patient contact, is among the most effective measures to prevent the spread of infections.
    • Isolation Precautions: Patients known to be infected or colonized with MBL-producing bacteria might require isolation to prevent transmission to other patients.
    • Environmental Cleaning: Regular cleaning and disinfection of patient care areas and equipment to reduce the risk of transmission.
    • Personal Protective Equipment (PPE): Use of PPE like gowns and gloves, especially when handling body fluids or when in contact with patients known to be infected or colonized.
    • Education & Training: Healthcare workers should be educated about MBL-producing bacteria, their implications, and prevention strategies.
  2. Antibiotic Stewardship:
    • Prudent Use of Antibiotics: Only use antibiotics when they are necessary, and select the most appropriate antibiotic for the infection.
    • Regular Review of Prescriptions: Start with broad-spectrum antibiotics if needed, but once culture and sensitivity results are available, switch to narrower-spectrum antibiotics.
    • Duration Control: Limit the duration of antibiotic treatment to the minimum necessary to treat the infection.
    • Educate Healthcare Professionals: Ensure that doctors, nurses, and other healthcare professionals understand the importance of judicious antibiotic use.
  3. Surveillance:
    • Monitoring & Reporting: Regularly monitor for the emergence of MBL-producing bacteria in clinical settings.
    • Data Sharing: Share data about the prevalence of MBLs with regional and national health bodies to understand and respond to trends.
  4. Patient Education:
    • Antibiotic Use: Advise patients to only take antibiotics when prescribed by a healthcare professional and to complete the full prescription, even if they feel better.
    • Hygiene Practices: Educate patients about the importance of regular hand hygiene and other practices to reduce the risk of infection.
  5. Limit Use in Agriculture:
    • Antibiotics, especially those critical for human medicine, should not be used in agriculture for growth promotion. Their use should be limited to treating diagnosed infections in animals.
  6. Research:
    • New Antibiotics: Invest in research to discover new antibiotics.
    • Alternative Therapies: Explore alternative therapies like bacteriophages, immunotherapies, and probiotics.
    • Rapid Diagnostic Tools: Develop and employ rapid diagnostic tools to quickly identify MBL-producing bacteria.
  7. Travel Precautions:
    • Travelers to regions with a high prevalence of MBL-producing bacteria should be advised about the risks and educated on prevention strategies.
  8. Public Health Campaigns:
    • Governments and health organizations should run campaigns to raise awareness about antibiotic resistance and the importance of antibiotic stewardship.

Keynotes on Metallo-beta-lactamases (MBLs)

Here are the keynotes on Metallo-beta-lactamase (MBL) producing bacteria:

  1. Definition: MBLs are enzymes produced by certain bacteria that confer resistance to a broad range of beta-lactam antibiotics, including penicillins, cephalosporins, and especially carbapenems.
  2. Mechanism: MBLs degrade antibiotics by hydrolyzing their beta-lactam ring, rendering them inactive. They require metal ions (typically zinc) for their enzymatic activity.
  3. Inhibitor Resistance: Unlike some other beta-lactamases, MBLs are not inhibited by available beta-lactamase inhibitors, such as clavulanic acid.
  4. Common Producers: Notable bacteria producing MBLs include Pseudomonas aeruginosa, Acinetobacter baumannii, and some members of the Enterobacteriaceae family like Klebsiella pneumoniae and E. coli.
  5. Global Concern: Certain types of MBLs, such as the NDM (New Delhi Metallo-beta-lactamase), have garnered international attention due to their rapid spread and association with travel and medical tourism.
  6. Clinical Implication: Infections caused by MBL producers are challenging to treat due to their broad antibiotic resistance profile, often leading to prolonged hospital stays, increased medical costs, and higher mortality rates.
  7. Detection: Laboratory detection can involve phenotypic methods, like the Imipenem-EDTA Double-Disk Synergy Test, and genotypic methods, such as PCR targeting specific MBL genes.
  8. Treatment: Options are limited and may include antibiotics like colistin, tigecycline, aminoglycosides, and fosfomycin. In some cases, combination therapy is used to enhance treatment efficacy.
  9. Prevention: Strategies encompass rigorous infection control measures in healthcare settings, antibiotic stewardship programs, patient education, surveillance, and research into new antimicrobial agents.
  10. Public Health Impact: The rise of MBL-producing bacteria underlines the global threat of antibiotic resistance, necessitating international collaboration, research, and public health initiatives to tackle their spread.

Further Readings on Metallo-beta-lactamases (MBLs)

  1. Books:
    • “Antibiotic Resistance: Mechanisms and New Antimicrobial Approaches” by Kateryna Kon and Mahendra Rai.
    • “Molecular Medical Microbiology” edited by Yi-Wei Tang, Max Sussman, Dongyou Liu, Ian Poxton, and Joseph Schwartzman offers comprehensive insights into various aspects of medical microbiology, including antibiotic resistance.
  2. Scientific Journals/Articles:
    • Journals such as “Clinical Microbiology Reviews”, “Antimicrobial Agents and Chemotherapy”, “The Journal of Antimicrobial Chemotherapy”, and “Infection and Drug Resistance” frequently feature research articles and reviews on MBLs and antibiotic resistance.
    • You might particularly look for review articles that focus on the epidemiology, mechanisms, clinical implications, and control of MBL-producing bacteria.
  3. Online Resources:
    • Centers for Disease Control and Prevention (CDC): They offer comprehensive information on antibiotic resistance and specific pathogens, including MBL producers.
    • World Health Organization (WHO): WHO provides global insights into antibiotic resistance, including data, reports, and strategies to combat MBL producers.
  4. Guidelines:
    • Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) offer guidelines on the detection and interpretation of MBL production in clinical isolates.
  5. Conferences & Workshops:
    • Consider attending international conferences on microbiology or infectious diseases. They often host sessions or workshops dedicated to antibiotic resistance, including discussions on MBLs.
  6. University Courses:
    • Numerous universities provide specialized courses or modules on antimicrobial resistance. This can be a great way to gain in-depth knowledge and stay updated with current findings.
  7. Research Databases:
    • Utilize databases like PubMed or Google Scholar to search for recent research articles, reviews, and case reports related to MBLs. Key terms such as “Metallo-beta-lactamase”, “MBL mechanisms”, “MBL epidemiology”, and “MBL treatment” can assist in narrowing down pertinent papers.

Leave a Comment