Legionella pneumophila- Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes


Legionella pneumophila is a gram-negative, facultative intracellular bacterium that is the primary causative agent of Legionnaires’ disease, a severe form of atypical pneumonia. Here’s a brief introduction:


  • Kingdom: Bacteria
  • Phylum: Proteobacteria
  • Class: Gammaproteobacteria
  • Order: Legionellales
  • Family: Legionellaceae
  • Genus: Legionella
  • Species: Legionella pneumophila


  1. Morphology: It’s a rod-shaped bacterium, often appearing slightly curved.
  2. Culture: Grows on buffered charcoal yeast extract (BCYE) agar but not on standard media unless supplemented with cysteine and iron.
  3. Aerobic: Requires oxygen for growth.
  4. Intracellular Pathogen: Predominantly infects and multiplies within protozoa in natural water sources and can infect alveolar macrophages in humans.


  • Found naturally in freshwater environments like lakes and streams.
  • Can colonize man-made water systems like cooling towers, hot water tanks, large plumbing systems, and decorative fountains.
  • Does not typically thrive in car/home air conditioning units as they don’t use water as a coolant.

Clinical Significance:

  1. Diseases:
    • Legionnaires’ disease: A severe form of pneumonia. Symptoms include cough, fever, chills, and muscle aches.
    • Pontiac fever: A milder illness caused by the same bacteria with flu-like symptoms but without pneumonia.
  2. Transmission: It’s acquired by inhaling aerosolized water containing the bacterium. It is not transmitted from person to person.
  3. Outbreaks: Outbreaks of Legionnaires’ disease have been linked to large or complex water systems, like those in hospitals, hotels, and cruise ships.


  1. Culture: Respiratory secretions or lung tissue can be cultured on BCYE agar.
  2. Urine Antigen Test: Detects the presence of Legionella antigen in the urine.
  3. Serology: Detects the rise in antibodies to L. pneumophila.
  4. PCR: Detects the presence of bacterial DNA in a patient sample.


  • Regular inspection, cleaning, and disinfection of water systems susceptible to harboring the bacteria.
  • Designing water systems in a way that reduces the creation and spread of water aerosols.
  • Maintaining hot water storage tanks at temperatures unfavorable for the bacteria’s growth.


  • Legionnaires’ disease is typically treated with antibiotics. Macrolides (e.g., azithromycin) or fluoroquinolones (e.g., levofloxacin) are commonly used.


Legionella pneumophila has specific morphological characteristics that help in its identification:

  1. Cell Shape and Size:
    • Legionella pneumophila is a small, slender, rod-shaped bacterium. The rods are often pleomorphic, meaning they can vary in shape and size. They can sometimes appear slightly curved.
  2. Gram Stain:
    • It is a Gram-negative bacterium. When subjected to a Gram stain procedure, it does not retain the crystal violet stain and instead takes up the counterstain, appearing pink or red under a microscope.
  3. Flagella:
    • Legionella pneumophila is motile and possesses a single, polar flagellum, which it uses for movement.
  4. Intracellular Nature:
    • While not a morphological trait that’s easily observed under a standard microscope, a defining characteristic of Legionella pneumophila is its facultative intracellular lifestyle. In natural environments, it can invade and replicate within various protozoa. In human infections, it primarily targets alveolar macrophages, where it can evade the host’s defense mechanisms and replicate.
  5. Culture Characteristics:
    • When cultured on specialized media like buffered charcoal yeast extract (BCYE) agar, Legionella pneumophila colonies appear grayish-white, moist, and can have a cut-glass appearance. They require certain supplements like cysteine and iron to grow, which is why BCYE agar is supplemented with these components.
  6. Growth Requirements:
    • Legionella pneumophila is aerobic, meaning it requires oxygen to grow. It has an optimal growth temperature of 35-37°C, which is consistent with its ability to cause human disease.


The pathogenicity of Legionella pneumophila refers to its ability to cause disease in humans. Several factors contribute to its virulence, enabling it to invade, evade the host immune system, and replicate within host cells. Here’s an overview of its pathogenic attributes:

  1. Facultative Intracellular Lifestyle:
    • Legionella pneumophila primarily infects and multiplies within host cells. In natural environments, it targets protozoa, while in human infections, it primarily targets alveolar macrophages in the lungs.
  2. Type IV Secretion System (Dot/Icm System):
    • This bacterial apparatus allows L. pneumophila to inject effector proteins into the host cell. These effectors modulate host cellular processes, facilitating bacterial entry, survival, and replication inside a protective vacuole, which avoids fusion with lysosomes.
  3. Evasion of Host Immunity:
    • The bacterium can inhibit the host cell’s ability to present antigens on its surface, thereby avoiding detection and attack by immune cells. It can also suppress the pro-inflammatory response of the infected cell.
  4. Biofilm Formation:
    • In natural and artificial water systems, L. pneumophila can form biofilms, which are communities of bacteria embedded within a matrix of extracellular polymeric substances. Biofilms provide protection against environmental stresses, including disinfectants.
  5. Endotoxin:
    • As a Gram-negative bacterium, L. pneumophila has lipopolysaccharide (LPS) in its outer membrane, which can act as an endotoxin. This can stimulate a strong inflammatory response in the host.
  6. Flagella:
    • The presence of a flagellum not only facilitates motility but can also play a role in the early stages of infection by promoting adherence to host cells.
  7. Surface Structures:
    • Specific structures on the bacterial surface, such as pili, play roles in adherence to host cells, facilitating the initial stages of infection.
  8. Metabolic Flexibility:
    • L. pneumophila can adapt to different environmental conditions by altering its metabolic pathways, enhancing its survival in various settings.
  9. Resistance to Environmental Stresses:
    • The bacterium possesses mechanisms that confer resistance to environmental stresses, such as temperature changes, pH fluctuations, and nutrient deprivation.

Lab Diagnosis


Laboratory diagnosis of Legionella pneumophila infections is crucial for effective treatment and control. Several methods can be employed to detect and identify the bacterium from clinical samples:

  1. Culture:
    • The gold standard for diagnosis.
    • Buffered Charcoal Yeast Extract (BCYE) Agar: Specimens, typically respiratory secretions like sputum or bronchoalveolar lavage fluid, are plated on BCYE agar, which is enriched with cysteine and iron. The colonies of L. pneumophila appear grayish-white and can have a cut-glass appearance.
    • The culture method allows for antimicrobial susceptibility testing if necessary.
  2. Urine Antigen Test:
    • A rapid test that detects L. pneumophila serogroup 1 antigen in the urine.
    • Offers quick results and has a high sensitivity for infections caused by serogroup 1, the most common cause of Legionnaires’ disease.
    • It doesn’t detect other serogroups or species of Legionella.
  3. Molecular Methods:
    • Polymerase Chain Reaction (PCR): Used to detect Legionella DNA in respiratory specimens. It’s fast and sensitive but requires specialized equipment.
    • Sequencing and typing: Useful for epidemiological investigations and outbreak tracing.
  4. Direct Fluorescent Antibody (DFA) Test:
    • Uses fluorescently labeled antibodies to detect Legionella in respiratory specimens. Requires expertise to interpret and is less commonly used now due to the availability of more sensitive and specific tests.
  5. Serology:
    • Measures the patient’s antibody response to L. pneumophila.
    • A significant rise in antibody titers between acute and convalescent-phase serum samples indicates a recent infection.
    • Not recommended for routine diagnosis because of its limited sensitivity and specificity but can be useful in epidemiological studies.
  6. Other Tests:
    • Immunohistochemistry: Can be used on lung tissue samples, especially post-mortem, to detect the presence of the bacterium.
    • Phage Typing: Used in epidemiological investigations.

For optimal diagnostic accuracy, clinicians often employ a combination of the above tests, especially culture in tandem with rapid methods like urine antigen testing or PCR.


Treatment of infections caused by Legionella pneumophila, such as Legionnaires’ disease, primarily involves antibiotics. It’s essential to initiate treatment promptly, especially in severe cases, to improve outcomes. Here are the recommended treatments:

  1. Antibiotics:
    • Macrolides:
      • Azithromycin: It is often the first choice for treating Legionnaires’ disease due to its efficacy and reduced side effects.
    • Fluoroquinolones:
      • Levofloxacin: Frequently used and can be given orally or intravenously.
      • Moxifloxacin and Ciprofloxacin: Other fluoroquinolones that can also be effective.
    • Rifampin: Sometimes, it’s used in combination with other antibiotics, especially in severe cases or when treating immunocompromised patients. However, it’s not used alone because of the potential for rapid development of resistance.
  2. Supportive Care:
    • Hydration: Patients with Legionnaires’ disease can become dehydrated due to fever and reduced fluid intake. Ensuring adequate hydration is crucial.
    • Oxygen and Ventilation: In severe cases or when patients have difficulty breathing, supplemental oxygen or even mechanical ventilation might be required.
    • Electrolyte Management: The disease can sometimes lead to electrolyte imbalances, which may need correction.
  3. Duration of Treatment:
    • The usual duration of antibiotic treatment for Legionnaires’ disease is 7-14 days, but the exact duration can vary based on the patient’s response to therapy and the severity of the disease.
  4. Monitoring:
    • Patients should be monitored for clinical improvement and potential side effects of medications.
    • In hospitalized patients, laboratory parameters, chest X-rays, and other relevant tests can be used to monitor progress.
  5. Considerations for Special Populations:
    • In immunocompromised patients or those with other underlying health conditions, the disease might be more severe, and treatment might need to be adjusted accordingly.
  6. Prevention:
    • While not a direct treatment, it’s worth noting that preventing Legionella contamination in water systems and ensuring proper maintenance and disinfection can reduce the risk of outbreaks and individual cases.


Preventing infections caused by Legionella pneumophila primarily involves controlling its growth and spread in man-made water systems. Since the bacterium thrives in warm water environments, many preventive measures focus on water system maintenance and treatment. Here are key strategies for prevention:

  1. Water Temperature Control:
    • Ensure that hot water systems maintain temperatures above 50°C (122°F). Legionella bacteria begin to thrive at temperatures between 20°C (68°F) and 45°C (113°F). However, water should be delivered to taps at a temperature that does not pose a scalding risk, so thermostatic mixing valves may be needed.
  2. Regular Flushing of Water Systems:
    • In buildings or facilities that are not in constant use, regularly flushing out the water systems can help reduce the risk of Legionella growth. This is especially true for showers, faucets, or other outlets that might not be used frequently.
  3. Disinfection:
    • Regular disinfection of water systems can help kill Legionella bacteria. Methods include chlorination, ultraviolet (UV) light, and the use of copper-silver ionization systems.
  4. Maintenance of Cooling Towers:
    • Regularly clean and disinfect cooling towers and evaporative condensers to prevent the growth of Legionella. Ensure drift eliminators are in place and functioning to reduce the spread of water mists.
  5. Avoid Stagnation:
    • Design and maintain water systems in a way that minimizes water stagnation. Stagnant water can promote bacterial growth.
  6. Regular Inspections and Maintenance:
    • Inspect, clean, and disinfect water storage tanks and water systems periodically.
    • Regularly replace components like filters or pipes that can harbor bacteria.
  7. Biofilm Control:
    • Biofilms in pipes or tanks can protect Legionella from disinfectants and provide a niche for growth. Using treatments that disrupt biofilms can help in control efforts.
  8. Monitoring and Testing:
    • Regularly monitor water parameters like temperature and pH. Periodic testing for the presence of Legionella can be useful, especially in high-risk settings like hospitals.
  9. Risk Assessment:
    • Conduct routine risk assessments of building water systems, especially in healthcare facilities, hotels, and other large complexes.
    • Implement a water safety plan that addresses the identified risks.
  10. Education:
  • Train facility managers, maintenance staff, and other relevant personnel on Legionella risks, prevention strategies, and appropriate response measures.
  1. Special Considerations for Healthcare Facilities:
  • Patients, especially those who are immunocompromised, are at a higher risk. Ensure stricter control measures in places like intensive care units or transplant units.
  • Implement point-of-use filters for high-risk patient areas.


Here are the keynotes on Legionella pneumophila:

  1. Taxonomy & Classification:
    • Gram-negative, rod-shaped bacterium.
    • Belongs to the genus Legionella.
  2. Ecology & Habitat:
    • Found primarily in freshwater environments.
    • Can colonize man-made water systems, including cooling towers, plumbing systems, and hot water tanks.
  3. Diseases:
    • Primary causative agent of Legionnaires’ disease, a severe form of pneumonia.
    • Also causes Pontiac fever, a milder, flu-like illness.
  4. Transmission:
    • Acquired by inhaling aerosolized water containing the bacterium.
    • Not spread person-to-person.
  5. Pathogenicity:
    • Facultative intracellular pathogen, primarily infecting alveolar macrophages.
    • Utilizes a Type IV secretion system to inject effector proteins into host cells, facilitating intracellular survival and replication.
  6. Diagnosis:
    • Culture on buffered charcoal yeast extract (BCYE) agar.
    • Urine antigen test (detects serogroup 1).
    • Polymerase chain reaction (PCR).
    • Direct fluorescent antibody (DFA) test.
  7. Treatment:
    • Macrolides (especially azithromycin) and fluoroquinolones (e.g., levofloxacin) are the primary antibiotics used.
  8. Prevention:
    • Regular maintenance and disinfection of water systems.
    • Temperature control: maintaining hot water systems above 50°C (122°F).
    • Risk assessments and water safety plans for building water systems.
  9. Outbreaks:
    • Often linked to contaminated large water systems like those in hotels, hospitals, or cruise ships.
    • Regular monitoring and prompt intervention are crucial during suspected outbreaks.

Further Readings

  1. Textbooks:
    • “Medical Microbiology” by Patrick R. Murray, Ken S. Rosenthal, and Michael A. Pfaller: This provides a comprehensive overview of various pathogens, including L. pneumophila.
    • “Principles and Practice of Infectious Diseases” by Gerald L. Mandell, John E. Bennett, and Raphael Dolin: A detailed guide to infectious diseases, including Legionnaires’ disease.
  2. Journals:
    • Clinical Microbiology Reviews: This journal often publishes comprehensive review articles on different pathogens.
    • Journal of Clinical Microbiology: Contains research articles and case reports about L. pneumophila and associated diseases.
  3. Specific Review Articles:
    • Fields BS, Benson RF, Besser RE. “Legionella and Legionnaires’ disease: 25 years of investigation.” Clinical Microbiology Reviews. 2002;15(3):506-526. This provides a historical and comprehensive review of the organism and disease.
  4. Online Resources:
    • Centers for Disease Control and Prevention (CDC): The CDC’s website on Legionella offers an overview, guidelines, prevention strategies, and other resources. The toolkit for developing a water management program to reduce Legionella growth is especially helpful.
    • World Health Organization (WHO): Offers guidelines on the prevention and control of Legionellosis.
  5. Scientific Databases:
    • PubMed: A vast database to find numerous research articles, reviews, and case reports on L. pneumophila.
  6. Guidelines and Recommendations:
    • “Legionellosis: Risk management for building water systems” by ANSI/ASHRAE Standard 188-2015. It provides guidelines for risk management concerning the growth and spread of Legionella in building water systems.
  7. Outbreak Reports:
    • Morbidity and Mortality Weekly Report (MMWR) from the CDC: Often features reports of outbreaks of Legionnaires’ disease, providing insights into real-world cases, their sources, and interventions.
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