Legionella Pneumophila  

 A pneumonia outbreak occurred in 1976 among American Legion members at a convention in Philadelphia. Subsequent investigations identified Legionella pneumophila as the causative organism for this outbreak. Legionella was an unknown organism before that outbreak because it exhibits poor gram staining and needs specific culture media for optimum growth. (named so: legion →legionella) [1,2]

Staining and microbiologic features:  

• Gram-negative aerobe and pleomorphic [1]  
• Facultative intracellular organism [3]
• Tests positive for catalase and oxidase [4]
• Charcoal yeast extract supplemented with L-cysteine and iron serves as a culture medium for Legionella pneumophila [1]
• Iron enhances the growth of Legionella, and it has equipped itself with mechanisms to extract iron from the host. [1]
• It can undergo rapid multiplication within amoeba, and the encysting of amoeba protects legionella from unfavorable environmental conditions. [2]
• The organism has the ability to transition into a state of reduced metabolic activity and can thrive within biofilms [2]
• Legionella pneumophila exhibits Hippurate hydrolysis (distinguishing it from other Legionella species.) [4]
• Silver staining can visualize this organism. [5]
• Found in aquatic environments and water systems (e.g., lakes, whirlpools, air conditioning systems, showerheads, etc.) [2]
• No animal reservoir [3]

Virulence:  

• Mip protein facilitates bacterial adhesion and invasion into macrophages. [6]
• Can use pili and flagella to attach and invade the host [2]
• Legionella has devised some mechanisms to evade host defense, such as:

1. Dot/Icm (a type 4 secretion system) impairs the fusion of lysosomes and phagosomal vacuoles containing Legionella. [7]
2. Macrophages produce reactive oxygen species such as superoxide and hydrogen peroxide as part of the oxidative burst in order to destroy engulfed microbes. However, Legionella pneumophila can produce Cu-Zn superoxide dismutase and catalase-peroxidase that can protect the bacteria during this oxidative stress. [7]

• Legionella can also produce and release RNAase, phospholipase A, and phospholipase C. [7]

Transmission:  

• Breathing in aerosolized microorganisms [3]
• Absence of demonstrated Person-to-person transmission [2]

Susceptible groups:  

Immunocompromised individuals [3]

Diseases:  

• Asymptomatic infection [3]

• Pontiac fever: A self-limited influenza-like illness in which a patient can present with fever, chills, headaches, fatigue, and myalgias. The condition is named Pontiac fever because workers at the Pontiac, Michigan, Public Health Department in 1968 experienced these symptoms, and subsequent investigations established Legionella pneumophila in the air conditioning system as the cause of these symptoms. [9]

• Legionnaires’ disease: The patient develops pneumonia and presents with rapid onset fever, chills, headaches, and nonproductive cough. It can involve multiple systems, such as gastrointestinal, CNS, and renal systems. [3] Chronic lung pathology, advanced age, and tobacco smoking are some of its risk factors. [5]

• Like pneumococcal pneumonia, legionella can also result in lobar consolidation. 

Legionella pneumonia by Pulmonary Pathology is licensed under CC BY-SA 2.0.

• However, patients with legionella infection can also have pulse temperature dissociation characterized by high body temperature with low heart rate, hyponatremia, hypophosphatemia, and elevated Creatine phosphokinase (CPK) due to rhabdomyolysis. Some patients can also have diarrhea, abdominal pain, confusion, and elevated liver enzymes. [9]

Diagnostic testing:  

• Culture (Gold-standard) [3]
• Most legionella community-acquired infections are due to Legionella pneumophila serogroup 1, and urine antigen test only detects this serogroup. [8, 10]
• Polymerase Chain Reaction (PCR) detects bacterial DNA [11]
• Nucleic acid amplification testing [3] 

References:  

1. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 297)
2. CMMRS edition 6, 2016-17 (page no: 99)
3. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 298) 
4. Jawetz, Melnick, & Adelberg’s Medical Microbiology Twenty-Seventh Edition (page no: 301)
5. First Aid for USMLE step 1, 2021 edition (page no: 143)
6. Jawetz, Melnick, & Adelberg’s Medical Microbiology Twenty-Seventh Edition (page no: 302)
7. CMMRS edition 6, 2016-17 (page no: 102)
8. CMMRS edition 6, 2016-17 (page no: 103)
9. CMMRS edition 6, 2016-17 (page no: 100)
10. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 299)
11. Robbins and Cotran pathologic basis of disease, 10th edition (page no: 707)