Ultrastructure of the Legionnaires' disease bacterium. A study using transmission electron microscopy

Area of Science:

• Microbiology
• Cell Biology
• Pathogen Ultrastructure

Background:

• Legionnaires' disease (LD) is a severe pneumonia caused by Legionella bacteria.
• Understanding the ultrastructure of the LD bacterium is crucial for its identification and characterization.
• Previous studies suggested gram-negative characteristics, but detailed ultrastructural analysis was needed.

Purpose of the Study:

• To investigate and describe the ultrastructural morphology of the Legionnaires' disease bacterium.
• To compare the ultrastructure of the bacterium in different environments: human lung, egg yolk membrane, and artificial culture media.
• To determine if unique ultrastructural features exist that could differentiate the LD bacterium from other gram-negative bacilli.

Main Methods:

• Transmission electron microscopy (TEM) was used to examine the LD bacterium.
• Samples were obtained from human lung tissue, developing chick embryos (yolk sac membrane), and standard artificial culture media.
• Ultrastructural features, including cell envelope, internal structures, and mode of division, were analyzed.

Main Results:

• The LD bacterium appeared as a rod-shaped prokaryote (0.3-0.9 µm diameter, ≥2.0 µm long) with consistent ultrastructure across all examined environments.
• Key features included electron-lucent nucleoids, ribosomes, cytoplasmic vacuoles/inclusions, and a double-layered, triple-membraned cell envelope (unit membrane, ~75 Å wide).
• Bacterial division occurred via a pinching, nonseptate process, characteristic of gram-negative bacteria, with no distinct peptidoglycan layer observed in the periplasmic space.

Conclusions:

• The ultrastructure of the Legionnaires' disease bacterium is consistent with its classification as a gram-negative bacillus.
• No unique ultrastructural markers were identified to distinguish the LD bacterium from other small gram-negative bacilli.
• The findings confirm previous reports on the bacterium's staining properties and gram-negative nature at the ultrastructural level.