Pseudomonas aeruginosa is a type of bacteria commonly found in soil, water, and vegetation. It is best known as an opportunistic pathogen that mainly causes infections in people with weakened immune systems, severe burns, or those using medical devices like catheters or ventilators. The name aeruginosa is derived from a Latin term meaning “copper rust” or “verdigris,” which hints at its distinctive visual presentation. Observing this organism requires looking at both the microscopic cellular level and the macroscopic level.
The Microscopic Structure
When viewed under a microscope, the individual Pseudomonas aeruginosa cell is rod-shaped, classifying it as a bacillus. These rods typically measure between 0.5 to 0.8 micrometers in width and 1.5 to 3.0 micrometers in length, often appearing singly or in pairs. Most strains are motile, utilizing a single, whip-like appendage called a polar flagellum that extends from one end of the cell.
Distinctive Features in Laboratory Growth
The most striking visual features of Pseudomonas aeruginosa appear when grown in large colonies on laboratory culture plates. The most common color associated with its growth is a vibrant blue-green, resulting from the production and diffusion of water-soluble pigments. This characteristic color comes from a combination of two compounds: pyocyanin (blue) and pyoverdin (yellow-green fluorescent pigment). Pyoverdin is notable because it fluoresces under ultraviolet light, a feature scientists use for identification.
Beyond the unusual coloration, colonies of Pseudomonas aeruginosa often emit a strong, easily recognizable smell. This odor is frequently described as sweet, fruity, or resembling grape juice or fresh tortillas, caused by the production of the aromatic compound 2-aminoacetophenone.
A common visual sign in a culture is the development of a slimy, mucus-like texture on the surface of the colonies. This is due to the bacteria producing a thick, protective layer known as a biofilm. Strains that form this mucoid appearance are frequently isolated from patients with chronic infections, such as those with cystic fibrosis. This dense growth structure helps the bacteria stick to surfaces and evade antibiotics and the host’s immune system.
Clinical Appearance During Infection
When Pseudomonas aeruginosa causes an infection, its distinctive characteristics become visible, especially in wounds. The blue-green pigment seen in a lab setting often stains dressings, bandages, or pus draining from an infected wound. This discoloration is a strong indicator of a pseudomonal infection. The drainage is frequently accompanied by the familiar sweet, fruity odor, which can sometimes be foul in a severe infection.
One severe, though less common, manifestation is Ecthyma Gangrenosum, which is almost exclusively associated with this bacterium. This lesion begins as a painless, round, red patch that quickly progresses to a pustule and then a blister with a hemorrhagic center. Within hours, the blister develops into a deep, necrotic ulcer with a central black or gray scab, known as an eschar, surrounded by a red halo.
Infections involving the eye, particularly in contact lens wearers, are very destructive and progress rapidly. A Pseudomonas corneal ulcer appears as a dense, “soupy” ulceration with a thick, yellowish-green discharge. The surrounding cornea often takes on a hazy, “ground glass” appearance due to severe inflammation and tissue necrosis. This rapid destruction and specific discharge color make the organism one of the most serious causes of bacterial keratitis.