Pseudomonas aeruginosa is a bacterium commonly found in diverse environments, including soil, water, and moist surfaces. This organism’s adaptability allows it to thrive even with minimal nutrients. While ubiquitous, it is also recognized as an opportunistic human pathogen, causing severe infections, particularly in individuals with compromised immune systems. Understanding its characteristics is necessary for identification in healthcare settings.
Cellular Structure and Motility
The individual P. aeruginosa cell is characterized as a Gram-negative rod, or bacillus, typically measuring between 0.5 to 1.0 micrometers wide and 1.5 to 3.0 micrometers long. The term “Gram-negative” refers to its cell wall structure, which includes a thin peptidoglycan layer situated between an inner cytoplasmic membrane and a distinct outer membrane. This outer membrane structure provides a protective barrier, contributing to the organism’s intrinsic resistance to certain compounds.
The bacterium is highly motile, primarily achieving movement through a single, whip-like appendage known as a polar flagellum. This flagellum facilitates “swimming” motility in liquid environments. The organism can also exhibit “twitching” motility, which is a surface-based movement mediated by small, hair-like structures called pili. These pili are involved in surface attachment and represent a first step in the formation of sessile communities on various materials.
Macroscopic Colony Appearance
When cultured on standard agar plates, P. aeruginosa colonies provide primary identification clues through their macroscopic appearance. Typical colonies are large, round, and flat with irregular, spreading edges, often having a smooth or slightly rough texture. A distinguishing feature is the characteristic metallic or iridescent sheen visible when light reflects off the surface, sometimes resembling a “beaten-copper” look.
Colony morphology frequently varies between non-mucoid and mucoid strains, a distinction with significant clinical implications. Non-mucoid colonies maintain the typical flat, spreading appearance with defined edges. In contrast, mucoid strains produce a large quantity of a viscous exopolysaccharide, alginate, resulting in colonies that appear wet, slimy, or gelatinous.
The mucoid phenotype is often observed in chronic infections, such as those in cystic fibrosis patients, because the alginate matrix forms a protective biofilm. This mucoid layer represents a phenotypic shift associated with increased resistance to the host immune response and certain antibiotics. Isolates recovered from clinical samples may also be larger and show more pronounced metallic patches than environmental strains.
Distinctive Pigment Production and Odor
A highly recognizable trait of P. aeruginosa is its production of water-soluble pigments that diffuse into the surrounding growth medium. The most well-known pigment is pyocyanin, a blue phenazine compound produced by a large majority of strains (90–95%). When pyocyanin is produced alongside pyoverdin, a yellow-green fluorescent pigment, the culture medium develops a classic blue-green coloration.
Pyoverdin functions as a siderophore, a molecule used to scavenge iron in low-iron environments, which is necessary for bacterial growth. Less common, but also produced by some strains, are pyorubin, which presents as a red pigment, and pyomelanin, which gives a brown-black coloration. The production of these pigments is directly linked to the organism’s fitness and sometimes its virulence. For example, pyocyanin is a toxin that can interfere with host cell functions, contributing to the organism’s persistence during infection.
In addition to its visual characteristics, P. aeruginosa cultures emit a distinctive odor, often described as sweet, grape-like, or sometimes tortilla-like. This unique aroma is due to the volatile organic compound 2-aminoacetophenone. The combination of the diffusing colored pigments and this recognizable scent provides a strong presumptive identification for the bacterium in a laboratory setting.
Growth Requirements and Metabolic Features
P. aeruginosa is highly adaptable due to its flexible metabolic capabilities, allowing it to survive in many different niches. It is typically classified as an obligate aerobe, meaning it requires oxygen for optimal energy production and growth. However, it also possesses the ability to function as a facultative anaerobe, capable of multiplying slowly in environments lacking oxygen if an alternative electron acceptor, such as nitrate, is available.
This bacterium is non-fastidious, meaning it has minimal nutritional requirements. It utilizes an extensive array of simple organic molecules as its sole sources of carbon and energy. This metabolic flexibility, demonstrated by its ability to metabolize over 100 different organic compounds, contributes significantly to its ubiquity. It can even grow using only acetate as a carbon source and ammonium sulfate as a nitrogen source.
The organism exhibits a remarkably broad temperature tolerance, thriving in a range from 4°C to 42°C. Its optimal growth temperature is around 37°C, aligning with human body temperature. Importantly, its capacity to multiply at 42°C is a distinguishing characteristic used in laboratory identification.