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Pseudomonas aeruginosa (page 1)
(This chapter has 4 pages)
© Kenneth Todar, PhD
Gram stain of Pseudomonas aeruginosa cells
Pseudomonas aeruginosa is member of the Gamma Proteobacteria
class of Bacteria. It is a Gram-negative, aerobic rod belonging to the
bacterial family Pseudomonadaceae. Since the revisionist taxonomy based
on conserved macromolecules (e.g. 16S ribosomal RNA) the family
includes only members of the genus Pseudomonas which are cleaved into
eight groups. Pseudomonas aeruginosa is the type species of its group.
which contains 12 other members.
Like other members of the genus, Pseudomonas aeruginosa is a
free-living bacterium, commonly found in soil and water. However, it
occurs regularly on the surfaces of plants and occasionally on the
surfaces of animals. Members of the genus are well known to plant
microbiologists because they are one of the few groups of bacteria that
are true pathogens of plants. In fact, Pseudomonas aeruginosa
is occasionally a pathogen of plants. However, Pseudomonas
aeruginosa has become increasingly recognized as an emerging
opportunistic pathogen of clinical relevance. Several different
epidemiological studies track its occurrence as a nosocomial pathogen
and indicate that antibiotic resistance is increasing in clinical
Pseudomonas aeruginosa is an opportunistic pathogen, meaning
that it exploits some break in the host defenses to initiate an
infection. In fact, Pseudomonas aeruginosa is the epitome of an
opportunistic pathogen of humans. The bacterium almost never infects
uncompromised tissues, yet there is hardly any tissue that it cannot
infect if the tissue defenses are compromised in some manner. It causes
urinary tract infections, respiratory system infections, dermatitis,
soft tissue infections, bacteremia, bone and joint infections,
gastrointestinal infections and a variety of systemic infections,
particularly in patients with severe burns and in cancer and AIDS
patients who are immunosuppressed. Pseudomonas aeruginosa infection is
a serious problem in patients hospitalized with cancer, cystic
fibrosis, and burns. The case fatality rate in these patients is near
Pseudomonas aeruginosa is primarily a nosocomial pathogen.
According to the CDC, the overall incidence of P. aeruginosa
in U.S. hospitals averages about 0.4 percent (4 per 1000 discharges),
the bacterium is the fourth most commonly-isolated nosocomial pathogen
accounting for 10.1 percent of all hospital-acquired infections.
Pseudomonas aeruginosa is a Gram-negative rod measuring 0.5 to
0.8 µm by 1.5 to 3.0 µm. Almost all strains are motile by
of a single polar flagellum.
The bacterium is ubiquitous in soil and water, and on surfaces
in contact with soil or water. Its metabolism is respiratory and
never fermentative, but it will grow in the absence of O2 if
NO3 is available as a respiratory electron acceptor.
Pseudomonas bacterium in nature might be found in
a biofilm, attached to some surface or substrate, or in a planktonic
form, as a unicellular organism, actively swimming by means of its
flagellum. Pseudomonas is one of the most vigorous,
bacteria seen in hay infusions and pond water samples.
In its natural habitat Pseudomonas aeruginosa is not
distinctive as a pseudomonad, but it does have a combination of
traits that are noteworthy and may relate to its pathogenesis.
• Pseudomonas aeruginosa has
very simple nutritional requirements.
It is often observed "growing in distilled water", which is evidence of
its minimal nutritional needs. In the laboratory, the simplest
for growth of Pseudomonas aeruginosa consists of acetate as a
source of carbon
and ammonium sulfate as a source of nitrogen.
• P. aeruginosa possesses the metabolic versatility for
pseudomonads are so renowned. Organic growth factors are not required,
and it can use more than seventy-five organic compounds for growth.
• Its optimum temperature for growth is 37 degrees, and it is able to
grow at temperatures as high as 42 degrees.
• It is tolerant to a wide variety of physical conditions, including
temperature. It is resistant to high concentrations of salts and
dyes, weak antiseptics, and many commonly used antibiotics.
• Pseudomonas aeruginosa has a predilection for growth
environments, which is probably a reflection of its natural existence
soil and water.
These natural properties of the bacterium undoubtedly contribute to
its ecological success as an opportunistic pathogen. They also help
the ubiquitous nature of the organism and its prominence as a
P. aeruginosa isolates may produce three colony types.
Natural isolates from soil or water typically produce a small, rough
colony. Clinical samples, in general, yield one or another of two
colony types. One type has a fried-egg appearance which is large,
with flat edges and an elevated appearance. Another type, frequently
from respiratory and urinary tract secretions, has a mucoid
which is attributed to the production of alginate slime. The
and mucoid colonies are presumed to play a role in colonization and
colonies on agar
P. aeruginosa strains produce two types of soluble pigments,
the fluorescent pigment pyoverdin and the blue pigment pyocyanin.
The latter is produced abundantly in media of low-iron content and
in iron metabolism in the bacterium. Pyocyanin (from "pyocyaneus")
to "blue pus", which is a characteristic of suppurative infections
by Pseudomonas aeruginosa.
soluble blue pigment pyocyanin
is produced by many,
but not all, strains of Pseudomonas aeruginosa