Bacteriology at UW-Madison
The Microbial World
Lectures in Microbiology by Kenneth Todar PhD University of Wisconsin-Madison Department of Bacteriology
Opportunistic Infections Caused by Pseudomonas aeruginosa
© 2009 Kenneth Todar PhD
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
Gram stain of Pseudomonas
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
groups of bacteria that are true pathogens of plants. In fact,
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,
that it exploits some break in the host defenses to initiate an
In fact, Pseudomonas aeruginosa is
the epitome of an opportunistic pathogen of humans. The bacterium
never infects uncompromised tissues, yet there is hardly any tissue
it cannot infect if the tissue defenses are compromised in some manner.
It causes urinary tract infections, respiratory system
tissue infections, bacteremia, bone and joint infections,
infections and a variety of systemic infections,
in patients with severe burns and in cancer and AIDS patients who are
Pseudomonas aeruginosa infection is a serious problem in patients
with cancer, cystic fibrosis, and burns. The case fatality rate in
patients is near 50 percent.
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, smooth,
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.
Pseudomonas aeruginosa is notorious for its resistance to
antibiotics and is, therefore, a particularly dangerous and dreaded
pathogen. The bacterium is naturally resistant to many antibiotics due
to the permeabiliity barrier afforded by its Gram-negative outer
its tendency to colonize surfaces in a biofilm form makes the cells
to therapeutic concentrations antibiotics. Since its natural habitat is
the soil, living in association with the bacilli, actinomycetes and
it has developed resistance to a variety of their naturally-occuring
Moreover, Pseudomonas maintains antibiotic resistance
both R-factors and RTFs, and it is able to transfer these genes by
means of the bacterial mechanisms of horizontal gene transfer (HGT),
mainly transduction and conjugation.
The soluble blue pigment
is produced by many,
but not all, strains of Pseudomonas
Only a few antibiotics are effective against Pseudomonas,
fluoroquinolones, gentamicin and imipenem, and even these antibiotics
not effective against all strains. The futility of treating Pseudomonas
infections with antibiotics is most dramatically illustrated in cystic
fibrosis patients, virtually all of whom eventually become infected
a strain that is so resistant that it cannot be treated.
Diagnosis of P. aeruginosa infection depends upon isolation and
laboratory identification of the bacterium. It grows well on most
media and commonly is isolated on blood agar or eosin-methylthionine
agar. It is identified on the basis of its Gram morphology, inability
ferment lactose, a positive oxidase reaction, its fruity odor, and its
ability to grow at 42°C. Fluorescence under ultraviolet light is
helpful in early identification of P. aeruginosa colonies.
is also used to suggest the presence of P. aeruginosa in wounds.
For an opportunistic pathogen such as Pseudomonas aeruginosa,
the disease process begins with some alteration or circumvention of
host defenses. The pathogenesis of Pseudomonas infections is
as suggested by the number and wide array of virulence determinants
by the bacterium. Multiple and diverse determinants of virulence are
in the wide range of diseases caused, which include septicemia,
tract infections, pneumonia, chronic lung infections, endocarditis,
Most Pseudomonas infections are both invasive and toxinogenic.
The ultimate Pseudomonas infection may be seen as composed of
distinct stages: (1) bacterial attachment and colonization; (2) local
(3) disseminated systemic disease. However, the disease process may
at any stage. Particular bacterial determinants of virulence mediate
of these stages and are ultimately responsible for the characteristic
that accompany the disease.
Although colonization usually precedes infections by Pseudomonas
aeruginosa, the exact source and mode of transmission of the
are often unclear because of its ubiquitous presence in the
It is sometimes present as part of the normal flora of humans, although
the prevalence of colonization of healthy individuals outside the
is relatively low (estimates range from 0 to 24 percent depending on
The fimbriae of Pseudomonas will adhere to the epithelial cells
of the upper respiratory tract and, by inference, to other epithelial
as well. These adhesins appear to bind to specific galactose or mannose
or sialic acid receptors on epithelial cells. Colonization of the
tract by Pseudomonas requires fimbrial adherence and
be aided by production of a protease enzyme that degrades fibronectin
order to expose the underlying fimbrial receptors on the epithelial
surface. Tissue injury may also play a role in colonization of the
tract, since P. aeruginosa will adhere to tracheal epithelial
of mice infected with influenza virus but not to normal tracheal
This has been called opportunistic adherence, and it may be an
step in Pseudomonas keratitis and urinary tract infections, as
as infections of the respiratory tract.
The receptor on tracheal epithelial cells for Pseudomonas pili
is probably sialic acid (N-acetylneuraminic acid). Mucoid strains,
produce an exopolysaccharide (alginate), have an additional or
adhesin which attaches to the tracheobronchial mucin
Besides pili and the mucoid polysaccharide, there are possibly
cell surface adhesins utilized by Pseudomonas to colonize the
epithelium or mucin. Also, it is possible that surface-bound exoenzyme
S could serve as an adhesin for glycolipids on respiratory cells.
The mucoid exopolysaccharide produced by P. aeruginosa is a
polymer of mannuronic and glucuronic acid referred to as alginate.
Alginate slime forms the matrix of the Pseudomonas biofilm
which anchors the cells to their environment and in medical situations,
it protects the bacteria from the host defenses such as lymphocytes,
the ciliary action of the respiratory tract, antibodies and complement.
Biofilm mucoid strains of P. aeruginosa are also less
to antibiotics than their planktonic counterparts. Mucoid strains of P.
aeruginosa are most often isolated from patients with cystic
and they are usually found in lung tissues from such individuals.
The ability of Pseudomonas aeruginosa to invade tissues depends
upon production of extracellular enzymes and toxins that break down
barriers and damage host cells, as well as resistance to phagocytosis
the host immune defenses. As mentioned above, the bacterial capsule or
slime layer effectively protects cells from opsonization by antibodies,
complement deposition, and phagocyte engulfment.
Two extracellular proteases have been associated with virulence
that exert their activity at the invasive stage: elastase and alkaline
protease. Elastase has several activities that relate to
The enzyme cleaves collagen, IgG, IgA, and complement. It also lyses
to expose receptors for bacterial attachment on the mucosa of the lung.
Elastase disrupts the respiratory epithelium and interferes with
Alkaline protease interferes with fibrin formation and
will lyse fibrin. Together, elastase and alkaline protease destroy the
ground substance of the cornea and other supporting structures composed
of fibrin and elastin. Elastase and alkaline protease together are also
reported to cause the inactivation of gamma interferon (IFN) and tumor
necrosis factor (TNF).
P. aeruginosa produces three other soluble proteins involved
in invasion: a cytotoxin (mw 25 kDa) and two hemolysins.
The cytotoxin is a pore-forming protein. It was originally named leukocidin
because of its effect on neutrophils, but it appears to be cytotoxic
most eucaryotic cells. Of the two hemolysins, one is a phospholipase
and the other is a lecithinase. They appear to act
to break down lipids and lecithin. The cytotoxin and hemolysins
to invasion through their cytotoxic effects on neutrophils, lymphocytes
and other eucaryotic cells.
One Pseudomonas pigment is probably a determinant of virulence
for the pathogen. The blue pigment, pyocyanin, impairs the
function of human nasal cilia, disrupts the respiratory epithelium, and
exerts a proinflammatory effect on phagocytes. A derivative of
is a siderophore that is produced under low-iron conditions to
iron from the environment for growth of the pathogen. It could play a
role in invasion if it extracts iron from the host to permit bacterial
growth in a relatively iron-limited environment. No role in virulence
is known for the fluorescent pigments.
Blood stream invasion and dissemination of Pseudomonas from
sites of infection is probably mediated by the same cell-associated and
extracellular products responsible for the localized disease, although
it is not entirely clear how the bacterium produces systemic illness. P.
aeruginosa is resistant to phagocytosis and the serum bactericidal
response due to its mucoid capsule and possibly LPS. The proteases
complement, cleave IgG antibodies, and inactivate IFN, TNF and probably
other cytokines. The Lipid A moiety of Pseudomonas LPS
mediates the usual pathologic aspects of Gram-negative septicemia, e.g.
fever, hypotension, intravascular coagulation, etc. It is also assumed
that Pseudomonas Exotoxin A exerts some pathologic
during the dissemination stage (see below).
P. aeruginosa produces two extracellular protein toxins, Exoenzyme
S and Exotoxin A. Exoenzyme S has the
characteristic subunit structure of the A-component of a bacterial
toxin, and it has ADP-ribosylating activity (for a variety of
proteins) characteristic of many bacterial exotoxins. Exoenzyme S is
produced by bacteria
growing in burned tissue and may be detected in the blood before the
are. It has led to the suggestion that exoenzyme S may act to impair
of phagocytic cells in the bloodstream and internal organs as a
invasion by P. aeruginosa.
Exotoxin A has exactly the same mechanism of action as the
toxin; it causes the ADP ribosylation of eucaryotic elongation factor
2 resulting in inhibition of protein synthesis in the affected cell.
Although it is partially-identical to diphtheria toxin, it is
It utilizes a different receptor on host cells than diphtheria toxin,
but otherwise it enters
cells in the same manner and has the exact enzymatic
mechanism. The production of Exotoxin A is regulated by exogenous iron,
but the details of the regulatory process are distinctly different in C.
diphtheriae and P. aeruginosa.
Exotoxin A appears to mediate both local and systemic disease processes
caused by Pseudomonas aeruginosa. It has necrotizing activity
the site of bacterial colonization and is thereby thought to contribute
to the colonization process. Toxinogenic strains cause a more virulent
form of pneumonia than nontoxinogenic strains. In terms of its systemic
role in virulence, purified Exotoxin A is highly lethal for animals
primates. Indirect evidence involving the role of exotoxin A in disease
is seen in the increased chance of survival in patients with Pseudomonas
septicemia that is correlated with the titer of anti-exotoxin A
in the serum. Also, tox- mutants show a reduced virulence in
Table 1 (below) is a summary of the virulence determinants of
aeruginosa. Table 2 is a brief description of the diseases
by Pseudomonas aeruginosa.
Summary of the Virulence
Determinants of Pathogenic Pseudomonas aeruginosa
fimbriae (N-methyl-phenylalanine pili)
polysaccharide capsule (glycocalyx)
alginate slime (biofilm)
hemolysins (phospholipase and lecithinase)
siderophores and siderophore uptake systems
pyocyanin diffusible pigment
Antiphagocytic surface properties
capsules, slime layers
Defense against serum bactericidal reaction
slime layers, capsules, biofilm
Defense against immune responses
capsules, slime layers, biofilm
genetic exchange by transduction and conjugation
inherent (natural) drug resistance
R factors and drug resistance plasmids
adaptability to minimal nutritional requirements
widespread occurrence in a variety of habitats
Scanning electron micrograph. CDC
Table 2. Diseases caused by
Endocarditis. Pseudomonas aeruginosa infects heart
of IV drug users and prosthetic heart valves. The organism establishes
itself on the endocardium by direct invasion from the blood stream.
Respiratory infections. Respiratory infections caused by Pseudomonas
aeruginosa occur almost exclusively in individuals with a
lower respiratory tract or a compromised systemic defense mechanism.
pneumonia occurs in patients with chronic lung disease and congestive
failure. Bacteremic pneumonia commonly occurs in neutropenic cancer
undergoing chemotherapy. Lower respiratory tract colonization of cystic
fibrosis patients by mucoid strains of Pseudomonas aeruginosa
common and difficult, if not impossible, to eradicate.
Bacteremia and septicemia. Pseudomonas aeruginosa causes
bacteremia primarily in immunocompromised patients. Predisposing
include hematologic malignancies, immunodeficiency relating to AIDS,
diabetes mellitus, and severe burns. Most Pseudomonas bacteremia
is acquired in hospitals and nursing homes. Pseudomonas accounts
for about 25 percent of all hospital acquired Gram-negative bacteremias.
Central nervous system infections. Pseudomonas aeruginosa
causes meningitis and brain abscesses. The organism invades the CNS
a contiguous structure such as the inner ear or paranasal sinus, or is
inoculated directly by means of head trauma, surgery or invasive
procedures, or spreads from a distant site of infection such as the
Ear infections including external otitis. Pseudomonas
is the predominant bacterial pathogen in some cases of external otitis,
including "swimmer's ear". The bacterium is infrequently found in the
ear, but often inhabits the external auditory canal in association with
injury, maceration, inflammation, or simply wet and humid conditions.
Eye infections. Pseudomonas aeruginosa can cause
infections in the human eye. It is one of the most common causes of
keratitis, and has been isolated as the etiologic agent of neonatal
colonize the ocular epithelium by means of a fimbrial attachment to
acid receptors. If the defenses of the environment are compromised in
way, the bacterium can proliferate rapidly through the production of
enzymes such as elastase, alkaline protease and exotoxin A, and cause a
destructive infection that can lead to loss of the entire eye.
Bone and joint infections. Pseudomonas infections of
and joints result from direct inoculation of the bacteria or the
spread of the bacteria from other primary sites of infection.
infections are most often seen in IV drug users and in conjunction with
urinary tract or pelvic infections. Pseudomonas aeruginosa has
particular tropism for fibrocartilagenous joints of the axial skeleton.
aeruginosa causes chronic contiguous osteomyelitis, usually
from direct inoculation of bone and is the most common pathogen
in osteochondritis after puncture wounds of the foot.
Urinary tract infections. Urinary tract infections (UTI) caused
by Pseudomonas aeruginosa are usually hospital-acquired and
to urinary tract catheterization, instrumentation or surgery. Pseudomonas
aeruginosa is the third leading cause of hospital-acquired UTIs,
for about 12 percent of all infections of this type. The bacterium
to be among the most adherent of common urinary pathogens to the
uroepithelium. As in the case of E. coli, urinary tract
can occur via an ascending or descending route. In addition, Pseudomonas
can invade the bloodstream from the urinary tract, and this is the
of nearly 40 percent of Pseudomonas bacteremias.
Gastrointestinal infections. Pseudomonas aeruginosa can
produce disease in any part of the gastrointestinal tract from the
to the rectum. As in other forms of Pseudomonas disease, those
the GI tract occur primarily in immunocompromised individuals. The
has been implicated in perirectal infections, pediatric diarrhea,
gastroenteritis, and necrotizing enterocolitis. The GI tract is also an
important portal of entry in Pseudomonas septicemia and
Skin and soft tissue infections, including wound infections,
and dermatitis. Pseudomonas aeruginosa can cause a variety
skin infections, both localized and diffuse. The common predisposing
are breakdown of the integument which may result from burns, trauma or
dermatitis; high moisture conditions such as those found in the ear of
swimmers and the toe webs of athletes, hikers and combat troops, in the
region and under diapers of infants, and on the skin of whirlpool and
tub users. Individuals with AIDS are easily infected. Pseudomonas has
also been implicated in folliculitis and unmanageable forms of acne
Most strains of P. aeruginosa are resistant to killing
in serum alone, but the addition of polymorphonuclear leukocytes
in bacterial killing. Killing is most efficient in the presence of
opsonizing antibodies, directed primarily at the antigenic determinants
of LPS. This suggests that phagocytosis is an important defense and
opsonizing antibody is the principal functional antibody in protecting
from P. aeruginosa infections.
Once P. aeruginosa infection is established, other antibodies,
such as antitoxin, may be important in controlling disease.
that patients with diminished antibody responses (caused by underlying
disease or associated therapy) have more frequent and more serious P.
aeruginosa infections underscores the importance of
immunity in controlling Pseudomonas
infections. unfortunately, cystic fibrosis is the exception. Most
cystic fibrosis patients have high levels of circulating antibodies to
bacterial antigens, but are unable to clear P. aeruginosa efficiently
from their lungs. Cell-mediated immunity does not seem to play a major
role in resistance or defense against Pseudomonas infections.
Epidemiology and Control of P. aeruginosa
Pseudomonas aeruginosa is a common inhabitant of soil, water,
and vegetation. It is found on the skin of some healthy persons and has
been isolated from the throat (5 percent) and stool (3 percent) of
patients. In some studies, gastrointestinal carriage rates
increased in hospitalized
patients to 20 percent within 72 hours of admission.
Within the hospital, P. aeruginosa finds numerous reservoirs:
disinfectants, respiratory equipment, food, sinks, taps, toilets,
showers and mops. Furthermore,
it is constantly reintroduced into the hospital environment on fruits,
plants, vegetables, as well by visitors and patients transferred from
facilities. Spread occurs from patient to patient on the hands of
personnel, by direct patient contact with contaminated reservoirs, and
by the ingestion of contaminated foods and water.
The spread of P. aeruginosa can best be controlled by observing
proper isolation procedures, aseptic technique, and careful cleaning
monitoring of respirators, catheters, and other instruments. Topical
of burn wounds with antibacterial agents such as silver sulfadiazine,
with surgical debridement, dramatically reduces the incidence of P.
aeruginosa sepsis in burn patients.
Pseudomonas aeruginosa is frequently resistant to many commonly
used antibiotics. Although many strains are susceptible to gentamicin,
tobramycin, colistin, and fluoroquinolins, resistant forms have
developed. The combination of gentamicin and carbenicillin is
frequently used to treat
severe Pseudomonas infections. Several types of vaccines are
tested, but none is currently available for general use.
Written and Edited by Kenneth Todar. All rights
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