Pseudomonas

[CLBernat]

As homeopaths we treat patients, but it is always wise to know the pathology
of the diseases our patients entertain.
Dr. Carol Bernat
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.
Pseudomonas aeruginosa is a Gram-negative, aerobic rod belonging to the
bacterial family Pseudomonadaceae. The family includes other genera, which,
together with certain other organisms, constitute the bacteria informally known as
pseudomonads. These bacteria are common inhabitants of soil and water. They
occur regularly on the surfaces of plants and occassionally on the surfaces
of animals. The pseudomonads 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. But
Pseudomonas aeruginosa and two former Pseudomonas species (now reclassified as
Burkholderia) are pathogens of humans. A general treatment of the
pseudomonads is presented in _The Genus Pseudomonas _
(http://textbookofbacteriology.net/Pseudomonas.etc.html) . This chapter deals specifically with Pseudomonas
aeruginosa as a pathogen of humans.
Pseudomonas aeruginosa is an opportunistic pathogen, meaning that it
exploits some break in the host defenses to initiate an infection. 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 50
percent.
Pseudomonas aeruginosa is primarily a nosocomial pathogen. According to the
CDC, the overall incidence of P. aeruginosa infections in US hospitals
averages about 0.4 percent (4 per 1000 discharges), and the bacterium is the fourth
most commonly-isolated nosocomial pathogen accounting for 10.1 percent of
all hospital-acquired infections.
Characteristics
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 means 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.
The typical 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, fast-swimming bacteria seen in hay infusions and pond
water samples.
In its natural habitat Pseudomonas aeruginosa is not particularly
distinctive as a pseudomonad, but it does have a combination of physiological 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 medium for growth of
Pseudomonas aeruginosa consists of acetate for carbon and ammonium sulfate for
nitrogen.
--P. aeruginosa possesses the metabolic versatility for which 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 in moist
environments, which is probably a reflection of its natural existence in soil and water.
These natural properties of the bacterium undoubtedly contribute to its
ecological success as an opportunistic pathogen. They also help explain the
ubiquitous nature of the organism and its prominance as a nosocomial pathogen.
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 smooth colony types. One type has a
fried-egg appearance which is large, smooth, with flat edges and an elevated
appearance. Another type, frequently obtained from respiratory and urinary tract
secretions, has a mucoid appearance, which is attributed to the production of
alginate slime. The smooth and mucoid colonies are presumed to play a role
in colonization and virulence.
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 functions in iron metabolism in the
bacterium. Pyocyanin (from "pyocyaneus") refers to "blue pus" which is a
characteristic of suppurative infections caused 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 outer membrane LPS. Also, its tendency to colonize surfaces in a
biofilm form makes the cells impervious to therapeutic concentrations
antibiotics. Since its natural habitat is the soil, living in association with the
bacilli, actinomycetes and molds, it has developed resistance to a variety of
their naturally-occuring antibiotics. Moreover, Pseudomonas maintains
antibiotic resistance plasmids, both R-factors and RTFs, and it is able to transfer
these genes my means of the bacterial processes of transduction and
conjugation.
Only a few antibiotics are effective against Pseudomonas, including
fluoroquinolones, gentamicin and imipenem, and even these antibiotics are 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 with a strain that is so
resistant that it cannot be treated.
Diagnosis
Diagnosis of P. aeruginosa infection depends upon isolation and laboratory
identification of the bacterium . It grows well on most laboratory media and
commonly is isolated on blood agar or eosin-methylthionine blue agar. It is
identified on the basis of its Gram morphology, inability to 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. Fluorescence is also used to suggest the presence of
P. aeruginosa in wounds.
Pathogenesis
For an opportunistic pathogen such as Pseudomonas aeruginosa, the disease
process begins with some alteration or circumvention of normal host defenses.
The pathogenesis of Pseudomonas infections is multifactorial, as suggested by
the number and wide array of virulence determinants possessed by the
bacterium. Multiple and diverse determinants of virulence are expected in the wide
range of diseases caused, which include septicemia, urinary tract infections,
pneumonia, chronic lung infections, endocarditis, dermatitis, and
osteochondritis.
Most Pseudomonas infections are both invasive and toxinogenic. The ultimate
Pseudomonas infection may be seen as composed of three distinct stages: (1)
bacterial attachment and colonization; (2) local invasion; (3) disseminated
systemic disease. However, the disease process may stop at any stage.
Particular bacterial determinants of virulence mediate each of these stages and are
ultimately responsible for the characteristic syndromes that accompany the
disease.
Colonization
Although colonization usually precedes infections by Pseudomonas aeruginosa,
the exact source and mode of transmission of the pathogen are often unclear
because of its ubiquitous presence in the environment. It is sometimes
present as part of the normal flora of humans, although the prevalence of
colonization of healthy individuals outside the hospital is relatively low (estimates
range from 0 to 24 percent depending on the anatomical locale).
The fimbriae of Pseudomonas will adhere to the epithelial cells of the upper
respiratory tract and, by inference, to other epithelial cells as well.
These adhesins appear to bind to specific galactose or mannose or sialic acid
receptors on epithelial cells. Colonization of the respiratory tract by
Pseudomonas requires fimbrial adherence and may be aided by production of a protease
enzyme that degrades fibronectin in order to expose the underlying fimbrial
receptors on the epithelial cell surface. Tissue injury may also play a role
in colonization of the respiratory tract since P. aeruginosa will adhere to
tracheal epithelial cells of mice infected with Influenza virus but not to
normal tracheal epithelium. This has been called opportunistic adherence, and it
may be an important step in Pseudomonas keratitis and urinary tract
infections, as well as infections of the respiratory tract.
The receptor on tracheal epithelial cells for Pseudomonas pili is probably
sialic acid (N-acetylneuraminic acid). Mucoid strains, which produce an
exopolysaccharide (alginate) have an additional or alternative adhesin which
attaches to the tracheobronchial mucin (N-acetylglucosamine). Besides pili and the
mucoid polysaccharide, there are possibly two other cell surface adhesins
utilized by Pseudomonas to colonize the respiratory epithelium or mucin. Also,
it is likely that surface-bound exoenzyme S could serve as an adhesin for
glycolipids on respiratory cells.
The mucoid exopolysaccharide produced by P. aeruginosa is a repeating
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, phagocytes, the ciliary action of the
respiratory tract, antibodies and complement. Biofilm mucoid strains of P. aeruginosa
are also less susceptible to antibiotics than their planktonic counterparts.
Mucoid strains of P. aeruginosa are most often isolated from patients with
cystic fibrosis and they are usually found in post mortem lung tissues from
such individuals.
Invasion
The ability of Pseudomonas aeruginosa to invade tissues depends upon
production of extracellular enzymes and toxins that break down physical barriers and
damage host cells, as well as resistance to phagocytosis and 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 virulence. The enzyme cleaves
collagen, IgG, IgA, and complement. It also lyses fibronectin to expose receptors for
bacterial attachment on the mucosa of the lung. Elastase disrupts the
respiratory epithelium and interferes with ciliary function. 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 for most eukaryotic cells. Of the two
hemolysins, one is a phospholipase and the other is a lecithinase. They appear to
act synergistically to break down lipids and lecithin. The cytotoxin and
hemolysins contribute to invasion through their cytotoxic effects on eukaryotic
cells.
One Pseudomonas pigment is probably a determinant of virulence for the
pathogen. The blue pigment, pyocyanin, impairs the normal function of human nasal
cilia, disrupts the respiratory epithelium, and exerts a proinflammatory
effect on phagocytes. A derivative of pyocyanin, pyochelin, is a siderophore that
is produced under low-iron conditions to sequester iron from the environment
for growth of the pathogen. No role in virulence is known for the
fluorescent pigments.
Dissemination
Blood stream invasion and dissemination of Pseudomonas from local 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 inactivate complement, cleave IgG antibodies,
and inactivate IFN, TNF and probably other cytokines . The Lipid A moiety of
Pseudomonas LPS (endotoxin) mediates the usual pathologic aspects of
Gram-negative septicemia, e.g. fever, hypotension, intravascular coagulation, etc.
It is also assumed that PseudomonasExotoxin A exerts some pathologic activity
during the dissemination stage (see below).
Toxinogenesis
P. aeruginosa produces two extracellular protein toxins, Exoenzyme S and
Exotoxin A. Exoenzyme S is probably an exotoxin. It has the characteristic
subunit structure of the A-component of a bacterial toxin, and it has
ADP-ribosylating activity (for a variety of eukaryotic proteins) characteristic of
exotoxins. Exoenzyme S is produced by bacteria growing in burned tissue and may be
detected in the blood before the bacteria are. It has been suggested that
exoenzyme S may act to impair the function of phagocytic cells in the
bloodstream and internal organs to prepare for invasion by P. aeruginosa.
Exotoxin A has exactly the same mechanism of action as the diphtheria toxin,
it causes the ADP ribosylation of eukaryotic elongation factor 2. It is
partially-identical to diphtheria toxin, but it is antigenically-distinct. It
utilizes a different receptor on host cells, but otherwise it enters cells in
the same manner as the diphtheria toxin and it has the exact enzymatic
mechanism. The production of Exotoxin A in 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 at 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 including 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 antibodies in the serum. Also, tox- mutants show a
reduced virulence in some models.
Table 1 (below) is a summary of the virulence determinants of Pseudomonas
aeruginosa. Table 2 is a brief description of the diseases caused by
Pseudomonas aeruginosa.
Table 1. Summary of the Virulence Determinants of Pathogenic Pseudomonas
aeruginosa
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Adhesins
fimbriae (N-methyl-phenylalanine pili)
polysaccharide capsule (glycocalyx)
alginate slime (biofilm)
Invasins
elastase
alkaline protease
hemolysins (phospholipase and lecithinase)
cytotoxin (leukocidin)
siderophores and siderophore uptake systems
pyocyanin diffusible pigment
Motility/chemotaxis
flagella
Toxins
Exoenzyme S
Exotoxin A
Lipopolysaccharide
Antiphagocytic surface properties
capsules, slime layers
LPS
Defense against serum bactericidal reaction
slime layers, capsules
LPS
protease enzymes
Defense against immune responses
capsules, slime layers
protease enzymes
Genetic attributes
genetic exchange by transduction and conjugation
inherent (natural) drug resistance
R factors and drug resistance plasmids
Ecologic criteria
adaptability to minimal nutritional requirements
metabolic diversity
widespread occurrence in a variety of habitats
Table 2. Diseases caused by Pseudomonas aeruginosa
Endocarditis. Pseudomonas aeruginosa infects heart valves 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 compromised lower
respiratory tract or a compromised systemic defense mechanism. Primary pneumonia
occurs in patients with chronic lung disease and congestive heart failure.
Bacteremic pneumonia commonly occurs in neutropenic cancer patients undergoing
chemotherapy. Lower respiratory tract colonization of cystic fibrosis patients by
mucoid strains of Pseudomonas aeruginosa is common and difficult, if not
impossible, to treat.
Bacteremia and Septicemia. Pseudomonas aeruginosa causes bacteremia
primarily in immunocompromised patients. Predisposing conditions include hematologic
malignancies, immunodeficiency relating to AIDS, neutropenia, 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 from a contiguous structure
such as the inner ear or paranasal sinus, or is inoculated directly by means
of head trauma, surgery or invasive diagnostic procedures, or spreads from a
distant site of infection such as the urinary tract.
Ear infections including external otitis. Pseudomonas aeruginosa is the
predominant bacterial pathogen in some cases of external otitis including
"swimmer's ear". The bacterium is infrequently found in the normal 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 devastating infections in
the human eye. It is one of the most common causes of bacterial keratitis, and
has been isolated as the etiologic agent of neonatal ophthalmia. Pseudomonas
can colonize the ocular epithelium by means of a fimbrial attachment to
sialic acid receptors. If the defenses of the environment are compromised in any
way the bacterium can proliferate rapidly and, through the production of
enzymes such as elastase, alkaline protease and exotoxin A, cause a rapidly
destructive infection that can lead to loss of the entire eye.
Bone and joint infections. Pseudomonas infections of bones and joints result
from direct inoculation of the bacteria or the hematogenous spread of the
bacteria from other primary sites of infection. Blood-borne infections are most
often seen in IV drug users, and in conjunction with urinary tract or pelvic
infections. Pseudomonas aeruginosa has a particular tropism for
fibrocartilagenous joints of the axial skeleton. Pseudomonas aeruginosa causes chronic
contiguous osteomyelitis, usually resulting from direct inoculation of bone,
and is the most common pathogen implicated in osteochondritis after puncture
wounds of the foot.
Urinary tract infections. Urinary tract infections (UTI) caused by
Pseudomonas aeruginosa are usually hospital-acquired and related to urinary tract
catheterization, instrumentation or surgery. Pseudomonas aeruginosa is the third
leading cause of hospital-acquired UTIs, accounting for about 12 percent of
all infections of this type. The bacterium appears to be among the most
adherent of common urinary pathogens to the bladder uroepithelium. As in the case
of E. coli urinary tract infection can occur via an ascending or descending
route. In addition, Pseudomonas can invade the bloodstream from the urinary
tract, and this is the source of nearly 40 percent of Pseudomonas bacteremias.
Gastrointestinal infections. Pseudomonas aeruginosa can produce disease in
any part of the gastrointestinal tract from the oropharynx to the rectum. As
in other forms of Pseudomonas disease, those involving the GI tract occur
primarily in immunocompromised individuals. The organism has been implicated in
perirectal infections, pediatric diarrhea, typical gastroenteritis, and
necrotizing enterocolitis. The GI tract is also an important portal of entry in
Pseudomonas septicemia.
Skin and soft tissue infections, including wound infections, pyoderma and
dermatitis. Pseudomonas aeruginosa can cause a variety of skin infections, both
localized and diffuse. The common predisposing factors 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 and combat troops, in the perineal region and under diapers of infants,
and on the skin of whirlpool and hot tub users. Individuals with AIDS are
easily infected. Pseudomonas has also been implicated in folliculitis and
unmanageable forms of acne vulgaris.
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Host Defenses
Most strains of P. aeruginosa are resistant to killing in serum alone, but
the addition of polymorphonuclear leukocytes results in bacterial killing.
Killing is most efficient in the presence of type-specific opsonizing
antibodies, directed primarily at the antigenic determinants of LPS. This suggests
that phagocytosis is an important defense and that opsonizing antibody is the
principal functioning antibody in protecting from P. aeruginosa infections.
Once P. aeruginosa infection is established, other antibodies, such as
antitoxin, may be important in controlling disease. The observation 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 antibody-mediated immunity in controlling
infections. 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 Infections
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 nonhospitalized patients. The
gastrointestinal carriage rates increase 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, and mops. Furthermore, it is
constantly reintroduced into the hospital environment on fruits, plants,
vegetables, as well by visitors and patients transferred from other facilities. Spread
occurs from patient to patient on the hands of hospital 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 and monitoring of
respirators, catheters, and other instruments. Topical therapy of burn wounds
with antibacterial agents such as silver sulfadiazine, coupled 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 amikacin, resistant forms have developed. The combination of
gentamicin and carbenicillin is frequently used to treat severe Pseudomonas
infections. Several types of vaccines are being tested, but none is currently
available for general use.
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