Sunday, December 04, 2005


Psuedomona Bacterial Infections


Pseudomonads are motile, Gram-negative rods that utilize glucose oxidatively. Members of this genus are classified into five groups based on ribosomal RNA homology. These bacteria are clinically important because they are resistant to most antibiotics and they are capable of surviving in conditions that few other organisms can tolerate. They also produce a slime layer that is resistant to phagocytosis. Pseudomonas is often encountered in hospital and clinical work because it is a major cause of hospital acquired (nosocomal) infections. Its main targets are immunocompromised individuals, burn victims, and individuals on respirators or with indwelling catheters. Additionally, these pathogens colonize the lungs of cystic fibrosis patients, increasing the mortality rate of individuals with the disease. Infection can occur at many sites and can lead to urinary tract infections, sepsis, pneumonia, pharyngitis, and a lot of other problems. Rarely will you find Pseudomonas as a cause of infection in healthy individuals. Its non-invasive nature limits its pathogenic capabilities.

P. aeruginosa

Pseudomonas aeruginosa is the most frequently isolated non-fermenter in the laboratory. It has several features that distinguish it from other species of Pseudomonas:

It can grow at 42 degrees celsius

Produces a bluish pigment (pyocyanin) and a greenish pigment

Characteristic fruity odor The basis of this organisms pathogenicity involves several toxins and chemicals which the bacterium secretes upon infection. The lipopolysaccharide layer helps the cell adhere to host tissues and prevents leukocytes from ingesting and lysing the organism. Lipases and exotoxins then procede to destroy host cell tissue which then leads to the complications associated with infection. P. aeruginosa prefers to inhabit moist environments but it can survive in a medium as deficient as distilled water. It will also grow on just about any laboratory medium and is beta-hemolytic on blood agar.

Treatment of Pseudomonas infection consists of a combination of two antibiotics: for example an anti-pseudomonal penicillin and an aminoglycoside. The best way to reduce the spread of P. aeruginosa in the hospital is to use good aseptic technique on hospital instruments and when in contact with patients.


Oxidase +
Characteristic odor and color

University of Texas - Houston Medical School


Pseudomonas Infections


A 51 year old man had been treated for the previous year for reflux oesophagitis. In addition the patient was found to have developed a small benign colonic polyp which was endo-scopically removed about a year previously.

The patient's present problem began about 3 months previously when he developed food poisoning manifested by diarrhoea about 12 hours after partaking of food at a local restaurant. Since that time, the patient had developed chronic diarrhoea manifested by the passage of several loose stools every day. The patient was investigated for his chronic diarrhoea but no aetiology was found, and stool cultures for enteric pathogens were negative. During this time the patient was empirically treated with antibiotics in the form of ciprofloxacin (Ciprobay®) and metronidazole (Flagyl®) for a period of 2 weeks, but there was only marginal improvement of his diarrhoea. He was also treated with cholestyramine (Questran®) but with no benefit.

Despite his chronic diarrhoea, the patient remained clinically quite well and was neither toxic nor systemically ill.

Because of his ongoing diarrhoea, the patient underwent a repeat colonoscopy procedure, at which time numerous intestinal biopsies were submitted for histo-logic and microbiologic analysis. The colonoscopy procedure was unremarkable and his bowel was found to be entirely normal. However, the histopathologic assessment of an antral biopsy showed Helicobacter pylori-associated chronic antritis. In addition, culture of a biopsy specimen from the patient's terminal ileum grew a profuse growth of Pseudo-monas aeruginosa (which was fully sensitive to all the antibiotics against which it was tested). Furthermore, culture of ileal biopsies from 2 other patients taken on the same day as the patient's biopsy, also grew a profuse growth of Pseudomonas aeruginosa with the identical antibiogram pattern.

The patient was not diabetic and a fasting glucose value from 3 days previously was within the normal range at 5.4 mmol/l. The patient's renal function was satisfactory and his urea from 3 days previously was 4.2 mmol/l and creatinine = 106mmol/l. The patient's activity tests were unremarkable and his C-reactive protein from 3 days before was 2mg/l and his ESR was 4mm/1 hour. The patient's liver functions were essentially normal showing the following values: bilirubin (total) = 12mmol/l, bilirubin (conjugated) = 5.6mmol/l, bilirubin (unconjugated) = 6.4mmol/l, gGT = 81U/l (range 0-65), alk. phos. = 83U/l, ALT = 57U/l (range 0-43), AST = 26U/l, total protein = 72g/l and albumin = 43g/l.

The patient's haematologic picture was unremarkable showing the following results: Hb =16.1g/dl, WCC = 7 710 x 109/l and platelets = 290 000 x 109/l. The patient's thyroid function tests from 3 days previously showed the patient to be biochemically euthyroid. A serum gastrin level from 3 days previously was still pending.


Antibiotic treatment is not indicated at present.

Repeat stool for microbiologic analysis as follows:-

a) Presence of faecal leukocytes

b) Culture for enteric pathogens and Pseudomonas aeruginosa

c) Clostridium difficile toxin assay

d) Microscopy and antigen detection for parasites

Follow-up serum gastrin result.

Exclude non-infectious causes of chronic diarrhoea.

Re-evaluate and perform an audit of current disinfection and sterilization practices of endoscopes and equipment in the gastroenterology department.

An immersion time of at least 20 minutes in 2% glutaraldehyde is a minimum requirement for adequate disinfection of endoscopes between patient procedures.

Repeat FBC, ESR, C-reactive protein.



1. Bacteriology:

a) Pseudomonas aeruginosa is a small, aerobic gram-negative rod belonging to the family Pseudomonadaceae.
b) It is motile by virtue of its single polar flagellum.
c) More than half of all clinical isolates produce the blue-green pigment called pyocyanin; this pigment is helpful in the identification of the organism and accounts for the species name aeruginosa, which refers to the distinctive colour of copper oxide.
d) Pseudomonas aeruginosa is readily identified in the clinical laboratory, and appears as a straight or slightly curved, non-sporulating, motile gram-negative rod that grows aerobically on most common culture media.

2. Epidemiology:-

a) Pseudomonas aeruginosa is ubiquitous in nature, inhabiting:-
b) The organism has a predilection for moist environments.c) The organism occasionally colonizes the following anatomic sites of healthy humans:-
external ear
upper respiratory tract
large bowel
d) Carriage rates:-
Carriage rates are relatively low in normal individuals.

However, the following patients are predisposed to an increased carriage rate of Pseudomonas aeruginosa:-
patients who have serious underlying disease
patients whose immune defences have been naturally or iatrogenically compromised.
patients who have previously received antibiotic therapy
patients who have been exposed to the hospital environment

In predisposed patients colonization with Pseudomonas aeruginosa frequently precedes invasive infection.

e) Hospital-associated Pseudomonas aeruginosa infections:-

i Most Pseudomonas aeruginosa infections are hospital-acquired (nosocomial).

ii Many potential reservoirs of infection have been identified, including:-

respiratory equipment
cleaning solutions
physiotherapy pools

iii Most reservoirs are associated with moisture.

iv It is assumed that the organism is transmitted to patients via the hands of hospital personnel or via fomites.

v While some infecting strains of Pseudomonas aeruginosa appear to be endemic within the hospital environment, others are traced to a common source associated with a specific outbreak or epidemic.

vi Epidemiologic investigation is facilitated by:-

serotyping (immunotyping) of strains on the basis of differences in lipopolysaccharide structure

the use of nucleic acid sequencing

3. Pathogenesis:-

a) That the pathogenesis of infections due to Pseudo-monas aeruginosa is complex is evidenced by:-

The clinical diversity of the diseases related to this organism.

The multiplicity of virulence factors it produces.

b) Pseudomonas aeruginosa rarely causes disease in the healthy host.
c) However, the organism may undergo a transformation to virulence when:-

i) Normal cutaneous or mucosal barriers have been breached or bypassed, such as occurs as a result of:-

burn injury
penetrating trauma
endotracheal intubation
urinary bladder catheterization
intravenous drug abuse

ii) Immunologic defence mechanisms have been compromised by factors such as:-

chemotherapy-induced neutropenia
extremes of age
diabetes mellitus
cystic fibrosis

iii) The protective function of the normal indigenous bacterial flora has been disrupted by broad-spectrum antibiotic therapy.

iv) The patient has been exposed to reservoirs associated with a hospital environment.

d) The following factors account for the frequency and success with which the organism acts as an opportunistic pathogen:-

The ubiquity of the organism.
Its flexible nutritional and metabolic requirements.
Its environmental resilience.
Its relative resistance to antibiotics

e) Sequence of events in Pseudomonas aeruginosa infections:-

i) Infection caused by Pseudomonas aeruginosa begins with bacterial attachment and superficial colonization of cutaneous or mucosal surfaces, which progresses to localized bacterial invasion and damage to underlying tissues.

ii) The infective process may advance and progress to:-
bloodstream invasion
systemic inflammatory response syndrome (SIRS)
multiple-organ dysfunction
ultimately, death

iii) Alternatively, the infection may:-
remain anatomically localized
spread by direct extension to contiguous structures

iv) The organism and its products may cause tissue injury at primary and secondary sites of infection.

v) The release of systemically acting pseudomonal toxins as well as inflammatory mediators liberated by the infected host, may contribute directly or indirectly to the sepsis syndrome

f) Attachment of Pseudomonas aeruginosa to tissues - the role of pseudomonal adhesins:-

i) The initial attachment of Pseudomonas aeruginosa to the respiratory epithelium and other epithelial surfaces is mediated by:-

bacterial organelles called pili (or fimbriae)
the bacterial mucoid exopolysaccharide termed alginate, which is produced by mucoid pseudo-monal strains

ii) Receptors for these bacterial adhesins (note: adhesins are bacterial surface antigens that usually exist in the form of filamentous projections termed pili or fimbriae, and that bind to specific receptors on epithelial cell membranes of the host) are found, for example, on tracheal epithelial cells and tracheobronchial mucin, and are composed of N-acetyl-neuraminic acid (sialic acid) and N-acetyl-glucosamine, respectively.

g) Pseudomonal virulence factors:-

i) Surface moieties of Pseudomonas aeruginosa such as exopolysaccharide and lipopolysaccharide, protect the organism from direct antibody- and complement-mediated bactericidal mechanisms and from opsonophagocytosis.

ii) The organism produces a number of host tissue-destroying extracellular enzymes, including:-

alkaline protease
exoenzymes/exotoxins A and S

iii) The breakdown of host tissues by these extra-cellular bacterial products creates conditions conducive to enhanced:-

bacterial proliferation
bacterial invasion
host tissue injury

h) Sepsis syndrome/systemic inflammatory response syndrome (SIRS):-

The enhanced bacterial proliferation and invasion resulting in host tissue injury, is particularly likely to culminate in bloodstream invasion and dissemin-ation in the face of immune compromise such as that resulting from profound neutropenia.

The sepsis syndrome/SIRS, due to Pseudomonas aeruginosa shares many of the features of gram-negative sepsis caused by other bacteria, and the lipopolysaccharide/endotoxin produced by this organism, like that produced by other bacterial species, is thought to play a pivotal role in the patho-genesis of the sepsis syndrome.

i) Exotoxin A:-

In addition to lipopolysaccharide (endotoxin), which is a structural component of the bacterial outer membrane, the extracellular enzyme called exotoxin A is produced by most clinical isolates of Pseudomonas aeruginosa.

Exotoxin A which is a diphtheria-like toxin, inhibits mammalian protein synthesis by transferring the adenosine diphosphate (ADP) ribose moiety of the nicotinamide adenine dinucleotide (NAD) into covalent linkage with elongation factor 2, an enzyme that catalyzes the elongation step in polypeptide assembly, but is inactivated by exotoxin-mediated ADP ribosylation.

Exotoxin A causes both local and systemic disease.

It is cytotoxic in vitro and necrotizing in vivo and produces fatal shock in experimental animals, including non-human primates.

Toxigenic clinical isolates of Pseudomonas aeruginosa are more virulent than non-toxigenic strains; moreover, the rate of survival among patients with Pseudomonas aeruginosa bacteraemia in the presence of adequate pre-existing levels of anti-exotoxin A-specific serum antibodies is higher than in patients with absent anti-exotoxin A antibodies.


Pseudomonas aeruginosa infections involving virtually every portion of the human gastrointestinal tract, from oro-pharynx to rectum, have been documented.

Pseudomonas aeruginosa-associated gastrointestinal disease is most common among:-

a) infants

b) adults with haematologic malignancies and neutropenia

Asymptomatic large bowel colonization resulting from prolonged exposure to the hospital environment and the selective pressure of antibiotics, may be a silent source of organisms that subsequently invade the bloodstream during severe chemotherapy-induced neutropenia or other forms of immunosuppression.

Necrotizing enterocolitis:-

a) Pseudomonas aeruginosa causes necrotizing entero-colitis in:-

i) infantsii) neutropenic patients with cancer

b) The most common sites of involvement are the:-

i) distal ileumii) caecumiii) colon

c) The pathologic lesions manifest as haemorrhagic and necrotic ulcers that begin in the bowel mucosa and extend into the submucosa.d) Vascular invasion by bacteria may be documented in the submucosa in association with bacteraemia, local spread to the muscularis and serosa, and subsequent perforation leading to peritonitis.e) Necrotic ulcers may also occasionally be found in the:-

i) oropharynxii) oesophagusiii) stomachiv) proximal small bowel

Caecitis (typhlitis):-

a) Caecitis, a disease developing most frequently in patients with leukaemia, involves localized lesions of the caecum that are associated with necrosis and gangrene, and that sometimes result in perforation, bacterial peritonitis, and early death.b) Pseudomonas aeruginosa is the agent most commonly identified in this condition.

Rectal abscess in neutropenic patients:-

a) Pseudomonas aeruginosa is among the pathogens most frequently isolated from rectal abscesses in neutropenic patients with cancer.

b) These lesions, which may be associated with few signs and symptoms, must be carefully sought in susceptible patients because they may give rise to life-threatening bacteraemic sepsis if not surgically drained.

Diarrhoeal disease due to Pseudomonas aeruginosa:-

a) Pseudomonas aeruginosa has been implicated in epidemics of moderate to severe diarrhoea in children, in a form of enteric fever referred to as Shanghai fever.

b) Pseudomonas aeruginosa has been incriminated in a cholera-like illness attributed to a putative but still unidentified Pseudomonas enterotoxin.


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