Pseudomonas

Pseudomonas (page 4)

© Kenneth Todar, PhD

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 eradicate.

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 through the production of enzymes such as elastase, alkaline protease and exotoxin A, and 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 and bacteremia.

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, hikers 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.

Host Defenses

Most strains of P. aeruginosaare 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 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.

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 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 Pseudomonas 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. 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 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 fluoroquinolins, 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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