Pseudomonas

• TRANSMISSION ROUTE: Direct contact, inhalation

Definition

Pseudomonas is a member of the group of pseudomonads, which are gram-negative, rod-shaped, mostly aerobic, bacilli that include similar organisms in the genus Burkholderia.

Natural Habitat and Features

The pseudomonads are commonly found in soil or water, where they play a significant role in the degradation of organic material. In humans, they are part of the normal skin flora and are found in intestinal and respiratory passages; they are generally considered to be harmless saprotrophs. Pseudomonads are distinguished from the enteric bacteria, which they physically resemble (as mostly aerobic and with a nonfermentative metabolism), and because they use the enzyme cytochrome oxidase in their respiratory pathways.

The pseudomonads produce a variety of water-soluble pigments, including the blue pigment pyocyanin and the red pigment pyorubin, and can be easily identified by the grapelike odor many types exhibit when grown on sheep’s blood agar. Some species also produce the greenish pigment pyoverdin, which fluoresces in the presence of ultraviolet light.

Pathogenicity and Clinical Significance

Pseudomonas is considered a harmless organism in healthy persons. However, in persons with compromised immune systems, it becomes an opportunistic pathogen. It also becomes a pathogen if introduced into areas of the body that are generally sterile. Pseudomonas species, in particular aeruginosa, are problematic pathogens in persons with burns and other wounds to the skin. Under these conditions, the production of pigments by the bacterium results in a bluish-green pus.

Infections may be difficult to treat because the organism frequently exhibits resistance to antibiotics. The infection in adults has the potential to become severe, while in infants, the danger significantly increases as the organism may pass into the bloodstream.

P. aeruginosa is among the organisms commonly associated with nosocomial (hospital-acquired) infections, in which bacteria are introduced into the body from respirators or through the use of catheters. The bacteria can develop a mucoid polysaccharide biofilm on catheters. The biofilm protects the bacterial cells from the body’s immune defenses. Urinary tract infections are not uncommon under these conditions, and as many as 15 percent of such nosocomial infections are caused by Pseudomonas.

A variety of factors are involved in the pathogenic properties of Pseudomonas once it is introduced into the body. Pili, protein extensions on the cell surface, allow the bacterium to attach to tissues. Once the bacterium has begun to colonize, it secretes several types of enzymes that are damaging to the host. These enzymes include an elastase, which is particularly damaging to respiratory epithelium; a cytotoxin, which can damage or kill white blood cells; and several hemolysins, which can break down red blood cells.

P. aeruginosa also produces a toxin called exotoxin A, which acts in a manner similar to that of diphtheria toxin. It inhibits protein synthesis in cells that incorporate the toxin. The result is a potentially systemic disease, as the toxin may be released into the bloodstream.

The pigments produced by many Pseudomonas strains may also contribute to the potential virulence of the organism. Pyocyanin, a bluish pigment, impairs the normal functions of respiratory cilia and may also damage white blood cells. The pigment may also be modified by the bacterium, allowing it to increase the uptake of iron necessary for the bacterium’s replication and growth.

Persons with underlying respiratory diseases, such as those with compromised immune systems, chronic lung diseases, or cystic fibrosis, are at particular risk of P. aeruginosa infection. Because these infections are often caused by strains that produce mucoid layers on the bacterial cell surface, they are difficult to treat. Bacteremia and the dissemination of Pseudomonas may spread the organism to the heart (causing endocarditis) and to the central nervous system (causing meningitis).

A more common infection is that of otitis externa, an infection of the ear more commonly known as swimmer’s ear, which may result from contaminated water. Swimmer’s ear may also lead to an endogenous infection because Pseudomonas is commonly found among the microbiota already in the ear. Untreated middle- or inner-ear infections have the potential to develop into meningitis. An infection of the eye, keratitis, is less common but may become severe if the immune system has been compromised.

The species P. fluorescens exhibits many of the same features as P. aeruginosa. However, it grows poorly at body temperature (98.6° Fahrenheit, or 37° Celsius) and is rarely pathogenic.

Drug Susceptibility

Pseudomonas is naturally resistant to most common antibiotics, largely because of its own efflux pumps, which efficiently prevent the internalization of such drugs, and because of the type of outer membrane it produces on the surface of the cell. Many strains of Pseudomonas also possess resistance transfer factors in the form of plasmids, circular extrachromosomal pieces of deoxyribonucleic acid (DNA), which contain genes that confer the resistance to antibiotics. These plasmids may also be passed to other bacteria, spreading the danger of antibiotic resistance.

Surface infections such as otitis externa may be treated with polymyxin. However, this antibiotic is too toxic for internal use. Most therapy for Pseudomonas infections utilizes combinations of drugs that act at different levels of metabolism. Although Pseudomonas is resistant to penicillin, combinations of the penicillin derivative piperacillin, which inhibits cell wall formation, and the aminoglycoside tobramycin, an inhibitor of protein synthesis, have proven effective. Other antibiotics useful in the treatment of Pseudomonas infections include gentamicin, imipenem, aztreonam, and quinolones such as ciprofloxacin. Carbapenems like meropenem and doripenem may also be options, and newer antibiotics like ceftolozane-tazobactam and ceftazidime-avibactam have been developed to treat multidrug-resistant pseudomonas. Strains may differ in their susceptibility.

Bibliography

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