The genus Pseudomonas is made up of more than 300 species of Gram-negative bacterium. However, only a few cause disease in humans. The most frequently responsible for disease are P. cepacia, P. maltophelia, P. aeruginosa, P. pseudomallei, and P. mallei.
P. aeruginosa is a nonfermentive bacilli that causes more than two-thirds of the infections. This particular organism is a motile obligate aerobe that grows well on most culture media. The bluish green pus gives off an odor reminiscent of sweet grapes or corn tortillas, and only attacks tissues that are already compromised. It is responsible for more than twice as many infections within hospitals than in the community. Statistics show that this organism surfaces 30-40% of the time in ordinary hospital patients, but rises to 90% if that patient spends more than days days in the ICU. It is a particular scourge for patients in the burn unit or with open wounds. The development of P. aeruginosa pneumonia or bacteremia can have serious consequences with mortality rates reaching 70%.
Commonly found on plants and as part of the normal flora, this same organism is also responsible for “swimmer’s ear” and for a more serious eye infection that often leads to the perforation of the cornea and subsequent loss of the eye. P. aeruginosa infections are very resistant to drug therapy, but there are new antibiotics that have been specifically designed for the treatment of pseudomonal infections. How long this will last before mutant strains emerge is anyone’s guess.
Pseudomonas pseudomallei is a bacterium found in the soil and water in tropical areas of Southeast Asia. It is thought to be the agent responsible for a life-threatening human disease called melioidosis. The small number of cases diagnosed is thought to be just the tip of the iceberg, however. In some areas 5-20% of the agricultural workers have the antibody to it – meaning that they have been exposed at some point in time. Old age and a lowered immunity predispose people to this serious infection. The disease is similar to human glanders, presenting either as a rapidly fatal seticaemia, or it may run a more chronic course resembling tuberculosis with pneumonia, or multiple abcesses and osteomyelitis. P. pseudomallei can remain latent for a long time until something triggers it to begin to multiply.
The disease can also affect wild and domestic animals. Such was the case when four gorillas were imported into the Singapore zoo. They died from melioidosis within a few months of arrival. It has also been estimated that over 200,000 of the 2.5 million Americans who served in Vietnam were affected, but only about 343 cases and 36 deaths have been reported. The fact that 200,000 people may be carriers of the organism has led to the adoption of the name “the Vietnam time bomb.” Treatment starts with ceftazidime, followed by other antimicrobial drugs and given for an extended period of time. It is for such reasons as this that further deplete an immune system already in trouble. This particular infection is environmental and not contagious – meaning that it must be contracted from something in environment and not from another person. However, like other Pseudomonas strains, it can multiply in dilute solutions of disinfectants, causing outbreaks in hospitals.
