Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Pseudomonas

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find an associated nucleic acid. Despite the strong convictions

of many, none was ever found.

In 1983, the protein of the prion was found in Prusiner’s

laboratory and the following year, a portion of the amino acid

sequence was determined by Leroy Hood. With that knowl-

edge, molecular biological studies of prionsensued and an

explosion of new information followed. Prusiner collaborated

with Charles Weissmann on the molecular cloningof the gene

encoding the prion protein (PrP). Work was also done on link-

ing the PrP gene to the control of scrapie incubation times in

mice and on the discovery that mutationswithin the protein

itself caused different incubation times. Antibodies that pro-

vided an extremely valuable tool for prion research were first

raised in Prusiner’s lab and used in the discovery of the nor-

mal form of PrP protein. By the early 1990s, the existence of

prions as causative agents of diseases like CJD in humans and

bovine spongiform encephalopathy (BSE) in cows, came to be

accepted in many quarters of the scientific community. As pri-

ons gained wider acceptance among scientists, Prusiner

received many scientific prizes. In 1997, Prusiner was

awarded the Nobel Prize for medicine.

See alsoBSE and CJD disease; Infection and resistance; Viral

genetics

PPseudomembranous colitisSEUDOMEMBRANOUS COLITIS

Pseudomembranous colitis is severe inflammationof the colon

in which raised, yellowish plaques, or pseudomembranes,

develop on the mucosal lining. The plaques consist of clumps

of dead epithelial cells from the colon, white blood cells, and

fibrous protein.

Pseudomembranous colitis is usually associated with

antibiotic use. When the normal balance of the flora in the

colon is disturbed, pathogenic strains of the bacillus

Clostridium difficilemay proliferate out of control and produce

damaging amounts of cytotoxins known as cytotoxins A and B.

C. difficiletoxins often cause diarrhea and mild inflam-

mation of the colon. Less frequently, the condition may

progress further, causing ulceration and formation of the

pseudomembranous plaques. Pseudomembranous colitis is

most common in health care facilities such as hospitals and

nursing homes, where an individual is most likely to be

immune-compromised and to come into contact with persist-

ent, heat-resistant C. difficilespores by the fecal-oral route.

Thus, the best way to prevent it is meticulous cleanliness, cou-

pled with avoiding the overuse of antibiotics.

Mild symptoms such as diarrhea often disappear spon-

taneously soon after the antibiotics are discontinued.

Ironically, severe antibiotic-associated colitis must generally

be treated with additional antibiotics to target the C. difficile

pathogen. Benign intestinal flora such as lactobacillusor non-

pathogenic yeast may be administered orally or rectally.

Supportive therapies such as intravenous fluids are used as in

other cases of ulcerative colitis. In rare cases, surgery to

remove the damaged section of colon may be required.

While antibiotic use is the most common precipitating

cause of pseudomembranous colitis, occasionally the condi-

tion may result from chemotherapy, bone marrow transplanta-

tion, or other causes.

See also Microbial flora of the stomach and gastroin-

testinal tract

PPseudomonasSEUDOMONAS

The genus Pseudomonasis made up of Gram-negative, rod-

shaped bacteria that inhabit many niches. Pseudomonas

species are common inhabitants of the soil, water, and vegeta-

tion. The genus is particularly noteworthy because of the ten-

dency of several species to cause infections in people who are

already ill, or whose immune systems are not operating prop-

erly. Such infections are termed opportunistic infections.

Pseudomonasrarely causes infections in those whose

immune systems are fully functional. The disease-causing

members of the genus are therefore prevalent where illness

abounds. Pseudomonasare one of the major causes of noso-

comial (hospital acquired) infections.

Bacteria in this genus not only cause infections in

man, but also cause infections in plants and animals (e.g.,

horses). For example, Pseudomonas malleicauses ganders

disease in horses.

The species that comprise the genus Pseudomonasare

part of the wider family of bacteria that are classified as

Pseudomonadaceae. There are more than 140 species in the

genus. The species that are associated with opportunistic

infections include Pseudomonas aeruginosa, Pseudomonas

maltophilia, Pseudomonas fluorescens, Pseudomonas putida,

Pseudomonas cepacia, Pseudomonas stutzeri, and

Pseudomonas putrefaciens. Pseudomonas aeruginosais prob-

ably the most well-known member of the genus.

Pseudomonasare hardy microorganisms, and can grow

on almost any available surface where enough moisture and

nutrients are present. Members of the genus are prone to form

the adherent bacterial populations that are termed biofilms.

Moreover, Pseudomonas aeruginosaspecifically change their

genetic behavior when on a surface, such that they produce

much more of the glycocalyxmaterial than they produce when

floating in solution. The glycocalyx-enmeshed bacteria

become extremely resistant to antibacterial agents and

immune responses such as phagocytosis.

In the hospital setting Pseudomonas aeruginosacan

cause very serious infections in people who have cancer, cystic

fibrosis, and burns. Other infections in numerous sites in the

body, can be caused by Pseudomonas spp.Infections can be

site-specific, such as in the urinary tract or the respiratory sys-

tem. More widely disseminated infections (termed systemic

infections) can occur, particularly in burn victims and those

whose immune systems are immunosuppressed.

For those afflicted with cystic fibrosis, the long-lasting

lung infection caused by Pseudomonas aeruginosacan ulti-

mately prove to be fatal. The bacteria have a surface that is

altered from their counterparts growing in natural environ-

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