Diseases
These organisms primarily cause urinary tract infections, both community- and hospital-acquired.
Important Properties
These gram-negative rods are distinguished from other members of the Enterobacteriaceae by their ability to produce the enzyme phenylalanine deaminase. In addition, they produce the enzyme urease, which cleaves urea to form NH3 and CO2. Certain species are very motile and produce a striking swarming effect on blood agar, characterized by expanding rings (waves) of organisms over the surface of the agar (Figure 1).
Fig1. Proteus species—swarming motility on blood agar. Arrowhead points to the site where Proteus bacteria were placed on the blood agar. Short arrow points to the edge of the first ring of swarming motility. Long arrow points to the edge of the second ring of swarming motility. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
The cell wall O antigens of certain strains of Proteus, such as OX-2, OX-19, and OX-K, cross-react with antigens of several species of rickettsiae. These Proteus antigens can be used in laboratory tests to detect the presence of antibodies against certain rickettsiae in patients’ serum. This test, called the Weil Felix reaction after its originators, is being used less frequently as more specific procedures are developed.
In the past, there were four medically important species of Proteus. However, molecular studies of DNA relatedness showed that two of the four were significantly different. These species have been renamed: Proteus morganii is now Morganella morganii, and Proteus rettgeri is now Providencia rettgeri. In the clinical laboratory, these organisms are distinguished from Proteus vulgaris and Proteus mirabilis on the basis of several biochemical tests.
Pathogenesis & Epidemiology
The organisms are present in the human colon as well as in soil and water. Their tendency to cause urinary tract infections is probably due to their presence in the colon and colonization of the urethra, especially in women. The vigorous motility of Proteus organisms may contribute to their ability to invade the urinary tract.
Production of the enzyme urease is an important feature of the pathogenesis of urinary tract infections by this group. Urease hydrolyzes the urea in urine to form ammonia, which raises the pH, producing an alkaline urine. This encourages the formation of stones (calculi) called struvite composed of magnesium ammonium phosphate. Struvite stones often manifest as staghorn calculi in the renal pelvis. They obstruct urine flow, damage urinary epithelium, and serve as a nidus for recurrent infection by trapping bacteria within the stone. Because alkaline urine also favors growth of the organisms and more extensive renal damage, treatment involves keeping the urine at a low pH.
Clinical Findings
The signs and symptoms of urinary tract infections caused by these organisms cannot be distinguished from those caused by E. coli or other members of the Enterobacteriaceae. Proteus species can also cause pneumonia, wound infections, and septicemia. P. mirabilis is the species of Proteus that causes most community- and hospital-acquired infections, but P. rettgeri is emerging as an important agent of nosocomial infections.
Laboratory Diagnosis
These organisms usually are highly motile and produce a swarming overgrowth on blood agar, which can frustrate efforts to recover pure cultures of other organisms. Growth on blood agar containing phenylethyl alcohol inhibits swarming, thus allowing isolated colonies of Proteus and other organisms to be obtained. They produce non–lactose-fermenting (colorless) colonies on MacConkey’s or EMB agar. P. vulgaris and P. mirabilis produce H2S, which blackens the butt of TSI agar, whereas neither M. morganii nor P. rettgeri does. P. mirabilis is indole negative, whereas the other three species are indole-positive—a distinction that can be used clinically to guide the choice of antibiotics. These four medically important species are urease positive. Identification of these organisms in the clinical laboratory is based on a variety of biochemical reactions.
Treatment & Prevention
Most strains are sensitive to aminoglycosides and trimethoprim sulfamethoxazole, but because individual isolates can vary, antibiotic sensitivity tests should be performed. P. mirabilis is the species most frequently sensitive to ampicillin. The indole positive species (P. vulgaris, M. morganii, and P. rettgeri) are more resistant to antibiotics than is P. mirabilis, which is indole negative. The treatment of choice for the indole-positive species is a cephalosporin (e.g., cefotaxime). P. rettgeri is frequently resistant to multiple antibiotics.
There are no specific preventive measures, but many hospital acquired urinary tract infections can be prevented by prompt removal of urinary catheters.
