The Proteeae constitute at least 3 genera of gram-negative organisms:
Proteus: P. mirabilis, P. vulgaris, and P. myxofaciens
Morganella: M. morganii
Providencia: P. rettgeri, P. alcalifaciens, and P. stuartii
However, P. mirabilis causes most human infections. These organisms are normal fecal flora and are present in soil and water. They are often present in superficial wounds, draining ears, and sputum, particularly in patients whose normal flora has been eradicated by antibiotic therapy. They may cause bacteremia and deep-seated infections, particularly in the ears and mastoid sinuses, peritoneal cavity, and urinary tract of patients with chronic urinary tract infections or with renal or bladder stones; Proteus organisms produce urease, which hydrolyzes urea, leading to alkaline urine and the formation of struvite (magnesium ammonium phosphate) stones.
P. mirabilis is often sensitive to ampicillin, carbenicillin, ticarcillin, piperacillin, cephalosporins, fluoroquinolones, and aminoglycosides but is resistant to tetracyclines and nitrofurantoin. Multidrug-resistant P. mirabilis is an emerging problem.
Indole-positive species (P. vulgaris, M. morganii, P. rettgeri) tend to be more resistant but generally are sensitive to fluoroquinolones, carbapenems, piperacillin/tazobactam, 3rd-generation cephalosporins, and cefixime.