Proteus: Infections and Diseases

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Louise Hay

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Three species of high pathological interest belong to the genus Proteus: Proteus penneri, Proteus mirabilis and Proteus vulgaris; these bacteria are involved in various infections, especially of a nocosomal nature and affecting the urinary tract.

Microbiological description

Microorganisms of the genus Proteus are members of the Enterobacteriaceae family: they are gram negative, aerobic, motile, rod-shaped (elongated) bacteria. Normally, the bacilli of the genus Proteus have a size between 0,5 and 1,0 µm in width, and can be from 0,6 to 6 µm long. They are bacteria that habitually populate the gastrointestinal tract of humans and other animals (fish, reptiles, birds, mammals). Belonging to the Enterobacteriaceae family, Proteus bacilli are oxidase negative and urease / catalase / nitrase positive. The peculiar characteristic of metabolizing urea by developing ammonia (positive urease) differentiates Proteus from Salmonella.
Among the virulence factors, we remember the endotoxins, the flagella (which confer mobility and anchorage to the ureters in urinary infections) and the pili (able to adhere to the epithelia).
Microorganisms belonging to the genus Proteus usually inhabit soil, fertilizers and sewage. Except for the species P. rettgeri and P. morganii, many strains of Proteus carry out an acid-mixed fermentation and produce generous quantities of H2S (hydrogen sulphide).
Bacteria of the genus Proteus are sensitive to moist heat and dry heat. Outside the host, Proteus bacilli survive for a day or two on inanimate surfaces, although they can easily replicate in soil, water and sewers.

Proteus infections

Although they behave like commensal microorganisms in the human intestinal tract, bacilli of the genus Proteus can cause harm when they spread to other sites. In fact, once in the urinary tract, the bacillus can cause local infection: a subject appears more sensitive to these infections when his defenses are no longer sufficient to protect the body from bacterial insults.
Bacteria of the genus Proteus can be transmitted through contaminated catheters, or by accidental parenteral inoculation. Although the precise method of transmission has not yet been identified with certainty, the possibility of direct transmission is excluded.
Cystitis, pyelonephritis, and urolithiasis (stone formation in the bladder or kidneys) are the most common Proteus-mediated infections. However, following a Proteus insult, some particularly sensitive patients may also develop bacteremia and septicemia.
The most common symptoms associated with Proteus infections are:

  • alkalization of urine
  • stone formation
  • persistence of infection
  • renal failure (advanced stage)

The involvement of other organs is less frequent, although possible: in these circumstances, complications can also be documented

  • abdominal abscesses
  • cholangitis
  • surgical wound infections
  • purulent meningitis: diagnosed only in the newborn
  • pneumonia
  • septicemia (in case of severity)
  • sinusitis

The close relationship between the onset of Proteus infections and the presence of diabetic bedsores and ulcers has been observed: the pathogens, which entered the body through these lesions, can also infect the bone.


We have analyzed that bacteria of the genus Proteus are often involved in urinary tract infections and in nocosomal diseases (contracted in health-hospital structures). In Europe and America, it is estimated that 4-6% of Proteus infections are community acquired and an estimated 3-6% are nocosomal in nature.
The rate of infection is higher among the elderly, especially if catheterized or on antibiotic therapy for long periods; patients with structural urinary tract abnormalities also appear to be more at risk of this type of infection. Furthermore, it seems that Proteus infections are more frequent in uncircumcised patients.
Proteus mirabilis is the species most involved in urinary system diseases: it is estimated that 90% of Proteus infections are due to this species.

Proteus infections: therapy

Before proceeding with the therapy, the diagnostic assessment is essential, which fortunately is quite simple. Most of the Proteus strains are lactose-negative and, on agar medium, they show the typical phenomenon of swarming. Swash describes a particular phenomenon in which the Proteus colonies - grown on agar medium - do not remain confined, but form a peculiar growth film.
Most of the infections caused by Proteus are sensitive to the action of cephalosporins, imipenem and aminoglycosides: this means that these drugs are the most suitable for the treatment of the infections they carry. Proteus vulgaris and like P. penneri cannot be removed with these antibiotics, since they have developed resistance, especially to cefoxitin, cefepime, aztreonam, piperacillin, amoxicillin, ampicillin, cefoperazone, cefuroxime, and cefazolin.
Proteus mirabilis, unlike the latter and similar to E. coli, is rather simple to eradicate, since it is also sensitive to trimethoprim-sulfamethoxazole, amoxicillin, ampicillin and piperacillin. This bacterium is resistant to nitrofurantoin.
In case of respiratory tissue involvement, it is recommended to follow an antibiotic therapy with a higher posology than for mild Proteus infections. For example, ciprofloxacin should be taken at a dose of 1 gram per day for mild Proteus infections; the dosage should be doubled when Proteus bacteria also spread to the lungs.

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