Proteus mirabilis

Epidemiology

Bacteria of the species Proteus mirabilis are widely distributed in soil and water in the natural environment. In humans, Proteus is found as part of the normal flora of the gut. Its main pathological role is in infections of the urinary tract, but it can also cause wound infections and septicaemia. About a quarter of the human population will have Pr. mirabilis in their faeces. It is not an important cause of urinary tract infections in the anatomically normal urinary tract, but it commonly invades when the normal function of the tract is disturbed by instrumentation such as catheterisation. Infection is often by the strains resident in the patientís faeces. It can also be transmitted between catheter users by care staff responsible for catheter management.

Virulence

The most important virulence factor of Pr. mirabilis in relation to catheter encrustation is urease. Although several other species can produce this enzyme, Pr. mirabilis has by far the greatest ability to produce ammonia from urea.

Physiology

Proteus can exist in two distinct morphological and physiological forms, known as swimmer cells and swarmer cells. In aqueous suspension Pr. mirabilis is found in the swimmer state, small rod-like cells1 to 2 μm in length motile by 8 to 10 flagella. On contact with a surface, Proteus has the ability to convert to the swarmer state. The bacterial cells elongate dramatically to form highly flagellated filaments 20 to 80 μm in length. These cells line up in parallel to form rafts that are able to move rapidly over surfaces en masse.

Electron micrograph of Proteus mirabilis swarmer cells moving across a surface

Physiologically, there is a significant increase in protein synthesis and urease is produced in amounts 30 to 80 times higher than in the swimmer state. Polysaccharides are secreted in large amounts from the cells and this is thought to facilitate both enhance adhesion and swarming.

On agar surfaces, Pr. mirabilis produces coordinated bursts of swarming activity interspersed with a consolidation to the swimmer state. This produces characteristic concentric rings of growth.

Swarming of Proteus mirabilis on an agar plate showing the typical growth rings

This ability to form swarmer cells seems to allow rapid colonisation of solid surfaces and the establishment of extensive Proteus biofilms. It has been shown that Pr. mirabilis can swarm rapidly over the surfaces of all the major types of catheter. Swarming may thus play roles in both the initiation of catheter associated infection and the subsequent spread of the biofilm over the catheter surface.

Further reading on Proteus mirabilis :-
BioFilms and Encrustation »
Catheter Associated Urinary Tract Infection »

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