Proteus species: Properties, Diseases, Identification

Last updated on June 21st, 2021

Proteus are Gram-negative rod-shaped bacteria of the family EnterobacteriaceaeThey are widely distributed in nature and also occur as normal intestinal flora of humans. An opportunistic pathogen, they are one of the common causes of urinary tract infections (UTIs) and are associated with infection-induced renal stones. Other infections caused by Proteus species are pyogenic lesions, infection of ear, respiratory tract infections, and nosocomial infections.

Proteus species have pili (fimbriae). Pili are associated with adhesive properties and, in some cases, are correlated with virulence.

The word ‘Proteus’ was derived from Greek mythology, which described ‘Proteus’ as an early sea-god, noted for being versatile and capable of assuming many different forms. Plemorphic nature of this organism and its rapid swarming motility might have persuaded its discoverer Gustav Hauser to rename it as Proteus.

Antigenic characteristics

The bacilli possess thermostable, ‘O’ (somatic) and thermostable ‘H’ (flagellar) antigens, based upon which several serotypes have been recognized.

Certain strains of Proteus vulgaris (OX-19, OX-2, and OX-K) produce O antigens that are shared by some rickettsiae. These Proteus strains are used in an agglutination test (the Weil-Felix test) for serum antibodies produced against rickettsiae of the typhus and spotted fever groups.

General Properties

  1. Gram-negative
  2. Non-spore-forming rods
  3. Facultative anaerobes
  4. Urease positive (strong)
  5. Oxidase test: Negative
  6. Nitrates are reduced to nitrites  
  7. Ferments glucose but does not ferment lactose
  8. Deaminates phenylalanine to phenyl pyruvic acid

Urease enzyme produced by Proteus species is thought to play a major role in the production of infection-induced urinary stones. The ammonia produce after the breakdown of the urea results in struvite (magnesium ammonium phosphate) stone formation. Recurrent urinary tract infections with a urease-producing organism (mostly Proteus species) results in formation of staghorn calculi in the kidney.

Stag (male deer) and Staghorn calculi (source)

Biochemical Properties of Proteus mirabilis and Proteus vulgaris

Organisms that swarm on 5% sheep blood agar, exhibit a characteristics odor, and are oxidase negative can be presumptively identified as Proteus spp.  With further testing by spot indole, the positive isolates may be presumptively reported as Proteus vulgaris and the negative ones as Proteus mirabilis.

Properties Proteus mirabilis Proteus vulgaris
Colony characteristics in MacConkey Agar    
Pale or colourless (NLF) colonies
 
Pale or colourless (NLF) colonies
Motility Swarming motility Swarming motility
Lactose fermentation No No
Indole production No Yes
Urease production Yes Yes
H2S production Yes Yes

Laboratory Diagnosis & Identification

The sample used for the isolation and identification of the Proteus species depends on the nature of the disease/site of infections. For UTI, a midstream urine sample is used, and for pyogenic lesions, it is the pus aspirate. Sample should be collected in the sterile container maintaining aseptic conditions and should reach the laboratory within an hour of collection.

Culture: The choice of the culture media used for the isolation of the etiological agents depends on the nature of the specimen and suspected pathogens. For pus & urine samples, blood agar and MacConkey agar are commonly used. Proteus grow on the Blood agar plate in successive waves to form a thin filmy layer of concentric circles ( swarming). Proteus do not swarm in the MacConkey agar medium and form smooth, pale or colourless (NLF) colonies.


Swarming in Blood Agar (Source)

Swarming properties of Proteus presents problems in the diagnostic laboratory when mixed growth is present in which Proteus is one of the isolate. Several methods have been used to inhibit swarming. These are

  • increasing the concentration of agar in the medium, raising it to 6% instead of 1-2%.
  • incorporation of cholral hydrate (1:500), sodium azide (1:500), boric acid (1:1000) in the medium
  • Using cysteine lactose electrolyte deficient (CLED) agar as a sole medium instead of MacConkey agar and blood agar for the processing of urine samples.

Dienes Phenomenon

Proteus mirabilis is well known for its ability to differentiate into hyperflagellated, motile, and elongated swarmer cells that rapidly spread over a surface.

When two different strains of P. mirabilis swarm on the same agar plate, a visible demarcation line with lower cell density forms at the intersection, and this line is known as a Dienes line (after Louis Dienes, who described the phenomenon in 1946) BUT when two identical isolates meet, the swarming edges merge without formation of a Dienes line.

This phenomenon is of value in differentiating the two strains of Proteus for epidemiological purposes. Find more about Dienes phenomenon in “small things considered blog

Identification:

Gram staining, colony characteristics in culture media, and above mentioned bio-chemical tests (indole production, urease production, H2S production and more importantly phenyl pyruvic acid (PPA) etc. ) when used in combination are sufficient to identify an isolate as Proteus species.

About Acharya Tankeshwar 473 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.