Proteus are Gram-negative rod-shaped bacteria of the family *Enterobacteriaceae.*They 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 Proteusspecies are pyogenic lesions, infections of the 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.Plemorphicnature of this organism and its rapid swarming motility might have persuaded its discoverer Gustav Hauser to rename it asProteus.
Characteristics ofProteusspp
- Gram-negative
- Non-spore-forming rods
- Facultative anaerobes
- Urease positive (strong)
- Oxidase test: Negative
- Nitrates are reduced to nitrites
- Ferments glucose but does not ferment lactose
- 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 produced after the breakdown of the urea results in struvite (magnesium ammonium phosphate) stone formation. Recurrent urinary tract infections with a urease-producing organism (mostly I species) result in the formation of staghorn calculi in the kidney.
Antigenic characteristics
The bacilli possess thermostable, ‘O’ (somatic) and thermostable ‘H’ (flagellar) antigens, based upon which several serotypes have been recognized.
Certain strains of Pvulgaris(OX-19, OX-2, and OX-K) produce O antigens that some rickettsiae share. 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.
Biochemical Properties ofProteus mirabilisandProteus vulgaris
Organisms that swarm on 5% sheepblood agar, exhibit a characteristic 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 |
| H 2 S production | Yes | Yes |
Laboratory Diagnosis & Identification
The sample used for the isolation and identification of the Proteusspecies 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. The sample should be collected in a sterile container maintaining aseptic conditions and reach the laboratory within an hour of collection.
Culture: The choice of the culture media used to isolate 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 grows on the Blood agar plate in successive waves to form a thin filmy layer of concentric circles ( swarming). Proteus does not swarm in the MacConkey agar mediumand forms smooth, pale or colorless (NLF) colonies.
Swarming properties of Proteuspresent 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
- increase in agar concentration in the medium, raising it to 6% instead of 1-2%.
- incorporation of chloral hydrate (1:500), sodium azide (1:500), and 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 to process urine samples.
Dienes Phenomenon
Proteus mirabilisis well known for its ability to differentiate into hyperflagellated, motile, and elongated swarmer cells rapidly spreading 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)*BUTwhen 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 biochemical 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.
References
- Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
- Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition