Proteus Bacilli: Introduction, Morphology, Biochemical and Treatment

Proteus Bacilli


Proteus bacilli are normal intestinal commensals and opportunistic pathogens like coliforms. The name ‘Proteus’ refers to their pleomorphism, after the Greek god Proteus who could assume any shape.

Genus Proteus has four species: P. mirabilis, P. vulgaris, P. myxofaciens and P. penneri. P. mirabilis, P. vulgaris are widely recognized as human pathogens. These are motile, gram-negative bacilli, characterized by swarming growth on agar.


They are gram-negative coccobacilli, 1–3 μm long and 0.6 μm wide. Pleomorphism is frequent—short coccobacilli to long filaments. In young swarming cultures, many of the bacteria are long, curved and filamentous. They may be arranged singly, in pairs or in short chains. They are actively motile with peritrichous flagella. They also have more type of fimbriae and are noncapsulated.

Cultural Characteristics

They are aerobe and facultative anaerobes. All grow well on laboratory nutrient media. Proteus organisms are usually first recognized by their characteristic putrefactive odor described as ‘fishy’ or ‘seminal’ and swarming appearance on noninhibitory solid media, such as nutrient agar and blood agar. Swarming is a striking feature of Pr. mirabilis and Pr. vulgaris. Swarming of Proteus appears to be due to vigorous motility of the organisim although the exact cause is not yet established.

Swarming growth is a problem in the laboratory when mixed growth is obtained in which Proteus bacilli are present with other bacteria. A number of methods have been devised to inhibit swarming. Swarming of Proteus can be inhibited by (i) increasing concentration of agar (6%) and by (ii) incorporation of chloral hydrale (1 : 500), sodium azide (1:500), alcohol (5–6%), sulfonamide, surface. active agents or boric acid (1:1000).

Swarming does not occur on MacConkey’s medium, on which smooth colorless (NLF) formed. Proteus produces uniform turbidity with a slight powdery deposit and an ammonical odor in liquid medium (peptone water).

Dienes phenomenon: When two identical strains of Proteus are inoculated at different places of the same culture plate, the resulting swarms of growth coalesce and no line is formed between swarming culture of the same strain. When, however, two different strains of Proteus species are inoculated, the spreading films of growth fail to coalesce and remain separated by a narrow but easily visib1e furrow. This is known as Dienes phenomenon. It has been used to determine the identity or non-identity of various strains of Proteus.

Biochemical Reactions

The distinctive characters of this genus are:

  1. PPA test—Deamination of phenyl alanine to phenyl pyruvic acid (PPA test) is always positive.
  2. Urea hydrolysis – Urea hydrolysis by enzyme urease is another characteristic of Proteus, but is negative in some Providencia strains.
  3. All species of Proteus produce acid from glucose.
  4. Lactose is not fermented.
  5. They are malonate utilization negative.
  6. Indole is formed by Pr. vulgaris but is negative in Pr. mirabilis.
  7. They are MR positive and VP negative.
  8. H2S is produced by Pr. vulgaris and Pr. mirabilis.
  9. Nitrate reduction positive

Antigenic Structure

Proteus bacilli possess somatic O and flagellar H antigens, which are of considerable historical interest. Weil and Felix (1916) studying Proteus bacilli observed that flagellated strains growing on agar formed a thin surface film resembling the mist produced by breathing on glass and named this variety the ‘Hauch’ form (from Hauch, meaning film of breath).

Nonflagellated variants grew as isolated colonies without the surface film and were called ‘Ohne Hauch’ (meaning without film of breath). These names came to be abbreviated as the Hand 0 forms. Subsequently, the H and O were extended to refer to the flagellar and somatic antigens of other bacilli as well.

Weil and Felix also observed that certain nonmotile strains of Pr. vulgaris, called the ‘X strains’, were agglutinated by sera from typhus fever patients. This heterophilic agglutination due to the sharing of an alkali stable carbohydrate antigen by certain strains of Proteus (OX2, OX19 and OXK) and rickettsiae forms the basis of the Weil-Felix reaction for the diagnosis of some rickettsial infections.

Three nonmotile Proteus strains OX2, OX19 and OXK are used in the agglutination test. OX19, OX2 are the strains of P. vulgaris serotype 01 and serotype 02 and OXK is the strain of P. mirabilis serotype 03.

Typing methods: Phage typing, bacteriocin (proticin) typing and serotyping schemes have been developed for Proteus and Providencia species. Swarming Proteus strains exhibit the Dienes phenomenon and this forms the basis for a precise method of differentiation among such strains.


Proteus bacilli are widely distributed in nature as saprophytes, being found in decomposing animal matter, in sewage, in manured soil and in human and animal feces. They are frequently present on the moist areas of the skin. They are opportunistic pathogens, commonly responsible for urinary and septic infections, often nosocomial.

P. mirabilis accounts for the majority of human infections seen with this group of organisms. All members of the tribe can cause urinary tract infections (UTI), wound infections, pneumonia, infection of the ear, respiratory tract infection, septicemia and nosocomial infections. Strains of Pr. mirabilis are a prominent cause of urinary tract infection in children and in domiciliary practice.

UTI caused by Proteus tends to be more serious than that caused by E. coli and other coliforms. It produces urease which splits urea into carbondioxide and ammonia. Ammonia inactivates complement, damages renal epithelium and makes the urine alkaline. This increase in pH causes precipitation of calcium and magnesium salts from the urine and results in the formation of urinary calculi.

Labortory Diagnosis

Culture: Laboratory diagnosis of the infections caused by species Proteus can be carried out by culture of the specimen on MacConkey agar or DCA.

Identification: The isolate is identified by its morphological, biochemical and agglutination reactions.


Proteus bacilli are resistant to many of the common antibiotics. An exception is P. mirabilis which is sensitive to ampicillin and cephalosporins.

Reference and Sources


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