Genus Proteus

The genus *Proteus*, a member of the family Enterobacteriaceae, comprises five species: *P. vulgaris*, *P. mirabilis*, *P. myxofaciens*, *P. penneri*, and *P. hauseri*, along with three unnamed *Proteus* genomospecies.

The genera *Proteus*, *Morganella*, and *Providencia* are collectively known as the family Proteeae. Among these, *Proteus vulgaris*, *Proteus mirabilis*, *Providencia alcalifaciens*, and *Morganella morganii* are closely associated with clinical settings.

Proteus bacteria are widely found in the environment and also inhabit the intestines of humans and animals. Under certain conditions, Proteus can cause a variety of infections and foodborne illnesses. Notably, this bacterium is capable of producing enterotoxins in food. In recent years, there has been a relatively increasing trend in Proteus-related food poisoning cases.

Proteus species can cause a variety of infections, including urinary tract infections, wound infections, meningitis in newborns or infants, and rheumatoid arthritis. They also lead to infections following injuries or burns. Notably, Proteus infections of the urinary tract are second only to those caused by Escherichia coli. Additionally, Proteus mirabilis is commonly associated with foodborne illnesses. In immunocompromised mice, Proteus vulgaris has been linked to intestinal mucosal damage, sepsis, pyelonephritis, splenomegaly, and hepatitis—and can even influence specific mouse models used in research.

Collect swabs from the trachea, intestinal contents, or lesion secretions of the animals to be tested for inoculation and culture; enrichment is required if necessary. Proteus bacteria are Gram-negative bacilli that lack spores and a capsule, possess peritrichous flagella, and appear as small, blunt-ended rods with motility. The bacterial cells measure approximately (0.4–0.6) × (1.0–3.0) micrometers in size and are facultatively anaerobic, exhibiting migratory growth on solid surfaces. On DHL agar, Proteus forms circular, flat, colorless to pale pink, semi-transparent colonies with smooth surfaces; on BA (blood agar), they develop as gray, flat, and migratory colonies. As shown in Figure 1:

Figure 1.

Our laboratory primarily uses biochemical and mass spectrometry methods for the identification of the genus Proteus.

1. Biochemical Identification

The biochemical identification instrument is the bioMérieux Vitek Compact 2, and the identification card used is the GN card, which can identify Proteus vulgaris, Proteus mirabilis, Proteus penneri, and Proteus hauseri. For cases with inconclusive identification results, further manual biochemical testing is required. Partial biochemical comparison results are shown in Table 1.

Table 1. Biochemical Tests for Differentiation Among Species of the Genus Proteus.

Note: "+" indicates 90–100% positivity, while "-" indicates 0–9.9% positivity; S = susceptible, R = resistant, and V = variable.

2. Mass Spectrometry Identification

The principle behind mass spectrometry-based identification is laser-induced excitation of bacteria on a target plate, along with the matrix, causing bacterial proteins to travel through a vacuum flight tube. A detector then measures the differences in protein flight times, generating a characteristic profile that can be compared against a database to identify the most likely bacterial species. This method is simple to perform, offers high throughput and speed, and allows for the simultaneous analysis of multiple samples. The process for a single sample can take as little as 1 to 2 minutes.

Proteus species are normal flora of the human and animal intestines, but they also serve as significant opportunistic pathogens, particularly in animals with compromised immune systems. Therefore, eliminating these opportunistic pathogens is crucial for safeguarding animal health and ensuring reliable research outcomes with immunodeficient mice. To prevent Proteus infections in animals, it is essential to house them in strictly pathogen-free environments—this is especially critical for immunocompromised mice.

Most disinfectants commonly used in animal facilities are effective against Proteus bacteria. Any chemical disinfectant or physical sterilization method can be employed to eliminate Proteus from the environment. However, treating mice that carry the bacteria is generally not recommended, as while antimicrobial agents can alleviate clinical symptoms, they often fail to eradicate the persistent carrier state. Moreover, antibiotic treatments typically cannot fully clear bacteria from bedding materials or cage surfaces. Therefore, therapeutic interventions are advised only to ease clinical symptoms or as an additional precautionary measure before initiating a decontamination process. To obtain animals free of Proteus, it’s essential to cleanse them through embryo transfer or hysterectomy into a mother that is already free of the bacteria.

References

1. Róalski, Antoni, Torzewska A, Moryl M, et al. *Proteus* sp. – an opportunistic bacterial pathogen – classification, swarming growth, clinical significance, and virulence factors[J]. *Folia Biologica et Oecologica*, 2012, 8(1):1-17. DOI: 10.2478/fobio-2013-0001.

2. Tierce R K, Winn A A, Albers T M, et al. Detection and Transmission of Proteus mirabilis in Immunodeficient Mice[J]. Journal of the American Association for Laboratory Animal Science, 2022(3):61.

3. Jones JB, Estes PC, Jordan A E. Proteus mirabilis infection in a mouse colony.[J]. Journal of the American Veterinary Medical Association, 1972, 161(6):661-4.

4. SNT 2524.1-2010 "Method for Detecting Proteus Species in Imported and Exported Food"

5. GB/T 14926.43-2001 *Laboratory Animals: Bacteriological Testing, Staining Methods, Media, and Reagents*