Rod-shaped Bacillus of the Mouse

Corynebacterium kutscheri, a pathogen commonly recognized as a mandatory testing item for rats and mice under national standards in the control of infectious agents in laboratory animals, is well-known to researchers. This bacterium can cause illness or even death in rats and mice when their immune systems are weakened, potentially complicating the interpretation and evaluation of experimental results. Therefore, it is crucial that C. kutscheri tests remain negative during microbiological grading and monitoring of laboratory animals. In this article, we will provide a detailed overview of its etiology, epidemiology, as well as clinical symptoms and pathological changes.

Corynebacterium species are zoonotic pathogens that cause a variety of infectious diseases. Among them, Corynebacterium ulcerans, also known as C. kutscheri, is a Gram-positive bacterium. Pure cultures of the isolated strains, when subjected to Gram staining and microscopic examination, reveal small Gram-positive bacilli—often rod- or club-shaped, occasionally slightly curved—arranged singly, in pairs, or in a characteristic "palisade" pattern. These bacteria are non-motile and do not form spores. On routine culture media or 5% blood agar, they typically produce white colonies approximately 1 mm in diameter after incubation at 37°C for 48 hours; these colonies are raised, hemispherical, translucent, with a smooth surface and exhibit no hemolytic activity.

Corynebacterium rodentium primarily colonizes the upper respiratory tracts of mice and rats and is globally distributed. Notably, latent infections are highly prevalent, especially in conventional mouse colonies. The bacterium is mainly transmitted via the fecal-oral route but can also infect through skin wounds. Different mouse strains exhibit varying susceptibilities to C. rodentium colonization and disease development: C57BL/6 mice are generally tolerant, while BALB/c mice are more susceptible. Additionally, gender also plays a role in influencing susceptibility to this infection. Once C. rodentium establishes itself within a colony, the infection tends to become persistent, and animals are unable to clear it completely. While laboratory animals may carry the bacterium without showing symptoms, they remain at risk of sudden outbreaks at any time. This pathogen poses a serious threat in open-housing systems, where an outbreak could lead to the collapse of an entire animal population. However, among laboratory rodents used in research settings, C. rodentium is not commonly detected at high rates. According to statistics from Suzhou Xishan Bio, a third-party testing organization in China, over the past three years (up to the first half of 2023), none of the live experimental mice tested showed positive for C. rodentium. But what about the global situation? A recent article published by the U.S.-based CRL Laboratories compared five-year data from North American animal quality assessments with three years of European data, revealing that the prevalence of C. rodentium in mice remained at 0% in both regions. These findings clearly indicate that C. rodentium is relatively rare as a pathogen in laboratory mice, both domestically and internationally.

*Mycobacterium rodentium* exhibits a high carriage rate in infected animal populations, yet it is often difficult to detect the bacterium in the respiratory tract and intestines of animals with latent infections during routine testing. Moreover, the clinical symptoms of this disease are typically mild or inconspicuous. Possible symptoms include weight loss, joint swelling, skin ulcers and abscesses, as well as respiratory signs. Typical pathological changes associated with *Mycobacterium rodentium* infection include: enlarged lungs showing caseous necrosis; liver lesions characterized by purulent nodules; necrotic foci in the spleen; yellowish nodules visible on the surface of the kidneys; and ulcerations in the gastric and small intestinal mucosa. In some cases, grayish-white, viscous, foamy fluid can also be observed within the trachea. The pathogen is readily identifiable at sites of purulent lesions.

The initial diagnosis involves microscopic examination of the affected tissue, revealing clusters of small bacilli at the lesion site after Gram staining or imprint smears observed under a microscope. The most common method for detecting Corynebacterium rodentium is bacterial isolation and culture; additionally, PCR offers advantages such as high specificity, exceptional sensitivity, and rapid turnaround time. Serological methods can also be used to screen for Corynebacterium rodentium, though their application remains relatively limited in scope.

Some stress factors—such as intense light exposure, surgical procedures, transportation, malnutrition, and disruptions to the immune system—can trigger this disease, leading to outbreaks in populations of mice that are already latent carriers. Conducting research experiments with these animals not only compromises the accuracy of the resulting data but may also force researchers to halt their studies prematurely, causing potentially irreparable losses. Therefore, any experimental mice found to be positive for Corynebacterium rodentium must be promptly removed from the study.

Commonly used disinfectants in animal facilities are highly effective at eliminating Corynebacterium rodentium, and it is also relatively easy to eradicate the bacterium from the environment through chemical and mechanical disinfection methods. Antibiotic treatment can help alleviate clinical symptoms in infected animals. To prevent the disease from severely disrupting experiments, it is essential to establish a robust animal management system, effectively block transmission routes, and maintain rodent colonies free of Corynebacterium infections.

References

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