Mouse models are one of the mainstays of biological research and preclinical drug development. Mice are maintained and bred in colonies that are stringently maintained to ensure healthy animals are available for studies and strain maintenance. Breeding strategies are carefully developed so that the required number of mice are available on time for planned studies. In many cases, mouse colony management is somewhat straightforward where standard strains are maintained using widely published best practices1. However, depending on the study requirements or mouse strain characteristics, specialized mouse colonies are required to maintain and breed mice such as germ-free mice and their derivatives (gnotobiotic mice and conventionalized mice), and specific pathogen free mice.
Germ-free mice are unique animals that have no microbes and have not been exposed to microbes. In order to maintain the germ-free status, the mice are maintained in sterile isolators that include sterile bedding, water bottle, water and food and have to be handled through sterile gloves attached to isolators. Frequent testing is required to ensure that the mice maintain germ-free status. Due to a lack of microbiome species, these mice have been reported to have dysfunctional immune systems, low body weight, impaired metabolism and hormone levels, which contribute to behavioral issues2. Some of the behavior issues include anxiety3 and aggressive behavior towards unknown mice4 and the link between a lack of gut microbiome and neurological issues underscores the importance of the gut-brain axis. Despite the health and behavioral issues with germ-free mice, they are invaluable for research programs that study host-microbial interactions across various disease states. Germ-free mice that are exposed to clearly defined pathogens are called gnotobiotic animals and are used to study host interactions with specific microbes and the development of diseases caused by specific microbes. Gnotobiotic mice are generated by introducing microbes through oral gavage and monitoring the expansion of the gut microbiome5. Another method is to house germ-free mice with gnotobiotic mice so that the microbes can slowly colonize the germ-free mice through co-habitation5. It is important to note that gnotobiotic mice have to be maintained under sterile conditions as well. Conventionalized mice are developed by inoculating germ-free mice with the microbiome from non-germ-free mice6. The microbiome of regular mice varies between animals and contains many different bacterial species so conventionalized mice have essentially a “normal” microbiome6. Fecal matter is commonly to inoculate the germ-free mice to start the colonization process. Conventionalized mice are also used to study host-microbial interactions and may be preferred in some cases to gnotobiotic mice since the microbiome is more diverse.
Specific pathogen free (SPF) mice defined as mice that are proven to be negative for a specific set of infectious pathogens during multiple surveillance testing programs7. It is important to note that these mice may have microbial species that are not in the specified list of pathogens. They are also referred to as “clean” mice as they are largely free of zoonotic micro-organisms that can be transmitted to humans. SPF colonies are maintained in a controlled and monitored environment to ensure that the mice can be safely used in studies with minimal concerns on data being compromised due to infectious pathogens. The excluded pathogen list typically includes respiratory and gut pathogens including Salmonella and Clostridium piliforme8. Interestingly, the list of excluded pathogens is not completely standardized across institutions as some allow opportunistic pathogens while others do notsup>8. SPF mice are widely used across preclinical drug development studies and SPF colonies are maintained in most preclinical CROs and research institutes. In summary, establishing and maintaining specialized mouse colonies is now a standard practice thanks to published best practices and stringent regulations in animal facilities.
References:
1https://nc3rs.org.uk/3rs-resources/breeding-and-colony-management/colony-management-best-practice
3https://pmc.ncbi.nlm.nih.gov/articles/PMC3181531/
4https://www.sciencedirect.com/science/article/abs/pii/S0168010221000195
5https://febs.onlinelibrary.wiley.com/doi/10.1111/febs.17124#
