Bacterial infection of the brain: how bacteria penetrate the CNS by invading peripheral nerves

Abstract

Bacterial infections of the central nervous system (CNS), though uncommon, are associated with very high rates of morbidity and mortality. Recent research has also highlighted the correlation between pathogens and chronic diseases of the CNS, such as neurodegenerative disorders, particularly Alzheimer’s disease. Whilst some bacteria can cross the blood-brain/blood-cerebrospinal fluid barriers, to date, other pathways by which bacteria enter the CNS remain largely unknown. Identifying alternative paths by which pathogens can enter the CNS is thus important for developing novel strategies preventing CNS infection and potential long-term sequelae. Novel evidence suggests some bacterial species (as well as certain viruses and amoebae) can enter the brain via the cranial nerves innervating the nasal cavity, particularly the olfactory nerve that mediates the sense of smell and connects the nasal cavity with the olfactory bulb in the forebrain. The trigeminal nerve also innervates the nasal cavity and constitutes another invasion path. Only a handful of pathogens are thought to use cranial nerves to reach the brain; certain Chlamydia species (spp.) being amongst these. Chlamydia pneumoniae is to date the bacterium with the strongest established link to Alzheimer’s disease. Previous research by our laboratory has also demonstrated that the bacterium causing the tropical disease melioidosis, Burkholderia pseudomallei, can invade both the olfactory and trigeminal nerves, travel along these nerves, to then infect the CNS (the olfactory bulb and brainstem, respectively). We have also previously shown that in outbred mice, the olfactory nerve is resistant to B. pseudomallei infection. The nasal mucosa contains both innate and adaptive immune components and prevents many infections. If pathogens penetrate the mucosal barrier and reach nerves, glial cells of the nerves can also combat the infection. Whilst only a few macrophages are present inside the olfactory nerve fascicles, olfactory nerve glial cells, termed olfactory ensheathing cells (OECs), are powerful phagocytes with innate immune functions. Thus, in addition to the immune cells and other components of the immune system in the nasal mucosa, cranial nerve glia are thus thought to be key for preventing CNS infection, explaining why such infections are relatively rare. Some pathogens, however, can evade destruction by these cells and invade the nerves, however, it remains largely unknown which pathogens are capable of doing so. Furthermore, injuries to the nasal epithelium are common, and if the mucosal barrier is removed by injury, perhaps it is easier for pathogens to infect the underlying nerves (in particular the olfactory nerve) and then reach the CNS. With the exception of one bacterium (Staphylococcus aureus) for which injury has been shown to allow infection of the olfactory nerve, it also remains unknown whether epithelial injury increases the risk of pathogens invading the CNS via these paths. Thus, we need to determine which pathogens are capable of invading the CNS via nerves connecting the nasal cavity and the brain, and whether epithelial injury increases the risk of infection. Furthermore, determining the cellular mechanisms that protect against microbial invasion of the CNS via nerves, as well as why certain pathogens can evade destruction of the immune system may pave the way for the development of novel therapies preventing and treating CNS infections. The key aims of this thesis were to determine (1) whether prior injury to the nasal epithelium could allow B. pseudomallei to invade the olfactory nerve and bulb in the mouse strain where this nerve is usually resistant to this infection, (2) whether the bacterium Chlamydia muridarum (which infects mice and is commonly used to study Chlamydia spp. infection in rodents) can utilise cranial nerves that innervate the nasal cavity to invade the CNS and, if C. muridarum can invade the CNS, to then determine whether the bacteria remained viable and (3) whether C. muridarum can infect OECs, and how OECs respond to C. muridarum in vitro. This thesis demonstrated that injury to the olfactory epithelium allowed the invasion of the olfactory nerve and bulb by B. pseudomallei in S100β-DsRed Quackenbush mice, in which the olfactory nerve is otherwise typically resistant to infection. This work also showed that C. muridarum can rapidly (within 48 h) reach the CNS (olfactory bulb and cerebral cortex) via the olfactory nerve, as well as infect the trigeminal nerve, in mice. Immunohistochemistry showed the presence of C. muridarum inclusion bodies (membrane-bound components inside which the bacteria replicate intracellularly) and viable C. muridarum bacteria were also isolated from these regions. C. muridarum was shown to readily infect OECs in vitro, which led to the upregulation of a range of cytokines. The outcomes from this project will contribute to an increased understanding of how bacteria can reach the CNS and has revealed that injury to the nasal epithelium may increase the risk of CNS bacterial invasion via the olfactory nerve. The outcomes also include an increased understanding of how olfactory nerve glia become infected by and respond to bacteria. This work may also contribute towards the growing body of knowledge regarding the link between pathogens and certain diseases of the CNS, such as Alzheimer’s disease. Furthermore, with an increased understanding of how glial cells respond to bacteria, new therapies may be developed that stimulate bacterial degradation by the glia. Such therapies may provide valid future alternatives to antibiotics, also combating the growing problem of antibiotic resistance. Thus, this work may contribute to the foundation required to develop therapies to treat diseases that are currently not curable, as well as to better diagnose and identify susceptibilities to certain conditions.