Aspergillus fumigatus is an ubiquitous, saprophytic mould that forms and releases airborne conidia which are inhaled by humans on a daily basis. When the immune system is compromised (e.g. immunosuppressive therapy prior to organ transplantation) or there is pre-existing pulmonary malfunction (e.g. asthma, cystic fibrosis, TB lesions), A. fumigatus exploits weaknesses in the host defenses which can result in the development of saphrophytic, allergic or invasive aspergillosis. If not effectively eliminated by the innate immune response, conidia germinate and form invasive hyphae which can penetrate pulmonary tissues. The innate immune response to A. fumigatus is stage-specific and various components of the host's defenses are recruited to challenge the different cellular forms of the pathogen. In immunocompetent hosts, anatomical barriers (e.g. the mucociliary elevator) and professional phagocytes such as alveolar macrophages (AM) and neutrophils prevent the development of aspergillosis by inhibiting the growth of conidia and hyphae. The recognition of inhaled conidia by AM leads to the intracellular degradation of the spores and the secretion of proinflammatory mediators which recruit neutrophils to assist in fungal clearance. During the later stages of infection, dendritic cells activate a protective A. fumigatus-specific adaptive immune response which is driven by Th1 CD4(+) T cells.

Rosacea is a chronic inflammatory skin condition that predominantly affects the skin of the face and the eyes. Several factors are associated with the onset and persistence of the condition, including an altered immune response in the skin and elevated levels of Demodex mites. Alterations in the immune response include elevated levels of LL-37 in rosacea skin, increased expression of TLR-2 and increased amounts of vitamin D3 in epidermal tissue. The combined effect of these changes may make the skin more sensitive to external and internal stimuli. External stimuli that may trigger or sustain rosacea inflammation include exposure to ultraviolet light, while internal factors may include the presence of elevated numbers of Demodex mites. These mites may directly stimulate an immune response or release bacteria within the pilosebaceous unit that act as a trigger for inflammation. This review will highlight the changes that occur in the immune response of the skin and describe how Demodex mites and associated bacteria may activate this response and lead to the characteristics of rosacea.

Individuals with cystic fibrosis are susceptible to co-infection by Aspergillus fumigatus and Pseudomonas aeruginosa. Despite the persistence of A.  fumigatus in the cystic fibrosis lung P. aeruginosa eventually predominates as the primary pathogen. Several factors are likely to facilitate P. aeruginosa colonization in the airways, including alterations to the microbial environment. The cystic fibrosis airways are hypoxic, nitrate-rich environment, and the sputum has higher amino acid concentrations than normal. In this study, significant growth proliferation was observed in P. aeruginosa when the bacteria were exposed to A. fumigatus culture filtrates (CuF) containing a high nitrate content. Proteomic analysis of the A. fumigatus CuF identified a significant number of environment-altering proteases and peptidases. The molecular mechanisms promoting bacterial growth were investigated using Label-free quantitative (LFQ) proteomics to compare the proteome of P. aeruginosa grown in the A. fumigatus CuF and in CuF produced by a P. aeruginosa-A. fumigatus Co-culture, to that cultured in P. aeruginosa CuF. LFQ proteomics revealed distinct changes in the proteome of P. aeruginosa when cultured in the different CuF, including increases in the levels of proteins involved in denitrification, stress response, replication, amino acid metabolism and efflux pumps, and a downregulation of pathways involving ABC transporters. These findings offer novel insights into the complex dynamics that exist between P. aeruginosa and A. fumigatus. Understanding the molecular strategies that enable P. aeruginosa to predominate in an environment where A. fumigatus exists is important in the context of therapeutic development to target this pathogen.

The immunocompromised airways are susceptible to infections caused by a range of pathogens which increases the opportunity for polymicrobial interactions to occur. Pseudomonas aeruginosa and Staphylococcus aureus are the predominant causes of pulmonary infection for individuals with respiratory disorders such as cystic fibrosis (CF). The spore-forming fungus Aspergillus fumigatus, is most frequently isolated with P. aeruginosa, and co-infection results in poor outcomes for patients. It is therefore clinically important to understand how these pathogens interact with each other and how such interactions may contribute to disease progression so that appropriate therapeutic strategies may be developed. Despite its persistence in the airways throughout the life of a patient, A. fumigatus rarely becomes the dominant pathogen. In vitro interaction studies have revealed remarkable insights into the molecular mechanisms that drive agonistic and antagonistic interactions that occur between A. fumigatus and pulmonary bacterial pathogens such as P. aeruginosa. Crucially, these studies demonstrate that although bacteria may predominate in a competitive environment, A. fumigatus has the capacity to persist and contribute to disease.

The fungal pathogen Aspergillus fumigatus is frequently cultured from the sputum of cystic fibrosis (CF) patients along with the bacterium Pseudomonas aeruginosa. A. fumigatus secretes a range of secondary metabolites, and one of these, gliotoxin, has inhibitory effects on the host immune response. The effect of P. aeruginosa culture filtrate (CuF) on fungal growth and gliotoxin production was investigated. Exposure of A. fumigatus hyphae to P. aeruginosa cells induced increased production of gliotoxin and a decrease in fungal growth. In contrast, exposure of A. fumigatus hyphae to P. aeruginosa CuF led to increased growth and decreased gliotoxin production. Quantitative proteomic analysis was used to characterize the proteomic response of A. fumigatus upon exposure to P. aeruginosa CuF. Changes in the profile of proteins involved in secondary metabolite biosynthesis (e.g. gliotoxin, fumagillin, pseurotin A), and changes to the abundance of proteins involved in oxidative stress (e.g. formate dehydrogenase) and detoxification (e.g. thioredoxin reductase) were observed, indicating that the bacterial secretome had a profound effect on the fungal proteome. Alterations in the abundance of proteins involved in detoxification and oxidative stress highlight the ability of A. fumigatus to differentially regulate protein synthesis in response to environmental stresses imposed by competitors such as P. aeruginosa . Such responses may ultimately have serious detrimental effects on the host.