**5. Fungal infection in patients with cystic fibrosis**

Fungi are generally divided into molds or yeasts with the latter circumferentially shaped with a one-celled thallus. Molds also known as filamentous fungi grow as branching cylindrical hyphae [1]. Aspergillosis is the most well-characterized and well-recognized *Aspergillus* disease in CF, but reported prevalence varies significantly and is likely to be underdiagnosed. Aspergilli are saprophytic, sporeforming, filamentous fungi found ubiquitously in the environment. *A. fumigatus* is the most prevalent species causing human disease and is the species most frequently isolated from the respiratory secretions of CF patients [37].

Yet, the relative contribution of other emerging pathogens, such as *S. maltophilia*, *Aspergillus*, *Candida*, and *Scedosporium* species, remains less precisely defined. *Aspergillus fumigatus* is the only species that is associated with an increased risk for infection with *P. aeruginosa* [9]. *Candida* sp. is found in as many as 70% of patients with CF. The clinical significance of *Candida* in patients with CF is not clearly understood, but most clinicians discount these organisms as not significant [38]. Fungemia caused by *Trichosporon* has been reported in two patients with CF, who were double lung transplant recipients [39]. Some species such as *Exophiala dermatitidis* grow as unicellular fungi (yeast) at human body temperature but in the case of filamentous fungi at room temperature. *Trichosporon* species as of any *Candida* species may produce true mycelium in culture conditions or in host tissues. So, there are variations in isolation and purification of yeast and filamentous fungi. Additionally, some fungal strains are thermotolerant filamentous mycota such as *Aspergillus fumigatus* which are the most common, and others include *Scedosporium* species and *E. dermatitidis* [1]*. A. fumigatus* is frequently detected in respiratory secretions of both adults and children with CF. Once present in the airways, *Aspergillus* can exacerbate lung inflammation, establish infection, and trigger hypersensitivity responses [10].

In patients with CF, complications increased when exposed to *A. fumigatus* spores causing impaired mucociliary clearance and defective innate immune responses leading to accumulation and persistence of fungal spores within the smaller airways. Also, germination of spores leads to the formation of fungal hyphae and release of antigens, phospholipases, proteases, and other virulence factors which damage the airway of epithelial cells and allow a large dose of antigenic factors access to the interstitial and vascular compartments [40].

Also, exotic genera can be found in CF airway secretions, such as *Penicillium*, *Alternaria*, or *Scedosporium*. Some studies provide the first evidence that even healthy airways are not sterile and contain distinct fungi called "pulmonary mycobiome." The kinetics, dynamics, and disease relevance of the pulmonary mycobiome, however, are poorly understood [7]. The majority of yeasts in CF belong to *Candida* species and the most common species such as *Candida albicans* and other yeast strains recorded such as *Candida parapsilosis*, *Candida krusei*, *Candida glabrata*, and *Candida dubliniensis* [1].
