Factors affecting germination of ascospores in Aspergillus fumigatus

* Department of Basic Sciences, College of Dentistry, Hawler Medical University, Erbil, Iraq. Introduction A. fumigatus is a saprotrophic fungus that is involved in environmental carbon and nitrogen recycling is a ubiquitous filamentous fungus in the environment. It is abundant in soils and decomposing organic materials. It is essential that A. fumigatus survive and flourishes in nutrient limited environments, and the most widespread airborne human pathogen, accountable for approximately 90% of all invasive aspergillosis (IA) cases. Spores in A. fumigatus can be produced by two different forms of cellular reproduction, asexual reproduction (produce conidia) or sexual reproduction (produce ascospore). Sexual reproduction involves the production of eight ascospores normally produced in each ascus. These asci form within an ascocarp with a closed structure called cleistothecium. The species has a heterothallic breeding system, isolates of complementary mating types are necessary for sex to take place. Sexual reproduction may bring about increased genetic variation, due to recombination of genes from the parents that can occur by crossing over, gene conversion, or independent segregation of centromeres, permitting a species to react to a change in the environment and evolve. Germination consequently forms a first and critical stage Background and objective: Aspergillus fumigatus is the most common airborne fungal pathogen of humans. It is an opportunistic human pathogen in immunocompromised hosts. Following the recent discovery of the sexual reproductive cycle ascospores of this fungus were available and their germination was studied in the present study. This study aimed to study factors influencing germination of ascospores of A. fumigatus. Methods: A total of 12 environmental A. fumigatus were chosen in both mating type, examined for sexual reproduction by crossing complementary mating type, following removing of cleistothecia, preparation of ascospore suspensions in 0.05% Tween 20, then heat treated to kill any remaining conidia. Results: Ascospore germination of greater than 75% occurred at 28 °C on a wide variety of mycological media, 0.5% (ACM) was chosen for use in subsequent studies. Ascospores did not require a heat treatment prior to germination; however, heat treatment of ascospores at 75 °C for 1 hour previous to incubation enhanced germination to 89% and also served to inactivate contaminating conidia. Ascospores also exhibited a psychrotolerance following exposure at -80 °C for 96 hours. Inoculum concentration did not have a significant effect on germination rates between the range of 0.125×10 to 4×10 ascospores ml (P >0.05). Conclusion: This study demonstrated that ascospore germination of greater than 75% occurred at 28 °C on a wide variety of mycological media. Ascospores of A. fumigatus are thermotolerant also exhibited a psychrotolerance whilst at the same time retaining viable sexual ascospores, to assess percentage germination a drop of Lactophenol Cotton Blue was added.