Fungal Virulence in Eggs is Scrambled

The importance of a good infection model is immeasurable. The obvious choice to run clinical studies on for potentially pathogenic fungi is a mammal system, but those are becoming decreasingly possible due largely to moralistic reasons. While insects can be used for a lot of studies, testing virulence in warm blooded animals kinda requires something, well, a little warmer. For that reason many research groups turn to chicken embryos. A new study published on the virulence of fungi from the Lichtheimia genus has now established the chicken embryo model for this potentially deadly fungus group.

Lichtheimia corymbifera

 
Lichtheimia Species Exhibit Differences in Virulence Potential

Three of the five species in this genus (L. corymbifera, L. ramosa, and L. ornata) are some of the most common pathogens causing mucormycosis, a potentially fatal infection of the sinuses, lungs, or brain. While the fungi that cause this disease are common it generally only manages to take hold of those with compromised immune systems. Unfortunately, for those who contract mucormycosis, the mortality rate is near to fifty percent with large amounts of tissue invasion and destruction. This is why finding any relation to the causation of virulence is important. Any link could lead us to find ways to decrease, if not infections, at least death rates.

The group of scientists in this study infected embryonic eggs with 46 strains across all five Lichtheimia species as well as species from the closely related genus Dichotomocladium at different times of development and under different stresses.

In order to test the straight virulence levels each egg was infected with 10³ spores and suffered a 70-90% mortality rate, at 10⁴ spores the rates increased to 95-100%, and at 10⁵ and 10⁶ spores introduced all embryos died within 2 to 5 days.

As embryos mature, the immune system begins to develop, fighting off infection. To test the effects this increasingly complete defense system has a steady rate of 10³ spores were introduced at 8, 10, and 12 days of embryonic development which showed a respectively lower or delayed mortality rate.

The team also tested the growth rates by growing the 12 main representative strains of Lichtheimia as well as a member of Dichotomocladium on a simple and complex media. Of the more virulent strains of Lichteimia L. ornata showed a decrease growth rate on complex media; however L. corymbifera and L. ramosa continued rapid growth with L. ramosa being much faster. And as was expected when grown on minimal mediums there were further reduced growth rates. And when grown under exposure to a myriad of different carbon sources to test metabolic flexibility a distinct advantage of Lichteimia species over Dichotomocladium was noted.

To determine stress resistance, and thus survivability in both the environment and the host, the 12 representative strains were tested for sensitivity to osmotic, oxidative and cell wall stress. Under osmotic stress L. ramosa showed a reduced osmotolerance when compared to L. corymbifera and L. ornata. When exposed to cell wall stress and oxidative stress it mainly was again L. ramosa that showed sensitivity when compared to the other notable species.

Unfortunately in the end the combined data could not tie down an obvious virulence pattern. With some species growing slower and others showing sensitivity to different growth medium, or stresses. The group concluded that if there is a shared virulence among these pathogens it is either a complex set of physiological traits of some other feature they did not analyze in this study. However with the now established embryonic egg model, future testing will be more readily available.

While not a giant break through, studies such as this are just as important. They may not give us some amazing insight, but they build the necessary foundation for many future studies who will surely owe their results in part to this groundwork.



Study credit: Volker U. Schwartze, Kerstin Hoffmann, Ildikó Nyilasi, Tamás Papp, Csaba Vágvölgyi, Sybren de Hoog, Kerstin Voigt, Ilse D. Jacobsen
Photo source: http://www.life-worldwide.org/fungal-diseases/lichtheimia-corymbifera/