**5. Cutaneous lipids**

Four of the hibernating bat species found in North America as well as 11 hibernating bat species in Europe limit the degree of cutaneous *Pd* infection to the point where it does not result in WNS during hibernation. The physiological and biochemical mechanisms that enable these bat species to reduce *Pd* infections are poorly understood. Several recent studies have revealed that one factor which confers a resistance to *Pd* infections is the lipid composition of the epidermis. The outermost stratum of the epidermis is the first defense against fungal skin infections because the mycelium must initially invade it, and leukocytes are not present in these epidermal layers. The epidermis is composed chiefly of special epithelial cells called keratinocytes that occur in 4 strata; they are produced in the deepest stratum (the stratum basale) and migrate to the top stratum (the stratum corneum) as they age. Epidermal surface lipids are a mixture of compounds secreted by keratinocytes into the intracellular matrix, and sebum secreted onto it by the sebaceous glands. The lipid mixture secreted by keratinocytes contains free sphingosine bases, ceramides, cholesterol, and free fatty acids (FFAs), whereas the sebum is composed of triacylglycerols, diacylglycerols, FFAs, wax esters, squalene, cholesterol, and cholesterol esters [51, 52]. The epidermal lipids of bats also have cerebrosides and monoacylglycerols [53, 54]. Some free fatty acids (FFAs) are known to have antimicrobial effects [55].

It has been demonstrated that the wing epidermis of both *M. lucifugus* and *E. fuscus* contain the same 7 fatty acids: Myristic (14:0), pentadecanoic (15:0), palmitic (16:0), palmitoleic (16:1), stearic (18:0), oleic (18:1), and linoleic (18:2) acids. The wing epidermis of hibernating *E. fuscus* contains about twice as much myristic, palmitoleic, oleic, and linoleic acids than that of *M. lucifugus*, as well. Laboratory experiments with *Pd* cultures revealed that pentadecanoic, palmitoleic, oleic, and linoleic acids in the free fatty acid (FFA) form inhibit the growth of *Pd* [56, 57], with linoleic acid reducing it by more than 99%. The results of one of these experiments are summarized in **Figure 2**.

Epidermal free fatty acid composition thus appears to be one of the factors that enables *E. fuscus* to better resist *Pd* infections than *Myotis lucifugus*.

*The Physiological Ecology of White-Nose Syndrome (WNS) in North American Bats DOI: http://dx.doi.org/10.5772/intechopen.100369*

#### **Figure 2.**

*Mean (± SE) surface areas of Pd colonies at various growth stages on media containing 1% of either oleic (18:1), linoleic (18:2) or stearic (18:0) acids, while being incubated at 5.0 (blue symbols) and 10.6°C (red symbols). Mean within the same temperature treatment sharing a common lower-case letter are not significantly different at the P < 0.05 level. Data are from Frank et al. [56].*

Wax esters consist of an alcohol linked to a fatty acid molecule with an ester bond [58]. About 120 different wax esters have been found in the sebum of *Myotis myotis* during hibernation [59]. A recent study used laboratory *Pd* culture experiments to determine the effects of some of these wax esters on *Pd* growth [60]. These experiments have revealed that 4 of the wax esters found in the sebum of *M. myotis* inhibit *Pd* growth by over 90%. These anti-*Pd* wax esters are: behenyl linoleate, palmityl linoleate, arachidyl linoleate, and behenyl palmitoleate. One factor that enables *M. myotis* to resist *Pd* infections is therefore presence of these anti-*Pd* wax esters in their epidermis. Changes in epidermal lipid composition may also be one of the adaptations that permit some populations of *M. lucifugus* to now have a higher resistance to cutaneous *Pd* infections.

### **6. Conclusions**

Four species of North American bats develop severe *Pd* infections during hibernation which result in WNS, whereas 4 other bat species develop only moderate *Pd* infections during this period and do not display the abnormal torpor patterns and mortality associated with WNS. It has been also demonstrated that within the same population of *M. lucifugus*, individuals with a high density of *Pd* lesion in their skin suffer from WNS, whereas those with a much lower density of *Pd* lesions do not. Cutaneous infections with *Pd* thus do not always result in WNS, and a certain density of *Pd* lesions must be present in the skin before WNS develops. The precise minimum *Pd* lesion density for WNS is not known, and it should be determined to better understand both the ecology and potential impacts of WNS on the hibernating bat populations of North America, as the mere presence of some *Pd* lesions alone does not indicate that a bat species/population is afflicted with WNS. The effects of these lesions on torpor bout length must be taken into consideration when assessing potential WNS. The full extent to which *Pd* can infect the skin and causes WNS in 13 of 21 bat species listed in **Table 1** is not known. Most of these 13 bat species occur in Western North America. It is thus unclear how WNS will affect most Western bat species as *Pd* spreads across North America. If current trends in *Pd* susceptibility continue, then it is likely that WNS will severely affect 7–8 additional bat species in North America. Analyses of preserved museum specimens suggest that *Pd* has been associated with hibernating bats in Europe for over 100 years, whereas they reveal no evidence that *Pd* was present in North America bats between 1861 and 1971 [61]. It therefore appears that bats across Europe have adapted to *Pd* for over 100 years, and they are now highly resistant to *Pd* infections, thereby avoiding WNS. The physiological/biochemical basis of this resistance is largely unknown but warrants further investigation to better predict which New World bat species will be severely affected by WNS.
