Resistance of melanized feathers to bacterial degradation: is it really so black and white?

Melanins are common feather pigments that contribute to signaling and crypsis. Melanins may also help feathers resist feather-degrading bacteria (FDB). Two recent studies (Goldstein et al. 2004, Grande et al. 2004) tested the resistance of melanized versus unmelanized feathers to FDB using in vitro experiments, but draw opposite conclusions. Goldstein et al. (2004) concluded that melanized feathers resist FDB more than unmelanized feathers, while Grande et al. (2004) concluded that unmelanized feathers resist FDB more than melanized feathers. To resolve this conflict in the literature, we replicated previous studies but included additional tests not previously used. We inoculated melanized and unmelanized feathers of domestic geese Anser anser domesticus, with the FDB Bacillus licheniformis and measured bacterial activity every two days over two weeks. Three metrics of bacterial activity on feathers were measured: soluble protein content around feathers in solution, bacterial growth on feathers, and loss of feather mass. The latter two metrics were not considered in the aforementioned studies, which indirectly measured bacterial activity. We conducted two trials, one in which feathers were sterilized by autoclaving before inoculation (Goldstein et al. 2004, Grande et al. 2004), and a second in which feathers were sterilized by ethylene oxide gas. This allowed us to test whether autoclaving, done in previous studies, influences bacterial activity on feathers and could confound results. In both trials, unmelanized feathers degraded earlier, supported greater bacterial growth, and lost more mass than melanized feathers. These results support the findings of Goldstein et al. (2004); melanized feathers are more resistant to FDB than unmelanized feathers. Thus, using direct metrics of bacterial activity, we resolve a current conflict in the literature. We also found that autoclaving feathers influences FDB activity on them, and thus autoclaving should be avoided in future studies.