Symbioses between animals and bacteria are ubiquitous. To better understand these relationships, it is essential to unravel how bacteria evolve to colonize hosts. Previously, we serially passaged the free-living bacterium, Shewanella oneidensis, through the digestive tracts of germ-free larval zebrafish (Danio rerio) to uncover the evolutionary changes involved in the initiation of a novel symbiosis with a vertebrate host. After 20 passages, we discovered an adaptive missense mutation in the mshL gene of the msh pilus operon, which improved host colonization, increased swimming motility, and reduced surface adhesion. In the present study, we have determined that this mutation was a loss-of-function mutation and found that it improved zebrafish colonization by augmenting S. oneidensis representation in the water column outside larvae through a reduced association with environmental surfaces. Additionally, we found that strains containing the mshL mutation were able to immigrate into host digestive tracts at higher rates per capita. However, mutant and evolved strains exhibited no evidence of a competitive advantage after colonizing hosts. Our results demonstrate that bacterial behaviors outside the host can play a dominant role in facilitating the onset of novel host associations.

Malignant melanoma is one of the most aggressive and resistant tumor types which is often accompanied by poor prognosis when spread to nearby or distant tissues. Even though an increasing number of second-line therapies are available for cancer patients, the 5-year survival rate of melanoma patients with regional and distant metastases is still rather low. Therefore, there is an urgent need to develop new, highly efficient treatments. Melanocortin-1-receptor (MC1R) is a cell surface receptor for α-MSH. It is responsible for melanogenesis and is shown to be overexpressed on the surface of cancer cells in a subset of melanoma patients. Here, we designed and synthesized α-MSH analogs and conjugated them to daunomycin through oxime linkage. Using our peptide-drug conjugates (Dau-α-MSHs), we performed comparative in vitro and in vivo evaluations to reveal their suitability for tumor targeting. Firstly, the expression level of MC1R was determined in various melanoma cell lines via RT-qPCR. Then, we examined the antiproliferative effect of our Dau-α-MSHs, moreover we determined the cellular uptake profile and intracellular localization of Dau-α-MSHs. The most efficient Dau-α-MSHs were investigated in vivo using a murine xenograft melanoma model. We found that various cell lines show differential expression on the mRNA levels. We also showed that healthy primary cells have lower expression of MC1R compared to tumor cell lines. Moreover, the concentration-dependent cellular uptake profile of Dau-α-MSHs was detected and the compounds exhibited a great antiproliferative effect on melanoma cell lines. We also provided evidence that our novel Dau-α-MSHs can inhibit tumor expansion significantly and are suitable for targeted tumor therapy.