Offering applications as a service in the Cloud builds on the notion of application multi-tenancy. Multi-tenancy, the sharing of application instances and their underlying resources between users from different organizational domains, allows service providers to maximize resource utilization and reduce servicing costs per user. Realizing application multi-tenancy however requires suitable enabling mechanisms offered by their supporting middleware. Furthermore, the middleware itself can be multi-tenant in a similar fashion. In this work we focus on enabling multi-tenancy for one of the most important components in service-oriented middleware, the Enterprise Service Bus (ESB). In particular, we discuss the prototype realization of a multi-tenant aware ESB, using an open source solution as the basis. We then evaluate the performance of our proposed solution by an ESB-specific benchmark that we extended for multi-tenancy purposes.