Interoperability is one of the key requirements in the Internet of Things considering the diverse platforms, communication standards and specifications available today. Inherent resource constraints in the majority of IoT devices makes it very difficult to use existing solutions for interoperability, thus demanding new approaches. This paper presents the process of adapting a lightweight interoperability middleware for IoT, LISA, from RIOT to Contiki OS and evaluates memory and power overheads. The middleware follows a service oriented architecture and classifies devices according to available resources to assign different roles, such as Application, Service and Manager Nodes. These roles live in different tiers in a generic IoT architecture, where the Manager nodes are located in the intermediate Fog layer. To adapt to an event based kernel of Contiki, the middleware defines and handles a set of events that are used to communicate with the user application. A network of nodes is simulated to show the architecture promoted by the middleware and the results are presented.