Multimedia conferencing applications play a critical role in business and everyday life. However, scalability and elasticity remain quite elusive, even though they are the keys to efficiency in resource usage. A cloud-based approach could solve the scalability and elasticity issues and bring other benefits such as an easy introduction of new applications. This paper proposes a cloud infrastructure that relies on fine-grained conferencing substrates. These substrates are virtualized and shared by conferencing applications. They enable scalability and elasticity