Aggregation is a common phenomenon in microbiology. It manifests itself in different forms ranging from loose irregular structures such as effluent floc and marine snow to the ordered aggregates that show the reproducible patterns seen in many microbial colonies. Aggregation conveys advantages to microorganisms. These include transfer of chemical signals, exchange of genetic information, protection from adverse environmental conditions, metabolic cooperation between different species, as well as cell differentiation in some populations. Coaggregation is now recognized as a mechanism for allowing specific association between collaborating bacterial species. We describe a simple cellular automaton that illustrates structures that could form when different species interact. In particular the importance of neighbours is investigated.