cueO encodes a periplasmic multicopper oxidase, which is known to be involved in copper homeostasis and protection against oxidative stress in Escherichia coli K12. Transcriptional profiling showed that expression of genes associated with motility was lowered in a cueO mutant while expression of genes associated with autoaggregation was elevated. Increased aggregation was correlated with increased expression of cell surface proteins antigen 43 and curli. Changes in gene expression caused by the deletion of cueO were essentially independent of SoxR and OxyR, the global regulators of oxidative stress response.