Although the natural mode of bacterial growth in nature is as biofilm, almost all antimicrobial and immunological tests are routinely developed using planktonic inoculums. Bacterial biofilms protect the microbial community from external damage and promote the persistence of chronic infections. In this study, interactions between human macrophages and bacterial inoculums of planktonic and biofilm modes of growth have been explored using Escherichia coil (E. coli) K12. Human macrophages phagocytize planktonic E. coil more efficiently than bacteria grown in a biofilm. Moreover, they prefer to phagocytize planktonic bacteria. In this context, CD64 expression is involved. Our data indicate that bacteria with "a biofilm background" avoid phagocytosis by nave macrophages, which could create a favorable environment for chronic infection. Our findings were corroborated in a clinical O25b-ST131 ESBL-producer E. coil isolate, which caused urinary tract infections. (C) 2013 Elsevier Inc. All rights reserved.