Functions of type 3 fimbriae during K Functions of type 3 fimbriae during K. pneumoniae infection and biofilm formation. Functions of type 3 fimbriae during K. pneumoniae infection and biofilm formation. Type 3 fimbriae are helix-like, membrane-bound, adhesive structures on the surface of K. pneumoniae. They are composed primarily of MrkA subunits, with the MrkD subunit on the tip. Type 3 fimbriae have been found to be necessary for K. pneumoniae biofilm production and binding to medical devices. MrkD has specifically been found to bind extracellular matrix, such as that exposed on damaged tissues and coating indwelling devices, while MrkA binds abiotic surfaces, such as medical devices both prior to insertion into patients and after insertion when coated with host matrix. Type 3 fimbriae have been shown to have a possibly detrimental role, as their presence on K. pneumoniae increases reactive oxygen species (ROS) production by neutrophils. Michelle K. Paczosa, and Joan Mecsas Microbiol. Mol. Biol. Rev. 2016; doi:10.1128/MMBR.00078-15