other pieces of DNA are often co-transferred and this can result in dissemination of genetic traits throughout a bacterial population, such as antibiotic resistance. Not all bacteria can make conjugative pili, but conjugation can occur between bacteria of different species. Hyperthermophilic archaea encode pili structurally similar to the bacterial conjugative pili. However, unlike in bacteria, where conjugation apparatus typically mediates the transfer of mobile genetic elements, such as plasmids or transposons, the conjugative machinery of hyperthermophilic archaea, called Ced (Crenarchaeal system for exchange of DNA) and Ted (Thermoproteales system for exchange of DNA), appears to be responsible for the transfer of cellular DNA between members of the same species. It has been suggested that in these archaea the conjugation machinery has been fully domesticated for promoting DNA repair through homologous recombination rather than spread of mobile genetic elements. Fimbriae Fimbria (Latin for 'fringe', plural fimbriae) is a term used for a short pilus that is used to attach the bacterium to a surface, sometimes also called an "attachment pilus". The term "fimbria" can refer to many different (structural) types of pilus, as many different types of pili have been used for adhesion, a case of convergent evolution. The Gene Ontology system does not treat fimbriae as a distinct type of appendage, using the generic pilus (GO:0009289) type instead. This appendage ranges from 3–10 nanometers in diameter and can be as much as several micrometers long. Fimbriae are used by bacteria to adhere to one another and to adhere to animal cells and some inanimate objects. A bacterium can have as many as 1,000 fimbriae. Fimbriae are only visible with the use of an electron microscope. They may be straight or flexible. Fimbriae possess adhesins which attach them to some sort of substratum so that the bacteria can withstand shear forces and obtain nutrients. For example, E. coli uses them to attach to mannose receptors. Some aerobic bacteria form a very thin layer at the surface of a broth culture. This layer, called a pellicle, consists of many aerobic bacteria that adhere to the surface by their fimbriae. Thus, fimbriae allow the aerobic bacteria to remain both on the broth, from which they take nutrients, and near the air. Fimbriae are required for the formation of biofilm, as they attach bacteria to host surfaces for colonization during infection. Fimbriae are either located at the poles of a cell or are evenly