Facultative
Organisms that can grow under different conditions. Commonly used to describe bacteria that can grow oxidatively under aerobic or fermentatively under anaerobic conditions.

F-factor
An E. coli plasmid that encodes conjugative transfer ("fertility") functions. The F-plasmid may exist as an autonomous plasmid in the cytoplasm or may integrate into specific sites in the chromosome, producing an Hfr cell.

F+ cell
A cell that carries a F-factor as an autonomous plasmid, which enables the cell acting as a donor (male) to transfer the F-factor to a recipient (female) cell.

F- cell
A cell which does not contain the F-factor, and hence able to act as a recipient (female) in a conjugative DNA transfer in matings with F+ or Hfr strains.

Fimbriae
Thin, proteinaceous filaments that extend from the cell surface of microbial cells and facilate adhesion to solid surfaces or other cells.

Fingerprinting
A common term for methods that allow the rapid identification of particular types of bacteria (also known as "typing"). For example, plasmid fingerprinting involves determining the number and types of plasmids in a particular strain of bacteria.

F-pilus
A filamentous appendage encoded by the F-plasmid. The pilus promotes adhesion of F+ and F- cells and provides adsorption sites for male-specific phages.

F-prime (F')
An extrachromosomal F-plasmid that carries a fragment of chromosomal DNA.

Filamentous phage
A group of phage with the nucleic acid surrounded by a long, tubular protein coat. Examples of filaments phage include M13 and Ff.

Filter mating
A method where conjugation between donor and recipient bacteria is done on a millipore filter. Conjugation is often much more efficient on a solid surface, probably because the donor and recipient bacteria are fixed in close proximity facilitating stable mating bridge formation.

Flagella
Long, flexible, helical protein structures that extend from the surface of the cell. Rotation of flagella in one direction results in motility, and rotation of flagella in the opposite direction results in tumbling.

Flanking sequence or flanking region
The DNA sequence located on either side of a specific genetic locus.

Fluctuation Test
An experimental approach designed by Luria and Delbruck to determine whether mutations are random or directed, and to measure mutation rates (see http://www.sci.sdsu.edu/~smaloy/MicrobialGenetics/topics/mutations/fluctuation.html).

Footprinting
A method for identification of a protein-binding site on a DNA molecule. For example, DNase I footprinting identifies those phosphodiester bonds which are covered by a specific DNA-binding protein and thus protected from cleavage by DNase I.

Frameshift
A mutation which adds or deletes one or two base pairs (or any non-multiple of 3) from a coding sequence in a molecule of DNA, so that the genetic code is read out-of-phase. All codons translated downstream of a frameshift mutation will be misread, and frequently an out-of-frame stop codon will prematurely terminate translation.
Functional domain
A region of a polypeptide chain or RNA that performs a particular function. Functional domains are often folded into independant structures, separated from the rest of the polymer by a flexible hinge region.