The ease of rapidly accumulating a large number of mutants requires careful bookkeeping to avoid confusing one mutant with another. Each mutant constructed should be assigned a strain number. Strain numbers usually consist of two to three capital letters designating the lab where they were constructed and a serial numbering of the strains in a central laboratory collection. Every mutation should be assigned a name that corresponds to a particular gene or phenotype, and an allele number that identifies each specific isolate. When available for a particular group of bacteria, genetic stock centers are the ultimate resources for gene names and allele numbers. Examples include the Salmonella Genetic Stock Centre ( http://www.ucalgary.ca/~kesander/), and the E. coli Genetic Stock Center (http://cgsc.biology.yale.edu/). It is also important to indicate how the strain was constructed, the parental (recipient) strain, and the source of any donor DNA transferred into the recipient strain (Maloy et al., 1996).