The bacterial genome, that is, the entirety of all genes of a bacterium, was once viewed as a rather stable entity. However, the observation of spreading resistance to antibiotics led to the discovery of extra-chromosomal elements encoding this property. Obviously, plasmids are able to transfer genes from one bacterium to another not only among one species but also from one species to another. Such transfer of genes is not restricted to antibiotic resistance genes. Examples of further traits often encoded by plasmids include resistance to heavy metals and production of toxins.
Another example of mobile genetic elements is phages, the viruses of bacteria. Phages are not just able to infect and finally lyse the bacterial host cell. Certain phages infect and then integrate their whole genome into the bacterial chromosome and thereby become a prophage. This may add another important factor to the property of the infected bacteria. In the case of pathogenic bacteria the production of toxins is frequently encoded by a prophage. A few medically important examples are bacteriophage β of Corynebacterium diphtheriae encoding diphtheria toxin, phage C1 of Clostridium botulinum coding for the C1 neurotoxin, and phage H-19B of Escherichia coli, which harbors the gene for Shiga toxin Stx1 (for a recent review, see Brüssow et al., 2004).
Smaller but still important mobile genetic units are insertion sequence (IS) elements. IS elements mediate DNA rearrangements by transposition, resulting in off/on switching of gene expression by insertion into, and excision from, open reading frames (ORFs), respectively.