Transduction is another method for transferring genes from one bacterium to another; this time the transfer is mediated by bacteriophages (bacterial viruses, also called phages). [ A bacteriophage infection starts when the virus injects its DNA into a bacterial cell. The bacteriophage DNA may then direct the synthesis of new viral components assembled in the bacterium. Bacteriophage DNA is replicated and then packaged within the phage particles. Early in the infective cycle the phage encodes an enzyme that degrades the DNA of the host cell. Some of these fragments of bacterial DNA are packaged within the bacteriophage particles, taking the place of phage DNA. The phage can then break open (lyse) the cell. When released from the infected cell, a phage that contains bacterial genes can continue to infect a new bacterial cell, transferring the bacterial genes. Sometimes genes transferred in this manner become integrated into the genome of their new bacterial host by homologous recombination. Such transduced bacteria are not lysed because they do not contain adequate phage DNA for viral synthesis. Transduction occurs in a wide variety of bacteria and is a common mechanism of gene transfer.
Some bacteriophages contribute to the virulence of bacterial infections. Certain phages can enter an alternate life cycle called lysogeny. In this cycle, all the virus's DNA becomes integrated into the genome of the host bacterium. The integrated phage, called a prophage, can confer new properties to the bacterium. For example, strains of Corynebacterium diptheriae, which have undergone lysogenic conversion, synthesize the toxin in diphtheria that damages human cells. Clostridium botulinum and Streptococcus pyogenes, when lysogenized by certain phages, also manufacture toxins responsible for illness, causing botulism and scarlet fever respectively. Strains lacking the prophage do not produce the damaging toxins.
Conjugation is another means of gene transfer in many species ... (More)