13.4: Bacterial growth and reproduction is fast and efficient.

The time it takes for a bacterium to reproduce can be very short; most bacteria have generation times between 1 and 3 hours, and some are even shorter. Escherichia coli, for example, has a generation time of 20 minutes in optimal conditions, so a single E. coli cell could give rise to a population of 20 billion cells in less than 12 hours.

Bacteria carry genetic information in two structures: the chromosome and plasmids. The genes that provide instructions for all of the cell’s basic life processes are usually located in a circular DNA molecule, the bacterial chromosome. Most bacteria have just one chromosome, but some have more than one. A bacterial chromosome is organized more efficiently than a eukaryotic chromosome, in two ways. First, in bacteria, the genes that code for proteins with related functions—enzymes that play a role in a pathway that breaks down food for energy, for example—are often arranged right next to one another on the chromosome. This makes it possible to efficiently control the transcription of all the genes together. Second, almost all the DNA in a bacterial chromosome codes for proteins, so bacteria do not use time and energy transcribing mRNA that will not be translated. As we saw in Chapter 5, as much as 90% of the DNA in the chromosomes of eukaryotes does not code for genes, and after transcription, this is edited out of the mRNA before it is translated into protein.

A second type of structure that carries genetic information in bacteria is a circular DNA molecule called a plasmid. Plasmids carry genes for specific functions. For example, metabolic plasmids carry genes enabling bacteria to break down specific substances, such as toxic chemicals; resistance plasmids carry genes enabling bacteria to resist the effects of antibiotics; and virulence plasmids carry genes that control how sick an infectious bacterium makes its victim. Many bacteria have one or more (sometimes more than a hundred) plasmids. The strain of E. coli that has made news headlines after causing illness among many patrons of some fast-food restaurants carries a virulence plasmid that magnifies the effects of a gene for a sometimes-lethal toxin. (E. coli strains without this virulence plasmid are normal components of the bacterial community of the human intestine.)

When a bacterium divides, it creates two new daughter cells, each “offspring” carrying the genetic information that was present in the chromosome of the mother cell. Thus, binary fission transmits genetic information from one bacterial generation to the next (FIGURE 13-7). However, bacteria can also transfer genetic information laterally—to other individuals within the same generation—through any of three different processes: conjugation, transduction, and transformation (FIGURE 13-8).

Conjugation is the process by which one bacterium transfers a copy of some of its genetic information to another bacterium—even when the two bacteria are different species. It’s very much like plugging your phone or iPod into a computer to transfer songs from the computer to the mobile device. This is not reproduction, because you start with one iPod and one computer, and that’s what you have when you finish. But the iPod now contains songs that it did not have before. Plasmid transfer does exactly that for a bacterium—you still have just two bacterial cells, but the newly acquired plasmid has given the recipient bacterium genetic information that it did not have before. These genes could enable the bacterium to make an enzyme that allows it to metabolize a new chemical or to defend itself against a new antibiotic.

Transduction occurs when a kind of virus called a bacteriophage (one type is shown in Figure 13-1) infects a bacterial cell. The virus reproduces inside the bacterial cell, and sometimes, inadvertently, the new virus particles contain pieces of bacterial DNA in addition to or instead of the viral DNA. When these viruses are released and infect new bacterial cells, the bacterial DNA can be inserted into the host bacterium’s chromosome, passing new bacterial genes to that bacterium.

Transformation is the process by which bacterial cells scavenge DNA from their environment. This DNA comes from other bacterial cells that have burst open, releasing their cellular contents. The circular chromosomes break into short lengths of DNA, which can then be taken up by living bacterial cells and inserted into their own chromosomes, potentially adding genes they did not originally have.