CHAPTER 4

Bacterial Culture, Growth, and Development

4.1 Microbial Nutrition

4.2 Nutrient Uptake

4.3 Culturing and Counting Bacteria

4.4 The Growth Cycle

4.5 Biofilms

4.6 Cell Differentiation

Where do bacteria find food, and how do they grow? Bacteria struggle to survive in natural habitats because they constantly compete for food. Over eons, however, evolution has enabled bacteria to find new nutritional niches that require the microbes to consume strange foods (mothballs, for instance), harness energy from light, or survive boiling temperatures around deep-sea thermal vents. In the process, single-celled organisms developed ways to chemically “talk” to each other and form intricate multicellular communities called biofilms. The one process that links all of these activities is growth.

Like all living things, microorganisms need sources of carbon and energy to grow. As those resources are depleted, organisms can do one of the following: they can die (most do), evolve to better use what little resource remains, or cannibalize other, less fortunate members of the community. All three of these possible outcomes were observed within the single colony described in the Current Research Highlight.

What is the evidence that cells taken from the growth islands in aging colonies have actually evolved? Claude Saint-Ruf (Fig 4.1A), working with Ivan Matic, isolated numerous variants from islands in aging colonies. She then tested how well these variant cells competed with their parental strains for growth in reconstructed aging colonies. Equal numbers of parental and variant cells were mixed, spotted onto solid growth media (agar), and incubated for 7 days. In Figure 4.1B, you can see in the control (top panel) that two parents labeled with different fluorescent proteins grew equally well on agar when mixed. However, the lower panel clearly shows that a variant strain (green) has outcompeted its parent (red) in the reconstructed aging colony. The results show that evolution in aging colonies will generate mutants that better utilize dwindling nutrients or the end products of metabolism.

FIGURE 4.1 Survival of the fittest in aging colonies. A. Claude Saint-Ruf examined growth competition between parental E. coli and evolved variants found within an aging colony (Current Research Highlight). B. 100 cells of a variant and 100 cells of its parent were mixed and spotted onto growth agar. Two parental strains labeled with different fluorescent proteins can be seen in the top panel; the bottom panel shows a parental strain (red) mixed with a variant strain (green).

Understanding how bacteria use food to increase cell mass and, ultimately, cell number enables us to control their growth and even manipulate them to make useful products. Yeast, for example, consume glucose and break it down to ethanol and carbon dioxide gas. These end products are mere waste to the yeast, but extremely important to humans who enjoy beer.

Chapter 4 provides a broad perspective of microbial growth, introducing topics that will be expanded on in later chapters. We start by discussing the nutrients bacteria need to grow and the ways nutrients are used. For example, how do different bacteria obtain carbon and nitrogen, where do they get their energy, and what mechanisms do they use to gather the nutrients they need? (The details of metabolism emerge in Chapters 13, 14, and 15.) Next we explain how scientists use our knowledge of microbial nutrition to culture bacteria in the laboratory and measure their growth. And we ponder, why do most bacterial species growing in the world fail to grow in the lab? We end by describing the communities that bacterial cells form while they grow and how some species differentiate into unique forms able to survive starvation or explore for new sources of food—all so they can grow again.