Microbial Growth and Nutrition Chapter 5 Microbial Growth and Nutrition

5.1 Microbial Reproduction Is Part of the Cell Cycle Binary Fission is Part of the Cell Cycle B period: Cell increases in mass and size C period: DNA replicates and the two strands separate. D period: Synthesis of a septum forms two identical cells. Figure 05.02A: Bacterial Cell Cycle.

Figure 05.03: A skyrocketing bacterial population. Bacterial and archaeal cells reproduce asexually. The generation (or doubling) time is the interval of time between successive binary fissions. In pathogens, a shorter doubling time means a shorter incubation period of disease. Figure 05.03: A skyrocketing bacterial population.

5.2 Microbial Growth Progresses Through 4 Distinct Phases 1. Lag phase: no cell division occurs while bacteria adapt to their new environment. 2. Logarithmic (log) phase is when exponential growth of the population occurs, human disease symptoms usually develop. Figure 05.04: The Growth Curve for a Bacterial Population.

Figure 05.04: Bacterial growth curve. 3. Stationary phase is when reproductive and death rates equalize. 4. Decline (exponential death) phase is when the accumulation of waste products and scarcity of resources causes the population to die. Figure 05.04: Bacterial growth curve.

Bacterial Cells Can Exist in Metabolically Inactive States Dormancy is a response to potential or actual environmental change. Persister cells can stop dividing but maintain a low rate of metabolism under stress Figure 05.06: Persister Cells.

Endospores are a response to nutrient limitation. Endospores are a highly resistant structure formed by species of Bacillus and Clostridium when nutrient supplies are low. © Scott Camazine/Alamy Images Courtesy of CDC Figure 05.07A: Clostridium showing spore formation. Figure 05.07B: Bacillus spore.

Figure 05.08: The Formation of a Bacterial Spore by Bacillus subtilis. A stressed cell undergoes asymmetrical cell division, creating a small prespore and larger mother cell. The prespore contains: Cytoplasm, DNA and dipicolinic acid, which stabilizes proteins and DNA. The mother cell matures the prespore into an endospore, then disintegrates, freeing the spore. Endospores: are resistant to desiccation and heat. Figure 05.08: The Formation of a Bacterial Spore by Bacillus subtilis.

Figure 05.07C: Germinating spore. When environmental conditions are again favorable, protective layers break down and the spore germinates into a vegetative cell. Figure 05.07C: Germinating spore. Courtesy of Janice Carr/CDC

Optimal prokaryotic growth is dependent on several physical factors. Temperature Each species has an optimal temperature for growth and a range of acceptable temperatures. Figure 05.09: Growth Rates for Different Microorganisms in Response to Temperature.

Psychrophiles grow optimally below 15°C and make up the largest portion of all prokaryotes on Earth. Mesophiles live at the medium temperature range of 10° to 45°C, including pathogens in the human body. Thermophiles live best around 60°C, in compost heaps and hot springs. Hyperthermophiles are Archaea that grow optimally above 80°C, found in seafloor hot-water vents.

Figure 05.11: The Effect of Oxygen on Microbial Growth. Many prokaryotes are obligate aerobes, which require oxygen to grow. Microaerophiles live in low oxygen environments Thioglycollate broth can be used to test an organism’s oxygen sensitivity. Figure 05.11: The Effect of Oxygen on Microbial Growth.

Figure 05.10ABC: Bacterial Cultivation in Different Gas Environments. Anaerobes do not or cannot use oxygen; aerotolerant species are insensitive to oxygen, but obligate anaerobes are inhibited or killed by oxygen. Facultative anaerobes grow either with oxygen or in reduced oxygen environments. Capnophilic bacteria (microaerophils) require an atmosphere low in oxygen and rich in carbon dioxide. Figure 05.10ABC: Bacterial Cultivation in Different Gas Environments. © Scott Coutts/Alamy Images

Hydrostatic and Osmotic Pressure pH The majority of species grow optimally at neutral (~7.0) pH. Acidophiles are acid-tolerant prokaryotes. Ex), those used to turn milk into buttermilk, sour cream, and yogurt. Hydrostatic and Osmotic Pressure Barophiles can withstand incredibly high hydrostatic pressure. Ex), psychrophiles living at the bottom of the ocean. Halophiles are salt-tolerant prokaryotes. They can maintain optimal osmotic pressure Ex) Vibrio cholerae that causes cholera

5.3 Culture Media Are Used to Grow Microbes and Measure Their Growth Culture media are of two basic types. A complex medium is a chemical unidentified medium or complex medium such as a nutrient broth or nutrient agar. In a synthetic medium, the chemical composition of the medium is known. Figure from Microfocus 5.5 Colonies of L. pneumophila

Figure 05.13A: Selective and Differential Media. Culture media can be modified to select for or differentiate between microbial species. A selective medium contains ingredients to inhibit growth of certain species and allow the growth of others. A differential medium contains specific chemicals to indicate species that possess or lack a biochemical process. Some “fastidious” organisms require an enriched medium containing specific nutrients. Many microbes are viable but can’t be cultured in the lab Figure 05.13A: Selective and Differential Media. Courtesy of Jeffrey Pommerville

Population measurements are made using pure cultures. A pure culture is a population consisting of only one species of prokaryote. The pour-plate isolation method allows separation of species through dilution of a sample. Figure 05.14: Pour plate. Courtesy of Jeffrey Pommerville

Figure 05.15ABCDE: The streak plate method. The streak-plate isolation method spreads out individual cells to form discrete colonies of species. Courtesy of James Gathany/CDC Figure 05.15ABCDE: The streak plate method.

Figure 05.18: Using Turbidity to Measure Population Growth. Population growth can be measured in several ways. Figure 05.18: Using Turbidity to Measure Population Growth. Turbidity (cloudiness) of a sample can be measured with a spectrophotometer. Courtesy of Jeffrey Pommerville

Population growth can be measured in several ways. One can perform a direct microscopic count. Figure 05.16: Direct Microscopic Count.

Figure 05.17: The standard plate count. Number of cells can be estimated through the most probable number test. In the standard plate count procedure, colonies grow on a plate containing a sample of diluted broth. The number of colonies indicates the original number of viable (living) cells in the broth. Figure 05.17: The standard plate count. © R.A. Longuehaye/Photo Researchers, Inc.