Microbiology: Chapter 3 — Nutrition and growth

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3.1 Nutrients and nutritional types #

3.1.1 What must be supplied #

Macronutrients are C, H, O, N, P, and S, plus K, Mg, Ca, and Fe. Mn, Zn, Co, Mo, Ni, and Cu are cofactors. A growth factor is an organic molecule the cell cannot make — a vitamin, amino acid, or base. Haemophilus influenzae needs haemin and NAD for that reason. No medium is given here.

Energy, electrons, and carbon are separate choices. Photo- versus chemo- names the energy. Litho- versus organo- names the electron donor. Auto- versus hetero- names the carbon. Dropping the donor confuses a nitrifier with a plant.

Purple non-sulfur bacteria can leave their box: photoheterotrophs in the light, chemoheterotrophs in the dark. E. coli stays a chemoorganoheterotroph whether it uses oxygen, nitrate, or fermentation. Only the acceptor changes. Lithotrophs often gain little per electron and spend proton-motive force pushing electrons uphill to NAD+. They turn over a lot of substrate per cell. Acid mine drainage is one result. The thermodynamics are biochemistry.

3.2 Population growth #

3.2.1 Four phases #

Growth here means more cells, usually by binary fission. A closed batch culture, on a log plot, lags, rises exponentially, levels, and falls. Lag is preparation — new enzymes, repair, or germination — and an already exponential inoculum in the same medium may show none. Exponential phase is balanced growth at constant specific rate, when wall-active antibiotics work best. Cells per hour still rise because the population is larger. Stationary phase means a nutrient has run out or waste has accumulated; division stops or matches death, and secondary metabolites and sporulation begin. Death phase is a fall in viable count. Some cells are metabolically alive and still form no colony. A microscope count includes dead cells; a plate count scores clumps as one colony-forming unit. The counts agree best in exponential phase.

3.2.2 Generation time and the equation #

Generation time g is the interval between divisions in exponential phase. In time t the number of generations is n = t/g, and

N = N0 × 2^n.

Also n = 3.32 log10(N/N0), and the specific growth rate μ = ln(2)/g, about 0.693/g. Outside exponential phase the equations do not apply, and they are not a yield.

Start at 1.0 × 10^3 cells, g = 30 minutes. After 5 hours, n = 10 and N = 1000 × 1024 = 1.024 × 10^6. Ten doublings multiply the population about a thousandfold. Under rich conditions E. coli can double in about 20 minutes; in a poor medium or a host it is slower. The 20 minutes belongs to the conditions. dN/dt is not the constant: it rises through exponential phase while μ stays put.

3.2.3 Batch culture and the chemostat #

Batch culture cannot hold exponential growth at a chosen rate, because success depletes the medium. A chemostat flows fresh medium in and culture out at equal rates. Dilution rate D is flow divided by volume. At steady state μ = D. One nutrient is limiting; biomass is set by the yield on that nutrient and by how much of it enters, until D approaches μmax. If D exceeds μmax the culture washes out. The Monod relation, μ = μmax S / (Ks + S), is why a higher D leaves more substrate unused. No value of Ks is given. A turbidostat instead holds density and lets cells grow near μmax. In a poor habitat the organism with the higher μ at low substrate keeps the niche. No vessel, medium, or flow is specified.

3.3 Environmental limits #

3.3.1 Temperature, pH, and water #

Cardinal temperatures are minimum, optimum, and maximum. The optimum sits nearer the maximum. Below the minimum, membranes gel and growth stops; the cell may still be alive. Above the maximum, proteins denature abruptly.

Thermophile proteins are more stabilised; their bacterial membranes favour long saturated chains, and hyperthermophile archaea use tetraethers. Psychrophile proteins are flexible and denature at moderate warmth. Cold and freezing preserve. They do not sterilise.

Acidophiles grow below about pH 5.5, neutrophiles between about 5.5 and 8, alkaliphiles above about 8, while internal pH stays nearer neutral. Some archaea grow near external pH 0. Alkaliphiles often use a sodium gradient because protons are scarce outside. Helicobacter pylori is a neutrophile that survives stomach acid by urease, locally generating ammonia from urea. Water activity is vapour pressure relative to pure water. Most bacteria need about 0.91 or above. S. aureus tolerates about 0.86, which is why salt and partial drying do not stop it. Many fungi reach about 0.80. Extreme halophiles (Halobacterium) require salt and accumulate KCl. Others use compatible solutes such as glycine betaine, ectoine, or glycerol. Drying and salting buy time. They do not sterilise.

3.3.2 Oxygen #

Oxygen is the best acceptor and a source of superoxide, hydrogen peroxide, and, with free iron, hydroxyl radical. Nothing removes hydroxyl radical once made. Superoxide dismutase makes peroxide and oxygen from superoxide. Catalase makes water and oxygen from peroxide. Peroxidases reduce peroxide without releasing oxygen. The pattern predicts the class; it is not a law for every species.

An aerotolerant organism can therefore sit in air without respiring, and an obligate anaerobe cannot be judged on an aerobic plate. Excluding oxygen is the principle of handling anaerobes, not a method written out here. Phagocytes make the same radicals on purpose. Survival inside them is an evasion (Chapter 8); the burst itself is immunology.

3.4 Key terms, summary, and self-check #

Key terms. Growth factor; photolithoautotroph; chemolithoautotroph; chemoorganoheterotroph; lag; generation time; chemostat; dilution rate; cardinal temperature; water activity; superoxide dismutase; catalase; facultative; aerotolerant.

Summary. Class is energy, electrons, and carbon. Only exponential phase obeys N = N0 × 2^n. A chemostat sets μ equal to D until washout. Cardinal temperatures, pH, and water activity describe habitats and preservation. Oxygen class follows from superoxide dismutase and catalase, which decide both use of oxygen and survival of it.

Self-check.

1. Place a purple sulfur bacterium, a nitrifier, and E. coli. What about E. coli does not change when it ferments instead of respiring?

2. Exponential culture, 2.0 × 10^4 cells, doubling every 20 minutes. How many after 2 hours? Why does a lag spoil the arithmetic?

3. What equals D at chemostat steady state, and what is washout?

4. Why can Streptococcus grow in air while Clostridium cannot? Answer with enzymes.

5. Why does a refrigerator preserve without sterilising, and which class still grows there?

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