Chapter Capsule

A. Source of Nutrients

Nutrition consists of taking in chemical substances (nutrients) and assimilating and extracting energy from them.

1. Substances required for survival are essential nutrients—usually containing the elements (C, H, N, O, P, S, Na, Cl, K, Ca, Fe, Mg). Essential nutrients are considered macronutrients (required in larger amounts) or micronutrients (trace elements required in smaller amounts—Zn, Mn, Cu). Nutrients are classed as either inorganic or organic.

2. A growth factor is an organic nutrient (amino acid and vitamin) that cannot be synthesized and must be provided.

A. An autotroph depends on carbon dioxide for its carbon needs. If its energy needs are met by light, it is a photoautotroph, but if it extracts energy from inorganic substances, it is a chemoautotroph.

B. A heterotroph acquires carbon from organic molecules. A saprobe is a decomposer that feeds upon dead organic matter, and a parasite feeds from a live host and usually causes harm. Disease-causing parasites are pathogens.

A. Temperature: An organism exhibits optimum, minimum, and maximum temperatures. Organisms that cannot grow above 20°C but thrive below 15°C and continue to grow even at 0°C are known as psychrophiles. Mesophiles grow from 10°C to 50°C, having temperature optima from 20°C to 40°C. The growth range of thermophiles is 45°C to 80°C.

B. Oxygen Requirements: The ecological need for free oxygen (O₂) is based on whether a cell can handle toxic by-products such as superoxide and peroxide.

1. Aerobes grow in normal atmospheric oxygen and have enzymes to handle toxic oxygen by-products. An aerobic organism capable of living without oxygen if necessary is a facultative anaerobe. An aerobe that prefers a small amount of oxygen but does not grow under anaerobic conditions is a microaerophile.

2. Strict (obligate) anaerobes do not use free oxygen and cannot produce enzymes to dismantle reactive oxides. They are actually damaged or killed by oxygen. An aerotolerant anaerobe cannot use oxygen for respiration, yet is not injured by it.

C. Effects of pH: Acidity and alkalinity affect the activity and integrity of enzymes and the structural components of a cell. Optimum pH for most microbes ranges approximately from 6 to 8. Acidophiles prefer lower pH, and alkalinophiles prefer higher pH.

D. Other Environmental Factors: Electromagnetic radiation and barometric pressure affect microbial growth. A barophile is adapted to life under high pressure (bottom dwellers in the ocean, for example).

A. A microbial cell must take on nutrients from its surroundings by transporting them across the cell membrane.

B. Passive transport involves the natural movement of substances down a concentration gradient and requires no additional energy (diffusion).

C. Osmosis is diffusion of water through a selectively permeable membrane. A form of passive transport that can move specific substances is facilitated diffusion.

D. Osmotic changes that affect cells are hypotonic solutions, which contain a lower solute concentration, and hypertonic solutions, which contain a higher solute concentration. Isotonic solutions have the same solute concentration as the inside of the cell. A halophile thrives in hypertonic surroundings, and an obligate halophile requires a salt concentration of at least 15%, but grows optimally in 25%.

E. In active transport, substances are taken into the cell by a process that consumes energy. In group translocation, molecules are altered during transport.

F. Phagocytosis and pinocytosis are forms of active transport in which bulk quantities of solid and fluid material are taken into the cell.

A. Microbes coexist in varied relationships in nature.

1. Types of symbiosis are mutualism, a reciprocal, obligatory, and beneficial relationship between two organisms, and commensalism, an organism receiving benefits from another without harming the other organism in the relationship. Parasitism occurs between a host and an infectious agent.

2. Synergism is a mutually beneficial but not obligatory coexistence. Antagonism entails competition, inhibition, and injury directed against the opposing organism. A special case of antagonism is antibiotic production.

A. The splitting of a parent bacterial cell to form a pair of similar-sized daughter cells is known as binary, or transverse, fission.

B. The duration of each division is called the generation, or doubling, time. A population theoretically doubles with each generation, so the growth rate is exponential, and each cycle increases in geometric progression.

C. A growth curve is a graphic representation of a closed population over time. Plotting a curve requires an estimate of live cells, called a viable count. The initial flat period of the curve is called the lag phase, followed by the exponential growth phase, in which viable cells increase in logarithmic progression. Adverse environmental conditions combine to inhibit the growth rate, causing a plateau, or stationary growth phase. In the death phase, nutrient depletion and waste buildup cause increased cell death.

D. Cell numbers can be counted directly by a microscope counting chamber, Coulter counter, or flow cytometer. Cell growth can also be determined by turbidometry and a total cell count.