Chapter 6

Distinguish between chemically defined and complex media

-Chemically defined media: Exact chemical composition is known (recipe of nutrients needed to grow culture) -Complex media: Extracts and digests of yeasts, meat, or plants (Nutrient broth or Nutrient agar); contains an ingredient that has unknown composition

Explain three indirect methods of measuring cell growth.

1. Turbidity: A spectrophotometer is used to determine turbidity by measuring the amount of light that passes through a suspension of cells (spectrophotometry). Metabolic activity: The measure of oxygen consumption and the production of an acid or carbon dioxide are examples of this indirect method. These products are in direct proportion, usually, to the number of bacteria present. 3. Dry weight: For filamentous organisms such as fungi, measuring dry weight is a convenient method of growth measurement

Define colony

A colony is a visible mass of microbial cells that theoretically arose from one cell.

explain how microbes are preserved by deep freezing and freeze drying

Although refrigeration can be used for the short-term storage of bacterial cultures, two common methods for preserving microbial cultures for long periods are deep-freezing and lypophilization. Deep-freezing is a process in which a pure culture of microbes is placed in a suspension liquid and quick-frozen at temperatures ranging from -50°C to -90°C. Lypophilization, also known as freeze-drying, is where a suspension of microbes is quickly frozen at temperatures ranging from -54°C to -72°C, and the water is removed by high vacuum (sublimation).

provide a use for each of the four elements (C, N, S, P) needed in large amounts for microbial growth

Carbon: Structural organic molecules, energy source. - Nitrogen: is needed for protein and nucleic acid synthesis. - Sulfur: is used to synthesize sulfur-containing amino-acids and vitamins such as thiamine and biotin. -Phosphorus: needed for the synthesis of ATP.

explain the importance of osmotic pressure to microbial growth

Growth of cell is inhibited if high concentration occurs. High salt and sugar concentrations draw water out any microbial cells that are present and thus prevent their growth. Hypertonic environments with increased salt or sugar, can cause plasmolysis of the cell (This concept is applied in food preservation)

Identify ways in which aerobes avoid damage by toxic forms of oxygen

In the phagolysosome of the phagocytic cell, ingested pathogens are KILLED by exposure to these toxic forms of oxygen. -Singlet oxygen: O2 boosted to a higher-energy state. Toxic form of oxygen, neutralized by pigments called **CAROTENOIDS. -Superoxide free radicals: Toxic form of oxygen that is detoxified by ** SUPEROXIDE DISMUTASE. -Peroxide anion: Toxic form of oxygen which is detoxified by catalase or peroxidase **CATALASE or PEROXIDASE. -Hydroxyl radical: Most reactive of the toxic forms of oxygen.

Differentiate between direct and indirect methods of measuring cell growth

It is not always necessary to count numbers. Indirect methods are available and tend to be more productive in industry and science where time is money. 1. Turbidity: A spectrophotometer is used to determine turbidity by measuring the amount of light that passes through a suspension of cells. See the figure below from your text. This is a common method of measuring many different things in science (spectrophotometry). Metabolic activity: The measure of oxygen consumption and the production of an acid or carbon dioxide are examples of this indirect method. These products are in direct proportion, usually, to the number of bacteria present. 3. Dry weight: For filamentous organisms such as fungi, measuring dry weight is a convenient method of growth measurement.

Identify how and why the ph of culture media is controlled

Most bacteria grow between pH 6.5 and 7.5. When bacteria are cultured in the laboratory, they often produce acids that eventually interfere with their own growth. To Neutralize the acids and maintain the proper pH, chemical buffers are included in the growth medium. The peptones and amino acids in some media act as buffers, and many media also contain phosphate salts. Phosphate salts have the advantage of exhibiting their buffering effect in the pH growth range of most bacteria. They are also nontoxic; in fact, they provide phosphorus, an essential nutrient.

Classify microbes into five groups on the basis of preferred temperature range

On the basis of preferred temperature ranges, microbes are classified as psychrophiles (cold-loving) 0-15 celsius,Psychotrophs: 20-30 celsius, mesophiles (moderate-temperature-loving) 25-40 celsius, and thermophiles (heat-loving) 50-60 celsius, Hyperthermophiles (80 and up).

Explain how microbes are classified on the basis of oxygen requirements

Oxygen Concentration: Obligate aerobe: dependent on oxygen for growth. Facultative anaerobes: do not require oxygen, but grow faster with it present (E.coli). Obligate anaerobe: do not grow under oxygen conditions (Clostridium). Aerotolerant: cannot use oxygen - but tolerates it. Microaerophiles: require lower concentrations of oxygen

explain four direct methods of measuring cell growth

Plate counts: A standard plate count reflects the number of viable microbes and assumes that each bacterium grows into a single colony. Plate counts are reported as a number of colony forming units (CFUs). A plate count may be done either by the pour plate method, or the spread plate method. Serial dilutions: Serial dilutions are performed in test tubes (dilution tubes), typically, where 1 milliliter of inoculum is transferred to 9 milliliters of solution for a 1:10 decrease in the number of cells. Filtration: In filtration, bacteria are retained on the surface of a membrane filter, and then transferred to a culture medium to grow and subsequently be counted. Most Probable Number: The most probable number (MPN) method can be used for microbes that will grow in a liquid medium; it is a statistical estimation.

Justify the use of each of the following; anaerobic techniques, living host cells, candle jars, selective media, differential media, and enrichment media.

The cultivation of anaerobic bacteria poses a special problem. Because oxygen will kill anaerobes, a special media called reducing media is used. Special ingredients in the media combine with oxygen to deplete the oxygen in the culture medium. Petri dishes can be incubated in an anaerobic jar, anaerobic chamber, or OxyPlate. See the microbial growth laboratory instruction (Lab #4) for a detailed discussion on this subject and methods. Selective and differential media are used to detect the presence of specific microorganisms associated with disease or poor sanitation. By inhibiting unwanted organisms with salts, dyes, or other chemicals, selective media allow the growth of only the desired microbes. Differential media are used to distinguish among different organisms growing on the same plate. Finally, an enrichment culture is used to encourage the growth of a particular microorganism in a mixed culture. This is often used for soil or fecal samples.

compare the phases of microbial growth and describe their relation to generation time

The generation time is the time required for a cell to divide or a population to double. It varies considerably among organisms and with environmental conditions, such as temperature or pH. Some bacteria have a generation time of one to three hours; others take over 24 hours per generation. Due to the large numbers resulting from exponential growth, understanding logarithmic representations of bacterial populations is required. Bacterial growth always follows a predictable pattern with four phases: Lag phase: very little to no bacterial growth. Log phase: the number of bacterial cells doubles at a constant, exponential rate. Stationary phase: population growth levels off as the rate of cell death equals the rate of cell division. Death phase

define bacterial growth including binary fission

The normal reproductive method of bacteria is binary fission (two identical cells are formed by splitting). Some bacteria reproduce by budding - a small initial outgrowth (a bud) that enlarges until its size approaches that of the parent cell and it separates by aerial spore formation or fragmentation.

Describe how pure cultures can be isolated by using streak plates

The streak plate method works well when the organism to be isolated is present in large numbers relative to the total population. An enrichment medium may need to be utilized when they are present in relatively low numbers relative to the total population