Lecture IIb Growth

Binary fission

The process by which one bacterial cell divides into two daughter cells.

Exponential growth

A pattern of bacterial growth in which the population doubles during each generation.

Generation

A single cycle of bacterial cell division.

Generation time

The time required for a bacterial population to double.

Culture medium

A nutrient-containing material used to grow microorganisms in the laboratory.

Culture media

Materials containing nutrients used for isolation, maintenance, and identification of bacteria.

Agar

A hydrocolloid derived from red algae that is commonly used as the solidifying agent in culture media.

Liquid medium

A culture medium without a solidifying agent that allows bacteria to grow in suspension.

Solid medium

A culture medium containing a solidifying agent such as agar.

Semisolid medium

A culture medium containing a lower concentration of agar, producing a soft gel.

Defined medium

A chemically defined medium in which the exact chemical composition is known.

Minimal medium

A defined medium containing only the nutrients required for growth of a particular organism.

Complex medium

A medium whose exact chemical composition is not completely known because it contains biological materials such as blood, milk, yeast extract, or beef extract.

Fastidious bacteria

Bacteria that have complex nutritional requirements and are difficult to grow.

Nonselective medium

A medium designed to support the growth of many different microorganisms.

Selective medium

A medium containing substances that allow certain bacteria to grow while inhibiting others.

SS agar

A selective medium used to isolate Salmonella and Shigella; bile salts inhibit many other bacteria.

Differential medium

A medium that allows different bacteria to be distinguished based on visible differences caused by their metabolic properties.

Indicator medium

Another term for differential medium because an indicator reveals differences in bacterial metabolism.

Blood agar

A differential medium used to distinguish bacteria based on their ability to lyse red blood cells.

Alpha hemolysis (α)

Partial or incomplete hemolysis of red blood cells producing a greenish discoloration around colonies.

Beta hemolysis (β)

Complete hemolysis of red blood cells around bacterial colonies.

Gamma hemolysis (γ)

No hemolysis of red blood cells.

Mannitol Salt Agar (MSA)

A medium that is both selective and differential and is commonly used to identify Staphylococcus species.

Selective property of MSA

The 7.5% NaCl inhibits most bacteria while allowing salt-tolerant organisms, especially Staphylococcus, to grow.

Differential property of MSA

Mannitol fermentation produces acid, causing the phenol red indicator to turn yellow.

Staphylococcus aureus on MSA

Typically ferments mannitol, producing acid and turning the medium yellow.

Staphylococcus epidermidis on MSA

Typically does not ferment mannitol, so the medium remains its original color.

Bacterial count

The process of estimating the number of bacterial cells in a sample.

Turbidity

The cloudiness of a bacterial suspension caused by cells scattering light.

Spectrophotometer

An instrument used to measure the amount of light absorbed or scattered by a bacterial suspension.

Relationship between turbidity and cell number

As bacterial cell number increases, turbidity and absorbance generally increase.

Wet weight

A method of estimating bacterial biomass by measuring the mass of cells containing water.

Dry weight

A method of estimating bacterial biomass after removing water from the cells.

Viable count

A method of estimating the number of living bacteria capable of forming colonies.

Colony-forming unit (CFU)

A viable bacterial unit capable of producing a colony on solid medium.

Serial dilution

A technique involving successive dilutions, commonly tenfold, to obtain a countable number of bacterial colonies.

CFU/mL

The number of viable colony-forming units present per milliliter of the original sample.

Bacterial growth curve

A graph showing changes in bacterial population over time in a closed culture.

Closed culture

A bacterial culture in which nutrients are not added and waste products are not removed.

Lag phase

The initial growth phase in which cells adapt to the environment and increase metabolic activity but show little or no cell division.

Log phase

The phase of rapid exponential cell division in which bacteria have a constant growth rate.

Why log-phase cells are antibiotic-sensitive

They are actively synthesizing DNA, proteins, and cell walls, making them vulnerable to antibiotics targeting these processes.

Stationary phase

The phase in which bacterial growth slows because nutrient depletion and waste accumulation limit cell division.

Causes of stationary phase

Nutrient depletion, accumulation of waste products, and accumulation of inhibitory metabolic products.

Secondary metabolites

Compounds produced especially during stationary phase, including some antibiotics.

Sporulation

The process by which certain bacteria form spores, often during nutrient depletion and stationary phase.

Death phase

The phase in which bacterial cell death exceeds cell division, causing an exponential decrease in viable cells.

Number of generations

The number of times a bacterial population doubles during a given time interval.

Population after n generations

b = a × 2ⁿ, where a is the starting population and b is the final population.

Generation number formula

n = 3.3 log(b/a), where a is the initial population and b is the final population.

Generation time formula

G = t/n, where t is elapsed time and n is the number of generations.

Relationship between generation time and growth rate

A shorter generation time indicates faster bacterial growth.

Most metabolically active growth phase

The log or exponential phase.

Phase with no significant cell division

Lag phase.

Phase with exponential cell division

Log or exponential phase.

Phase associated with nutrient depletion

Stationary phase.

Phase with greater cell death than cell division

Death phase.