Microbial Growth - Lecture Notes Review

Vocabulary terms and definitions regarding microbial growth, historical perspectives, environmental factors, nutritional requirements, and methods for laboratory cultivation and measurement.

Robert Koch

A historical figure ($1843$-$1910$) who studied disease causing bacteria, helped develop the Germ Theory of Disease, and developed methods of cultivating bacteria.

Agar

A solidifying agent for culture media recommended by the wife of an associate of Robert Koch; it has a high melting temperature and is not digestible for most microbes.

Binary Fission

The process by which prokaryotic cells divide, where one cell divides into two, leading to exponential growth.

Generation Time

The time it takes for a microbial population to double; also known as doubling time.

Biofilms

Polymer-encased communities of microorganisms that attach to surfaces, such as dental plaque or the scum in toilet bowls.

Extra Polymeric Substances (EPS)

Slimy polymers released by free-floating cells in a biofilm to allow unrelated cells to attach and grow.

Quorum Sensing

The process by which cells within a biofilm communicate with one another via chemical signals.

Type VI Secretion System

A needle-like structure used by some Gram-negative bacteria to inject toxic compounds directly into competing bacteria.

Pure Culture

A population of cells derived from a single cell, allowing for the study of a single species.

Aseptic Technique

A method used in laboratories to minimize the accidental introduction of other organisms when obtaining a pure culture.

Streak-Plate Method

The simplest and most commonly used method for isolating single cells by reducing the cell count through a series of streaks on an agar plate.

Lyophilization

A long-term storage method for stock cultures where cells are freeze-dried by being subjected to a strong vacuum and liquid nitrogen.

Closed System

A culture in which nutrients are not added and wastes are not removed, to which the bacterial growth curve applies.

Lag Phase

The first phase of the growth curve where the number of cells does not increase, but cells are synthesizing enzymes required for growth.

Log Phase

Also called the exponential growth phase, it is when cells divide at a constant rate and are most sensitive to antibiotics.

Primary Metabolites

Compounds such as amino acids produced by bacteria during the log phase of growth.

Secondary Metabolites

Compounds such as antibiotics produced as nutrients are depleted and wastes accumulate during the later stages of the log phase.

Stationary Phase

The phase where total cell numbers remain constant because nutrient levels are too low to sustain further growth.

Death Phase

The phase in which the total number of viable cells decreases at a constant, exponential rate.

Phase of Prolonged Decline

A stage after the death phase where some fraction of the population may survive by adapting to tolerate worsened conditions.

Extremophiles

Organisms, most of which are archaea, that live in harsh environments such as volcanic vents or polar regions.

Psychrophile

An organism with an optimum growth temperature between $-5 \text{ degrees Celsius}$ and $15 \text{ degrees Celsius}$, typically found in Arctic and Antarctic regions.

Psychrotroph

An organism with an optimum growth temperature between $15 \text{ degrees Celsius}$ and $30 \text{ degrees Celsius}$, often responsible for the spoilage of refrigerated foods.

Mesophile

An organism with an optimum growth temperature between $25 \text{ degrees Celsius}$ and $45 \text{ degrees Celsius}$; pathogens typically fall into this group with an optimum of $35 \text{ degrees Celsius}$ to $40 \text{ degrees Celsius}$.

Thermophile

An organism with an optimum growth temperature between $45 \text{ degrees Celsius}$ and $70 \text{ degrees Celsius}$, common in hot springs and compost heaps.

Hyperthermophile

An organism with an optimum growth temperature of $70 \text{ degrees Celsius}$ or greater, usually belonging to the domain Archaea.

Obligate Aerobe

An organism that requires $O_2$ for energy-harvesting and growth.

Facultative Anaerobe

An organism that grows best if $O_2$ is present but can also grow without it.

Obligate Anaerobe

An organism that cannot grow in the presence of $O_2$.

Microaerophile

An organism that requires small amounts of $O_2$ ($2$-$10$%) but is inhibited by higher concentrations.

Aerotolerant Anaerobe

Also called an obligate fermenter, this organism is indifferent to $O_2$ and can grow in its presence but does not use it.

Reactive Oxygen Species (ROS)

Harmful by-products of aerobic respiration, including superoxide ($O_2^-$) and hydrogen peroxide ($H_2O_2$).

Superoxide Dismutase

An enzyme produced by almost all organisms growing in the presence of oxygen to inactivate superoxide by converting it to $O_2$ and $H_2O_2$.

Catalase

An enzyme that converts hydrogen peroxide ($H_2O_2$) into $O_2$ and $H_2O$, used as a test for oxygen tolerance.

Neutrophile

A microorganism that multiplies within the pH range of $5$ to $8$ with an optimum near pH $7$.

Acidophile

An organism that grows optimally at a pH below $5.5$.

Alkaliphile

An organism that grows optimally at a pH above $8.5$.

Halotolerant

Organisms that can grow in relatively high-salt solutions up to approximately $10\% NaCl$, such as Staphylococcus on human skin.

Halophile

Microorganisms that require high levels of sodium chloride for growth; marine bacteria require approximately $3\%$, and extreme halophiles require $\ge 9\% NaCl$.

Heterotrophs

Organisms that use organic carbon as their carbon source.

Autotrophs

Organisms that use inorganic carbon in the form of $CO_2$ as their carbon source via carbon fixation.

Nitrogen Fixation

The process of converting $N_2$ gas to ammonia ($NH_3$) and then incorporating it into organic compounds.

Limiting Nutrients

Nutrients like phosphorus and iron that are available in the lowest concentrations relative to need, dictating the maximum level of growth.

Fastidious

Organisms like Neisseria species that require numerous growth factors, such as vitamins and amino acids, from their environment.

Phototrophs

Organisms that obtain their energy source from sunlight.

Chemotrophs

Organisms that extract energy from chemicals, whether organic (like sugars) or inorganic (like hydrogen gas).

Complex Media

Bacterial growth media containing a variety of ingredients where the exact composition is highly variable.

Chemically Defined Media

Media composed of exact amounts of pure chemicals, generally resulting in slower growth for cells.

Selective Media

Media that inhibit the growth of certain species in a mixed sample while allowing the growth of the species of interest, such as MacConkey agar.

Differential Media

Media containing substances that microbes change in an identifiable way, such as blood agar used to differentiate hemolysis types.

Beta Hemolysis

A reaction on blood agar that produces a clear zone around a colony.

Alpha Hemolysis

A reaction on blood agar that produces a greenish zone of partial clearing around a colony.

Capnophiles

Bacteria that grow best with increased levels of $CO_2$, such as Neisseria and Haemophilus.

Enrichment Culture

A technique used to isolate an organism that represents only a small fraction of a mixed population by providing conditions that promote its specific growth.

Colony-Forming Units (CFUs)

A measure used in plate counts to determine the number of viable cells, accounting for the fact that cells often attach to one another.

Turbidity

The cloudiness of a microbial suspension, which is proportional to the concentration of cells.

Spectrophotometer

An instrument used to measure turbidity, where the percentage of light reaching the detector is inversely proportional to the optical density (absorbance).