Microbial Growth

The Requirements for Growth

• Physical requirements : Temperature, pH, Osmotic pressure

Chemical requirements : Carbon, Nitrogen, Sulfur, and Phosphorous , Oxygen, Organic growth factors (vitamins, amino acids, purines, pyrimidines) ,Trace elements (elements needed in smaller quantities) e.g iron

There are 3 critical temperature values related to a microbe's growth:

• Minimum growth temperature (lowest temp. a strain can survive)

• Optimum growth temperature (the temp. that the strain grows best)

• Maximum growth temperature (highest temp. a strain can survive)

A strain's ability to grow within a certain temperature range can be described based on various terms:

Psychrophiles = cold-loving

Mesophiles = moderate-temperature-loving

Thermophiles = heat-loving

temperature response curves.

We can determine their thermal growth description, as well as their min., optimum, and max. growth temperatures experimentally using temperature response curves.

How fast cell division is happening.

Psychrophiles • Can grow at 0°C • Optimum below ~20 °C

Psychrotrophs • Can grow at 0°C • Optimum 20-30°C

Mesophiles • Optimum 25-40°C • Can't grow at 0°C

Thermophiles • Optimum 50-60°C

Hyperthermophiles • Optimum >80°C

Also, temperature response curves can also reflect a strain's ecology, for example:

Psychrophiles live in deep oceans and polar regions (colder)

• Psychrotrophs often cause food spoilage in refrigerators

• Mesophiles are often part of normal human microbiome and pathogens of animals.

• Thermophiles are found in hot springs and compost piles

The ability of microbes to grow at certain temperatures is also tied to food safety:

• >60°C destroys most microbes

• 15-50°C rapid growth (e.g. room temps)…some pathogens

• 0-4°C [common fridge temps] Psychrotrophs grow (few pathogens)… those that grow spoil food.

• Frozen temperatures = no significant growth

pH and Microbes

Most bacteria grow best between pH 6.5-7.5 (near neutral)

• Molds and yeasts (types of fungi) grow between pH 5-6 (slightly acidic)

• Some food preservation techniques can involve encouraging the production of acid by certain microbes during fermentation to inhibit the growth of other microbes. E.g. sauerkraut, certain pickling methods, some cheeses -add microbe and it will ferment, this affects the Ph and lowers it and inhibits other microbes from growing

• The microbes that live in acidic environments = acidophiles E.g. Lactobacillus is an acidophile

Osmotic pressure and Microbes

We've already talked about high salt outside cell can lead to shrinkage of cell's cytoplasm (plasmolysis).

• However, some cells can withstand high salt! How??

• They may take up salt molecules to equalize to salt concentration outside cell

• They may alter cell membrane to increase its rigidity (helps maintain integrity)

• Some microbes are adapted to high salt environments to the point that they require high salt concentrations = obligate halophiles

• Some only tolerate high salt concentrations = facultative halophiles

Chemical Requirements

• Some of the most important chemicals for microbial growth are C, N, S, and P

• Their uses by microbes are slightly different and they are often each acquired from specific molecules.

When microbes run out of molecules they cant replicate DNA but might get fatter

Carbon

• Backbone of organic molecules

• Present in CO2 (which many autotrophs use)

Nitrogen

• Important in proteins, DNA, ATP

• Bacteria can get N from: • decomposing protein • using inorganic nitrogen (e.g., NH4) • fixing N2 gas from atmosphere (called diazotrophs)

Sulfur

Used in amino acids, and certain B vitamins

• Bacteria get S typically from:

• Decomposing protein

• Using SO4 or H2S

Phosphorus

• Used in DNA, RNA, and ATP, and membranes

• Microbes get P typically from PO4

Trace Elements

Are inorganic elements required in small amounts

• Used generally as enzyme cofactors (like in nitrogenase enzyme)

• Includes iron, copper, molybdenum, and zinc

When growing microbes in lab you may or may not need to add these trace elements to media. • If using ultrapure water…you would need to add

• If using filtered tap water…you would likely not need to add

Researching Trace Metals in Microbiology

Also researching Trace Metals in Microbiology requires a lot of specialized techniques and strategies!

Trace metal bubble

Dr. Erin Bertrand Dalhousie University

Oxygen

• Is another element that has a large influence on microbial growth.

• A major element in our lower atmosphere (2% of dry air)

• Certain strains of microbes will prefer certain oxygen conditions

• Obligate aerobe = require oxygen

• Facultative anaerobe = can grow with or without oxygen when without they do fermentation or anaerobic respiration

• Anaerobes = cannot use oxygen (many are harmed by it)

• Aerotolerant anaerobes = tolerate oxygen but cannot use it

• Microaerophiles = require oxygen but at lower concentrations