Microbiology 1
The influence of Environmental factors and growth
5.1 Environmental Factors
5.1.1 Osmotic Concentration – Solutes and Water Activity
5.1.2 pH
5.1.3 Temperature
5.1.4 Oxygen Concentrations
5.1.5 Pressure
5.2 Growth Limitation by Environmental Factors
5.3 Bacterial Growth and Growth Requirements
5.4 Reproduction of Bacteria
5.5 The study of population growth
5.6 Counting Viable-But-Non-Culturable Vegetative Prokaryotes
The Influence of Environmental Factors and Growth
Growth of micro-organisms depends on nutrient limitation and the chemical and physical
nature of their surroundings;
Prokaryotes have the ability to adapt to extreme and inhospitable environments;
Bacillus infernus seem to be able to live over 1.5 miles below the earth’s surface, without
O2 and at temperatures above 60°C;
Micro-organisms that grow in such harsh conditions are often called extremophiles.
5.1 Environmental factors
5.1.1 Osmotic Concentration – Solutes and Water Activity
Osmosis – the movement of water across a selectively permeable membrane from a dilute
solution (higher water concentration) to a more concentrated solution; A selectively
permeable plasma membrane separates a micro-organism from its surroundings;
If a micro-organism is placed in a hypotonic (one with a lower osmotic concentration)
environment – water will enter the cell and cause it to burst unless something is done to
prevent the influx;
This can be prevented by the presence of a rigid cell wall, which maintains the shape and
integrity of the cell;
Inside the cell
Outside the
cell
1
, A. Paulse MIY100S-MIB151S Lecture 5_2018
When microbes with a cell wall are placed in a hypertonic environment, water leaves the
cell and plasmolysis occurs;
The cell dehydrates and the cell membrane is damaged
It becomes metabolically inactive and growth ceases
In order to survive, micro-organisms must maintain a high internal solute concentration
to retain water
Inside the cell
Outside the
cell
The amount of water available to micro-organisms can be reduced by interaction with
solute molecules (osmotic effect) or by adsorption to the surfaces of solids (the matrix
effect);
To survive, micro-organisms must maintain a high internal [solute] to retain water;
Most micro-organisms keep the osmotic concentration of their protoplasm above that of
their environment with the aid of compatible solutes so that the plasma membrane is
always firmly pressed against the cell wall:
Compatible solutes – solutes ( e.g., amino acids, polyols) that are compatible with the
metabolism and growth of the micro-organism when at high intracellular concentrations
Micro-organisms can increase their internal osmotic concentration by taking up or
synthesising amino acids, sugars, potassium and sodium ions and polyols, which don’t
normally disrupt enzyme structure and functions;
Micro-organisms can adapt to environments with varying concentrations of solutes;
Osmotolerant organisms – are micro-organisms that grow over wide ranges of water
activity:
Only a few organisms are osmotolerant
e.g. S. aureus can grow in solutions containing up to 3M NaCl
Halophiles – are micro-organisms that have adapted to high salt concentrations and now
require high levels of NaCl to grow;
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