Nutrient transport mechanisms


Osmosis (Fig. 7.4)

Diffusion of water through a selectively or differentially permeable membrane

Cell can block certain molecules dissolved in water

Water will diffuse at a faster rate from the less concentrated solution to the higher until equilibrium is reached

5% solute + 95% water ===> 70% solute + 30% water

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Osmotic conditions that affect cells (Fig. 7.5)


Isotonic

When the environment is equal in solute concentration to the cell's internal environment

 

Hypotonic

The solute concentration of the external environment is lower than that of the cell's internal environment (e.g. in pure water)

Water will flow to the inside of the cell

Some bacteria and amebas adapt by releasing excess of water out of the cell

 

Hypertonic

Environment has a higher solute concentration than the protoplasm (cytoplasm) forcing water out of cell

High osmotic pressure or potential is lethal to many microbes
(e.g. salt water, sugar solutions, honey, etc.)

Some halophilic bacteria restricts its loss of water or increase the salinity of their internal environment (Halobacterium)


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Types of transport (Table 7.5)

 
Passive

Follows physical laws that are unique to living systems and do not generally require direct energy input from the cell

Nutrients exist in a gradient from a high concentration outside the cell to a low concentration inside the cell

 

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Diffusion (Fig. 7.3)

 

Molecular movement in which atoms and molecules move in a gradient from an area of higher density to an area of lesser density

A fundamental property of atoms and molecules that exist in a state of random motion

It is the net movement of molecules down their concentration gradient by random thermal motion

Movement of small unchanged molecules across membranes

Examples: O2, CO2, H2O in cells release of waste out of cells


Facilitated Diffusion (Fig. 7.6)

Molecules bind to specialized membrane proteins (carriers)

Some examples: yeast transport of sugar, bacterial transport of glycerol, transport of calcium into bacterial endospores

Passive transport is slow and inefficient as compared to active transport mechanisms


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Active (Fig. 7.7)

Microbes must capture molecules in short supply and bring them inside cell against a gradient but at a faster rate than passive transport

Many microbes have effective and efficient active transport systems

Examples: monosaccharides, organic acids, phosphates and metal ions

Active transport also occurs within intracellular membranes such as the mitochondrion, chloroplast, and endoplasmatic reticulum (ER)

 

Carrier-mediated transport

Atoms or molecules are pumped into and out of the cell by specialized receptors

Driven by ATP or the proton motive force

Transports simple sugars, amino acids, inorganic ions (Na+, K+)

 

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Group translocation

Molecule is moved across membrane and converted into a metabolically useful substance.

An alternate system to transport sugars and amino acids

 

Bulk transport

The movement of large solids or masses of liquids enter the cell intact by engulfment and vesicle formation

 

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Endocytosis (Gr. to engulf)

For substances that do not pass physically through the membrane followed by vacuole formation

 

Phagocytosis(Gr. to eat)

In amebas, certain WBC ingest large solid matter particles

 

Pinocytosis (Gr. to drink)

Ingestion of liquids, such as oil droplets or large molecules in solution

The molecular size and concentration of a nutrient determine which method of transport is used


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Environmental factors affecting microorganisms


Microbial ecology focuses on ways that microorganisms deal or adapt to various environmental factors

Adaptation is a complex adjustment in biochemistry or genetics that enables long-term survival and growth

 

Main factors are:


Temperature

Gases

pH

Radiation

Osmotic pressure

Hydrostatic pressure

Other microorganisms


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Temperature

'Cardinal range' includes a minimum, maximum, and optimum temperature value for a given species

Typhus rickettsia

32-38°C

Leprosy bacillus

33-35°C 

Rhinovirus (flu)

33-35°C

Staphylococcus aureus

6-44°C

 

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Temperature adaptations

 Microorganisms are classified by their temperature requirements

 

The three main groups are:

Psychrophiles

Minimum -15°C

Optimum 0°

Maximum 15°


Snow fields, polar ice, deep sea, non-pathogenic bacteria

Listeria monocytogenes and Staphylococcus aureus can grow in refrigerated foods

Some are facultative psychrophiles or psychrotrophs (grow slowly in cold and an optimal temperature of 20°C

 

Mesophiles

Minimum 10°C

Optimum 30°C (range 20-40°C)

Maximum 50°C

Include most bacteria


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Thermoduric bacteria

Can survive short exposures to high temperatures but are normally mesophiles and contamiminats of heated or pasteurized foods (Giardia, Bacillus and Clostridium)


Thermophiles

Because of their heat-stable proteins, these bacteria can grow best at temperatures higher than 45°C

Range of growth is between 45 and 80° C

 

Hyperthermophiles

Grow between 80 and 250 °C

Spore forming, sunny areas and volcanic areas.

Thermal organisms are subject of intense research

These bacterial species below are essential for the polymerase chain reaction technique (PCR) for DNA amplification using DNA Taq polymerase at 65-72°C:

Thermus aquaticus

Thermococcus litoralis

Thermobaga


Most microorganisms will die at temperatures above 60°C



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Temperature, food and bacterial pathogens

 

Keep microbes out of optimum range for growth so food is not contaminated or deteriorated.

Food packed in vacuum or modified gaseous atmospheres excludes micro-aerophiles.



Bacterial contaminants of food include:

Campylobacter jejuni

One of the most common bacterial pathogens 

Occur naturally in intestine of livestock

More frequent than Salmonella in college campuses in the USA

Staphylococcus aureus

A major source of food intoxication particularly salty food (sausages) 

Heat sensitive 

Do not produce toxin at 4°C 

Toxin is resistance to heat

Pseudomonas spp.

Found in soil

Causes food spoilage of fruit and vegetables 

Can grow at 3-4°C

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Legionella pneumophila

Isolated from cooling towers (35-45°C)

A fastidious bacterial species

 


Salmonella  spp.

Do not grow below 10°C

6.5 million cases of illnesses in 2006 in USA

Few cells can cause infection!!

Most killed by mild heat treatment

 


Mycobacterium tuberculosis

Pathogen from air droplets, aerosols 

Main reason for pasteurization of milk


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Textbook: Foundations in Microbiology. K. Park Talaro. 6th edition. McGraw Hill.

Remember to read your textbook, study tables, graphs and illustrations.
Develop a strategy to administer your time so that when exams come you do not have to cram.
Attend lectures and ask questions.

Lecture notes are posted BEFORE lecture is given thereafter they will be removed.