Gas requirements of microorganisms


The atmospheric gases that most influence the growth of microorganisms are oxygen (O2) and carbon dioxide (CO2)

 

Oxygen concentration in air is about 20%

CO2 concentration is about 0.003% (3,000 ppm)

Capnophiles grow better at 3-10% concentrations of CO2

Neisseria, Brucella and Streptococcus pneumonia

 

Oxygen is essential for life but also a powerful oxidizing agent that exists in many toxic forms for life

 

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Microorganisms can be:

Oxygen users

Oxygen detoxifiers

Non-oxygen users or detoxifiers

Non-oxygen users but detoxifiers


Dangerous metabolic products of O2 include:

1O2 singlet oxygen  (extremely reactive molecule produced by both living and non-living processes

-O2 superoxide ion 

H2O2 peroxide molecule

OH- hydroxyl radicals


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Aerobes vs. anaerobes

Most aerobic bacteria have enzymes to neutralize O2 by-products


Aerobic bacteria

Superoxide dismutase catalyzes

- O2 + -O2+ 2H+ ==> H2O2 + O2

( -O2 from phagocytes)


Catalase catalyzes:

H2O2 + H2O2 ==> 2H2O + O2



Most bacteria, fungi, algae and protozoa are aerobes

 

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Anaerobic bacteria

Anaerobes lack superoxide dismutase and catalase to process deleterious forms of oxygen

Can not deal with the presence of oxygen and die due to its toxicity

Growing anaerobic bacteria usually requires special media, methods of incubation, and handling chambers that exclude oxygen (Fig. 7.10)

Live in deep mud, bottom of oceans, deep soil, and inside animal bodies



Facultative anaerobic bacteria

Their metabolism does not require O2 for growth and is not affected by its presence

Can adopt an anaerobic mode of metabolism (fermentation) or opt for aerobic respiration

They have both superoxide dismutase and catalase enzymes


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Aerotolerant anaerobes

Do not utilize oxygen but can tolerate its presence

Possess alternate mechanisms for the breaking down of peroxide and superoxide.

Certain lactobacilli and streptococci use manganese ions or peroxidases to perform this task



Microaerophilic bacteria 

Require only a very small amount of oxygen for growth

Treponema pallidum the causative agent of syphilis

In soil, water and some human bacterial microflora

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pH

 

The majority of microorganisms lives or grows in habitats between pH 6 and 8 because strong acids and bases can be highly damaging to enzymes and other cellular substances

Human pathogens grow between 6.5 - 7.5

Euglena mutabilis (algae) tolerates 0 - 1 (an acidophil)

Thermoplasma (an archea bacterium with no cell wall) tolerates 1-2 (lyses at 7)



Acidophiles

Molds and yeasts (fungi) can spoil pickled foods

A few species of algae and bacteria can actually survive at a pH near that of concentrated hydrochloric acid.



Alkalinophiles

Bacteria found in soils with high concentrations of ammonia (NH4)

Metabolism of urea is one way that Proteus spp. can neutralize the acidity of the urine to colonize and infect the urinary system

 

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Osmotic pressure

Most microbes exist under hypotonic or isotonic conditions


Osmophilic microorganisms

Resistant to high-salt environments


Halophiles

Halobacterium, Halococcus (10 - 25% NaCl)


Facultative halophiles

Resist or grow in high salt concentration but can also live in a non-salty environment Staphylococcus aureus (0.1 - 20% NaCl)

 

Barophiles

Stand high hydrostatic pressures (bottom of oceans up to 1000x atm). These bacteria are so adapted to high pressures that they will rupture or lyse when expose to normal atmosphere pressure


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Microbial interactions

Microorganisms coexist in varied relationships in nature

Symbiotic interactions

Mutualism

Reciprocal, obligatory and beneficial relationship between two organisms (Insight 7.5)



Commensalism

An organism receiving benefits from another without harming the other organism in the relationship (Satellitism: Fig. 7.12)

"Making a living" as intestinal normal flora

Could become a parasite or cause disease

Lactobacillus protects vagina against infections due to low pH

Staphylococcus epidermis lives on human skin

Many thrive in mouth and large intestine



Parasitism

Occurs between a host and an infectious agent; the host is harmed in the interaction

 

 

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Non-symbiotic interactions

Synergism

A mutually beneficial but not obligatory coexistence in which organisms cooperate to produce a reaction

 

Antagonism

Entails competition, inhibition, and injury directed against the opposing organism

A special case of antagonism is antibiotic production



Quorum sensing (Fig. 7.13)

An interaction among members of a biofilm that results in a coordinated reaction, such as secretion of inducer molecules

By behaving as a unit, the biofilm remains stable and organized

It can occupy and exploit a wide variety of habitats

 

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Microbial growth

Steps of the binary or transversal fission in bacteria (Fig 7.14)


Cell enlarges

Duplication of chromosomes

Formation of central transverse septum

Septum breaks into two daughter cells

 

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Generation time (GT)

Time needed for a bacterial population to double in number

The length of the GT is a measure of the growth rate of an organism

Compared to other living organisms bacteria are notoriously rapid

Population doubles by a factor of two (binary)

2, 4, 8, 16, 32, 64 and so on

2n (n = number of generations)

Population growth is geometric or exponential

Can grow at constant rate if all conditions are favorable

Microbial populations grow slow or fast according to species

The average GT is 30 to 60 min under optimum conditions, and longer GT require even days

Mycobacterium leprae ... 10-30 days/gen

Salmonella enteritidis ... 20-30 min/gen

Staphylococcus aureus ...20-30 min/gen

 

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Growth curve of a bacterial culture

Lag phase (few or no cells)

Log phase (exponential) - most cells are alive

Stationary phase - decrease growth

Death phase - curve dips downwards

 

Relative rates of these events change as the curve proceeds

 

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If the GT of a bacterial species is 20 min then each cell in the total population will reproduce every 20 min in its log phase


In a few hours, a population of bacteria can easily grow from a small number of cells to several million

A population may rise in 30 min from 1,000 to 1000,000,000 in 16 hrs!

Logarithmic (log) numbers are used to express bacterial populations in a shorter form

9,658,780 is transformed to a logarithmic form as:

 

log10 9,658,780 = 6.73

106.73 is equal to 9,658,780

 

 

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Enumeration of microorganisms

 

Cell numbers can be counted directly by

Microscope counting chamber (Fig. 7.18)

Coulter chamber (7.19)

Flow cytometer

Other new methods


Cell growth can be determined by turbidometry (Fig. 7.17) and a total cell count

 

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Practical importance of the growth curve

 

Populations grow slow or fast

Microbes in log phase are more sensitive to changes in the environment

Microbes in log phase are more numerous and virulent than those released later

An infected person sheds more bacteria in early and middle stages of infection

Faster multiplication rate may overwhelm the slower growth rate of the host's cellular defenses

Do not culture cells beyond stationary phase

Stain young cultures not old ones

Heat and disinfectants increase death rates

 

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Calculations


Size of a population over time:

Nf= Ni (2n)

Ni = initial number of bacteria

Nf = final number of bacteria

n = number of generations

2n = number of bacteria in the generation

 

Solve for n:

Log Nf = logNi + nlog2

Where n= Log Nf - logNi)/log2

Assume:

Ni = 6000 bacterial cells;

Nf = 38,000,000

n = (7.5798 - 3.7782)/0.301

n = 12.6 generations

 

If the time was 5 hrs (300 min) between Ni and Nf then

GT = 300 min/12.6 generations = 23.8 min/generation

 

==> One million bacteria are enough to spoil food (approximately equivalent to a population derived from 20 generations)

 

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Here is a problem:

How many bacteria (Nf) are present in your favorite taco after it sits in a very warm car for 4 hours?

Assume an initial population (Ni) of 10 bacteria

Ni= 10 bacteria

Time = 4 hrs (240 min)

GT = 20 min

n = 240/20 = 12 generations after 4 hrs of exposure

2n = 212 = 4,096 bacteria

 

Replace in formula and the answer is:

Nf = 10 x 4,096 = 40,960 bacteria



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Never understimate the power of a microbe!

 

When bacterium grows in a culture medium its log phase is relatively short.

It is limited by nutrient depletion, oxygen depravation, accumulation of inhibitory products, etc.

 

Imagine if an E. coli (GT = 20 min) cell is allowed to grow in the exponential phase for one day (24 hrs). What would the total final number (Nf) of the progeny be?

Answer: Ni = 1 and Nf= 4.72 x 1021 and this number of bacteria is equivalent to:

 

 

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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.