Bacteriology 1: Aerobes vs. Anaerobes, Pathologic Lesions, & Catabolism

Catabolism

Breakdown of larger molecules into smaller molecules with released energy captured as ATP

Bacteria can be classified according to both their ______ and _______ sources

energy; carbon

Two types of bacteria based on energy source

1.) Phototroph

2.) Chemotroph

Phototroph

Organism that uses light as its energy source

Chemotroph

Organism that obtains energy from organic or inorganic compounds

Two types of bacteria based on carbon source

1.) autotrophs

2.) heterotrophs

Autotroph

Organism that uses CO₂ as its primary carbon source

Heterotroph

Organism that requires organic compounds as its carbon source

What classification are all pathogenic bacteria based on their energy and carbon sources?

Chemoheterotrophs

Chemoheterotrophs

obtain both their energy and carbon from organic compounds within the host

Five bacterial classifications related to oxygen or CO₂ requirements

1.) Obligate aerobes

2.) Obligate anaerobes

3.) Facultative anaerobes

4.) Microaerophiles

5.) Capnophiles

Obligate aerobes

Bacterium that require oxygen for growth

Where would obligate aerobes grow in a culture tube with an oxygen gradient?

Near the surface where oxygen concentration is highest!

Two examples of obligate aerobes

1.) Mycobacterium spp

2.) Bordetella spp

What type of metabolism do obligate aerobes use?

Respiratory metabolism

Obligate anaerobes

Bacterium that cannot tolerate oxygen and require oxygen-free conditions for growth

Where would obligate anaerobes grow in a culture tube with an oxygen gradient?

Near the bottom where oxygen concentration is lowest

Two examples of obligate anaerobes

1.) Clostridium spp

2.) Bacteroides spp

What type of metabolism do obligate anaerobes primarily use?

Fermentation

What tissue do obligate anaerobes typically infect?

deep wounds and other poorly oxygenated tissues

Facultative anaerobe

Bacterium that can grow either with or without oxygen

Two examples of facultative anaerobes

1.) Staphylococcus spp

2.) E coli

Where would facultative anaerobes grow in a culture tube with an oxygen gradient?

Throughout the tube because they can grow with or without oxygen

What metabolism do facultative anaerobes use when oxygen is present?

Oxidative phosphorylation

What metabolism do facultative anaerobes use when oxygen is absent?

Fermentation

Microaerophile

Bacterium that require oxygen, but grow best at lower oxygen concentrations than atmospheric oxygen

What oxygen concentration do microaerophiles prefer?

Approximately 3-15% O₂

Where would microaerophiles grow in a culture tube with an oxygen gradient?

Slightly below the surface where oxygen is present at a reduced concentration

Example of a microaerophile

Borrelia spp

Capnophile

Bacterium that require or prefer increased environmental CO₂

What CO₂ concentration do capnophiles prefer?

Approximately 3-10% CO₂

Two examples of capnophilic bacteria

1.) Campylobacter

2.) Brucella

Why do capnophilic bacteria require additional environmental CO₂ for growth?

They do not produce enough CO₂ through their own metabolism to meet their metabolic needs

True or false: CO2 is required by most bacteria and is usually available as a product of their metabolism

true!

Why is CO₂ important to bacterial metabolism?

It is needed for synthesis of compounds such as fatty acids and amino acids

Anaerobes are found in oxygen _________ environments in nature

oxygen-poor

Examples of oxygen-poor environments:

-Deep wounds

-Intestinal tract

-Rumen

-Necrotic tissue

-Poorly perfused/devitalized -tissue

Aerobes are found in oxygen _________ environments in nature

rich

Examples of oxygen-rich environments:

-Upper respiratory tract

-Skin

-Blood

Although oxygen is necessary for aerobic life, it can also generate...

highly reactive and damaging molecules

Superoxide

Highly reactive oxygen radical that can damage cells

Superoxide can be converted into what molecule?

hydrogen peroxide

What enzyme converts superoxide into hydrogen peroxide?

superoxide dismutase (SOD)

hydrogen peroxide

toxic and can contribute to the production of even more reactive molecules

Reactive oxygen species

Highly reactive oxygen-containing molecules that can damage bacterial and animal cells; includes superoxide, H2O2, and the reactive molecules produced by H2O2

Two bacteria types that can be damaged by reactive oxygen species:

1.) aerobes

2.) facultative anaerobes

How can aerobes and facultative anaerobes be protected from reactive oxygen species?

through production of enzymes that eliminate hydrogen peroxide

Two enzymes that help eliminate hydrogen peroxide:

1.) Catalase

2.) Peroxidase

Why can aerobes tolerate oxygen?

They possess enzymes such as superoxide dismutase, catalase, and/or peroxidase that detoxify ROS

What bacteria type will NOT be able to tolerate oxygen?

obligate anaerobes

Why is oxygen toxic to obligate anaerobes?

They lack all the important enzymes that detoxify reactive oxygen species!

What happens when an obligate anaerobe is exposed to oxygen?

Oxygen exposure → ROS accumulate → anaerobe cannot detoxify ROS → bacterial damage or death

Because oxygen is toxic to obligate anaerobes, an important principle in treating anaerobic infection is to...

expose the infected tissue to oxygen!!

The oxygen requirement of a bacterium strongly influences how it performs _______, which in turn determines:

catabolism

-how much ATP it produces

-what byproducts it generates

-the appearance of infected tissue

-type of pathology

Two general types of bacterial catabolism based on the substrates used:

1.) Saccharolytic

2.) Asaccharolytic

Saccharolytic bacteria

Bacteria that use carbohydrates as their carbon and energy source

Asaccharolytic bacteria

Bacteria that use organic compounds as their carbon and energy source

What type of catabolism do most bacterial pathogens use?

Saccharolytic catabolism

Two metabolic processes saccharolytic bacteria use to metabolize carbohydrates

1.) Oxidative respiration

2.) Fermentation

Oxidative respiration

Energy-producing metabolism that uses an electron transport chain

What is the terminal electron acceptor in aerobic respiration?

Oxygen

Approximately how much ATP is produced per glucose through aerobic respiration?

Approximately 30-32 ATP

Why is aerobic respiration considered energy efficient?

It produces a large amount of ATP from a relatively small amount of substrate

Fermentation

Anaerobic metabolic process that generates ATP through glycolysis without using the aerobic electron transport chain

Two types of bacteria that use fermentation

1.) Obligate anaerobes

2.) Facultative anaerobes when oxygen is absent

Approximately how much ATP does fermentation produce per glucose?

Approximately 1-2 ATP

Aerobic respiration vs fermentation: ATP production

Aerobic respiration → approximately 30-32 ATP/glucose

Fermentation → approximately 1-2 ATP/glucose

Why must anaerobes metabolize more substrate than aerobes?

Fermentation produces much less ATP per glucose, so anaerobes must consume more substrate to obtain enough energy

Using 32 ATP vs 2 ATP, approximately how much more substrate may an anaerobe need to metabolize to obtain comparable energy?

Approximately 16× more substrate

Anaerobic metabolism contribute to extensive tissue ________. Why?

destruction; anaerobes must consume large amounts of host tissue to obtain enough energy because fermentation is inefficient

Pyruvate is considered the _____ of ___________

hub of fermentation

Why is pyruvate called the hub of fermentation?

It can be converted into many different fermentation products depending on the enzymes possessed by the bacterium

Three major types of fermentation products

1.) Organic acids

2.) Alcohols

3.) CO₂/gas

The byproducts of fermentation can contribute directly to...

the characteristic pathology of anaerobic infections

Three categories of characteristic pathology for anaerobic infections:

1.) odor

2.) gas

3.) tissue necrosis

Why can anaerobic infections have a characteristic odor?

Many fermentation products are small volatile molecules that produce characteristic smells

What types of odors can anaerobic infections produce?

Putrid/offensive or sometimes sweet odors depending on the organism

How can fermentation products contribute to tissue damage?

Acids and alcohols produced during fermentation can irritate and damage host tissue

Why can anaerobic infections contain gas?

Gas such as CO₂ can be produced as a product of fermentation and accumulate within infected tissue

Anaerobic respiration

Respiration that uses an electron transport chain but uses a terminal electron acceptor other than oxygen

Two types of bacteria that use anaerobic respiration

1.) facultative anaerobes

2.) anaerobes

How does anaerobic respiration differ from fermentation?

Anaerobic respiration uses an electron transport chain while fermentation does not

Will anaerobic respiration generate more or less ATP than fermentation? Why?

more; it uses an electron transport chain for energy production

Example of a bacterium capable of anaerobic respiration

Pseudomonas aeruginosa

Aerobic vs Anaerobic respiration

Aerobic respiration → ETC + O₂ as terminal electron acceptor

Anaerobic respiration → ETC + non-O₂ terminal electron acceptor

What determines whether an environment favors aerobic or anaerobic bacterial growth?

Oxygen availability

Three oxygen-rich sites where aerobic bacteria commonly cause infection

1.) Upper respiratory tract

2.) Skin

3.) Blood

Three oxygen-poor sites where anaerobic bacteria commonly cause infection

1.) Deep wounds

2.) Intestinal tract

3.) Rumen

Why do anaerobes commonly infect deep wounds?

Deep wounds can have poor oxygen delivery and create an oxygen-poor environment favorable for anaerobic growth

Where in nature/body environments are anaerobes generally found?

Oxygen-poor environments such as the intestinal tract, rumen, deep wounds, necrotic tissue, and poorly perfused tissue

Four classic signs of inflammation associated with aerobic infections

1.) Red

2.) Hot

3.) Painful

4.) Swollen

Why do aerobic infections tend to produce classic inflammatory lesions?

Oxygen-rich tissue supports recruitment, accumulation, and activation of inflammatory leukocytes

Seven characteristics of anaerobic infections

1.) Extensive tissue destruction

2.) Necrosis/blackened tissue

3.) Gas production

4.) Characteristic odor

5.) Acids and alcohols

6.) Poor blood supply

7.) Crepitus

Why do anaerobic infections cause extensive tissue destruction?

Inefficient fermentation requires extensive substrate use and fermentation products further damage tissue

Why can tissue become blackened during an anaerobic infection?

Extensive tissue destruction and necrosis cause tissue to become devitalized and blackened

Why do anaerobic infections commonly produce gas?

Gas is produced as a fermentation product and accumulates within infected tissue

Why can crepitus occur with an anaerobic infection?

Fermentation produces gas that accumulates within tissue and crackles when the tissue is palpated

What is the classic example of a severe gas-producing anaerobic infection?

Gas gangrene caused by Clostridium perfringens

Four findings are characteristic of gas gangrene caused by Clostridium perfringens:

1.) Blackened necrotic tissue

2.) Gas accumulation

3.) Characteristic odor

4.) Crepitus

Recall: Aerobic vs anaerobic infection appearance

Aerobic → red, hot, painful, swollen

Anaerobic → necrosis, blackened tissue, gas, odor, poor blood supply, crepitus