Microbiology Lab Quiz #2, Dr. Collins

Obligate aerobes

Organisms growing ONLY in the presence of O2

Obligate anaerobes

ONLY grow anaerobically and are harmed by the presence of O2

Facultative anaerobes

Can grow in the presence AND absence of O2

_____ _______ _______ are often used to create anaerobic conditions in a growth chamber.

Gas pack systems

How do gas pack systems work?

Hydrogen and Carbon dioxide are produced in the gas pack system. Hydrogen combines with oxygen inside the chamber to form water, while Carbon dioxide prevents oxygen from entering the chamber by creating pressure on the chamber wall.

Hypotonic solution

Water will enter the cell and cause it to become rigid

Hypertonic solution

Water will leave the cell and cause it to shrink

Plasmolysis

When the plasma membrane shrinks away from the cell wall due to a loss of water from the cell

Halotolerant

Bacteria that can TOLERATE high salt concentrations

Halophiles

Bacteria that THRIVE in high salt concentrations

Non-halophiles

Bacteria that CANNOT handle high salt concentrations (they grow from 0-1% NaCl)

Psychrophiles

Microorganisms growing at temperatures ranging from 0-15 degrees C

Psychrotolerant

Microorganisms that CAN grow in cold temperatures but grow optimally at temperatures between 25-45 degrees C

Mesophiles

Microorganisms ONLY growing at temperatures between 25-45 degrees C

Thermophiles

Microorganisms that grow at temperatures optimally above 55 degrees C

Pathogenic organisms are typically classified as _________. Why?

Mesophiles, because they only grow in temperatures between 25-45 degrees C, which includes normal body temperature.

Organisms that cause food spoilage are typically classified as ________________

Psychrotolerant

Antiseptics

Substances commonly applied on the skin to kill microbes.

Disinfectants

Substances used to kill microbes on inanimate objects (floor, table, sink, etc.)

Examples of antiseptics and their uses:

Alcohol and iodine are often applied to the site of a medical injection

Examples of disinfectants and their uses:

Chlorine/bleach is a disinfectant for the laundry, floors, toilet, sewage, or pool

Antibiotics

Chemicals produced by a living organism that kills other living organisms

Antibiotics display ________ _________ and are more limited in the types of organisms they kill than antiseptics and disinfectants are

Selective toxicity

Example of selective toxicity in antibiotics:

A given antibiotic will generally be effective in killing only some bacterial species while antiseptics/disinfectants are lethal to a wide range of microbes

How is antibiotic sensitivity determined?

By using the Kirby-Bauer method

What is the Kirby-Bauer method?

Antibiotics are infused into thin paper disks, which are then placed onto a plate via a mechanical dispenser or sterile forceps.

A small zone of inhibition or no zone indicates:

That the organism is RESISTANT to the antibiotic

A large zone of inhibition indicates:

That the organism is susceptible to the antibiotic

Selective media

Favors the growth of a particular organism while inhibiting the growth of other organisms. Selective media selects organisms by using dyes, bile salts, and nutritional sources.

Differential media

Distinguishes between different types of bacteria.

What does Eosin Methylene Blue Agar (EMB) select for?

EMB selects for gram negative bacteria by limiting the growth of gram positive bacteria via dyes

How is Eosin Methylene Blue Agar (EMB) differential?

EMB is differential because it can detect between lactose and non-lactose fermenting bacteria

Which two dyes are used in EMB to select for gram negative bacteria and differentiate between lactose and non-lactose fermenters?

Eosin and methylene blue.

Black or dark center colonies or green metallic colonies on an EMB plate means it is:

Lactose positive

Clear to white colonies that take on the color of the agar on an EMB plate means it is:

Lactose negative

What does MSA (Mannitol Salt Agar) select for?

MSA plates select for halotolerant bacteria such as Staphylococcus due to high salt concentrations

What two dyes are used in MSA plates to indicate pH differences?

Mannitol and phenol red

What does a yellow MSA plate indicate?

That it is a Mannitol fermenter. A yellow plate indicates that the mannitol is reduced and there is a DROP in pH

What does a red MSA plate indicate?

That it is a Non-mannitol fermenter. A red plate indicates that the mannitol is not reduced and there is NO CHANGE in pH

What do Blood Agar plates differentiate between?

They differentiate between hemolytic and non-hemolytic bacteria

Hemolysins:

Enzymes that destroy RBCs (red blood cells)

What does it mean when Blood Agar turns green?

It means that Alpha Hemolysis happened. This is a partial hemolysis that leaves the cells intact and causes a greening of the agar.

What does it mean when Blood Agar exhibits a clearing around the bacterial colony?

It means that Beta Hemolysis happened. This is a complete hemolysis that completely destroys the red blood cells and produces a clearing around the bacterial colony.

What does it mean when there is growth of the bacterial colony but no change to the Blood Agar?

It means that Gamma Hemolysis happened. This means that no hemolysis happened and there was no effect on the red blood cells.

What types of media are both selective and differential, and what type of media is only differential?

EMB and MSA are selective/differential, and Blood Agar is only differential

Penicillin mode of action:

Targets cell wall synthesis

Polymyxin B mode of action:

Targets plasma membrane synthesis

Chloramphenicol mode of action:

Targets protein synthesis

Erythromycin mode of action:

Targets protein synthesis

What is chloramphenicol classified as?

It is a broad spectrum drug, meaning it works for both Gram + and Gram -

Are Gram + or Gram - bacteria usually more resistant to antibiotics?

Gram - bacteria are more resistant to antibiotics than Gram + because of their LPS layer, and their thinner peptidoglycan layer doesn’t absorb the antibiotics as easily.