Bacterial Taxonomy

Question 0

What are the 4 steps to the gram stain?

Answer 0

1. Pour on crystal violet stain (a blue dye) and wait 60 seconds.
2. Wash of with water and flood with iodine solution. Wait 60 seconds.
3. Wash off with water and then “decolorize” with 95% alchohol.
4. Finally, counter-stain with safranin (a red dye). Wait 30 seconds and wash off with water.

Question 1

Why we use the gram stain?

Answer 1

1. We must use a stain because bacteria are colorless and invisible to light microscopy.
2. To separate the gram positive from the gram negative bacteria.
3. To determine whether the organism is round or rod-shaped.

Question 2

What color are the gram positive and what color are the gram negative bacteria?

Answer 2

Gram positive : BLUE

Gram negative : RED

Question 3

What is the layer outside the bacterial cytoplasmic membrane?

Answer 3

The peptidoglycan layer or cell wall.

Question 4

What are the main components of the peptidoglycan layer?

Answer 4

Repeating disaccharides with 4 amino acids in a side chain extending from each disaccharide.

Question 5

The amino-acid chains of the peptidoglycan covalently bind to other amino acids from neighboring chains. This results in a stable cross-linked structure. What is the enzyme that catalyzes the formation of this linkage? Where is it located?

Answer 5

A transpeptidase located in the inner cytoplasmic membrane.

Target of penicillin : for this reason it is also called penicillin binding protein.

Question 6

What is the main difference between the bacterial cytoplasmic membrane and the one of animals?

Answer 6

The bacterial cytoplasmic membrane has no cholesterol or other sterols.

Question 7

What is the important polysaccharide present in the gram positive cell wall?

Answer 7

Teichoic acid.

Question 8

As what does teichoic acid act?

Answer 8

As an antigenic determinant, so it is important for serologic identification of many gram-positive species.

Question 9

What are the 3 layer of gram negative cell envelope? (Not including the periplasmic space)

Answer 9

1. Cytoplasmic membrane
2. Peptidoglycan layer
3. A unique outer cell membrane

Question 10

Where is the periplasmic space in gram negative bacteria located and what are its components?

Answer 10

It is located between the cytoplasmic membrane and an extremely thin peptidoglycan layer.
It is filled with a gel that contains proteins and enzymes.

Question 11

Does the thin peptidoglycan layer in gram negative bacteria contain teichoic acid?

Answer 11

No, instead it contains a small helical lipoprotein called MUREIN lipoprotein.

Question 12

What is the importance of the MUREIN lipoprotein?

Answer 12

It is important because it originates from the peptidoglycan layer and extends outward to bind the unique third outer membrane.

Question 13

Although the outer membrane of gram negative bacteria is composed of a classic phospholipid bilayer, what makes it unique?

Answer 13

The outermost portion of the bilayer contains lipopolysaccharide (LPS).

Question 14

What are the components of the LPS?

Answer 14

1. Outer carbohydrate chains of 1-50 oligosaccharide units that extend into the surrounding media - specific for organism - antigen determinants : O-specific side chain or O-antigen.
2. The center part is a water soluble core polysaccharide.
3. Interior to the core : Lipid A - disaccharide with multiple fatty acid tails reaching into the membrane

Question 15

What is lipid A?

Answer 15

Lipid A is the gram negative endotoxin (toxic to humans).
When bacterial cells are lysed by our immune system, fragments of membrane containing lipid A are released into the circulation - causing fever, diarrhea, possibly fatal endotoxic shock (also called septic shock).

Question 16

What are the porin proteins?

Answer 16

Embedded in the gram negative outer membrane are porin proteins, which allow passage of nutrients - unique to gram negative bacteria.

Question 17

Mention the 6 major differences between gram positive and gram negative bacteria.

Answer 17

1. Two layers / Three layers
2. Low / High lipid content
3. No endotoxin / Endotoxin (LPS) - Lipid A.
4. No periplasmic space / Periplasmic space
5. No porin channel / Porin channel
6. Vulnerable to lysozyme and penicillin / Resistant to lysozyme and penicillin

Question 18

What are the 4 major shapes of bacteria?

Answer 18

1. Cocci : spherical
2. Bacilli : rods. Short bacilli are called coccobacilli.
3. Spiral forms : comma-shaped, S-shaped, or spiral-shaped.
4. Pleomorphic : lacking a distinct shape (like jello).

Question 19

What are the names of the more complex patterns that the different shaped bacteria organize into?

Answer 19

1. Pairs (diplococci)
2. Clusters
3. Strips
4. Single bacteria with flagella

Question 20

What are the major 7 gram-positive bacteria?

Answer 20

1. Streptococcus (forms strips of cocci)
2. Enterococcus (forms strips of cocci)
3. Staphylococcus (forms clusters of cocci)
4. Bacillus (Rod that produce spores)
5. Clostridium (Rod that produce spores)
6. Corynebacterium (Rod that do not produce spores)
7. Listeria (Rod that do not produce spores)

Question 21

What gram negative bacteria are cocci?

Answer 21

Only two groups - both diplococci :

1. Neisseria
2. Moraxella

Question 22

What is the only gram negative spiral shaped organism?

Answer 22

The spirochetes (e.g. Treponema Pallidum).

Question 23

What are the mycobacteria?

Answer 23

Weakly gram positive (stain better with acid-fast stain) - tuberculosis and leprosy.

Question 24

Describe briefly the spirochetes.

Answer 24

1. Gram negative cell wall, but too small to be seen with the light microscope - need a special darkfield microscope.
2. Besides the classic 3 layers, are surrounded by an additional phospholipid-rich outer membrane with few exposed proteins - protective from immune recognition (“stealth organisms”).
3. Periplasmic flagella : not protruding out of the outer membrane.

Question 25

What is a mycoplasma?

Answer 25

They do not have a cell wall.

They only have a simple cell membrane, so they are neither gram positive nor gram negative.

Question 26

What is the action of erythromycin and tetracycline?

Answer 26

Both inhibit protein synthesis preferentially at the bacterial ribosomal subunits.
Erythromycin : works at 50S subunit.
Tetracycline : works at 30S subunit.

Question 27

What are the properties for dividing the bacteria into groups based on their metabolic effects?

Answer 27

1. How the organism deals with oxygen.
2. What the organism uses as a carbon and energy source.
3. Other properties : different metabolic end-products that bacteria produce such as acid and gas.

Question 28

What are the three enzymes that bacteria possess to break down thee oxygen radical products?

Answer 28

1. Catalase - breaks down hydrogen peroxide.
2. Peroxidase - also breaks down hydrogen peroxide.
3. Superoxide dismutase - breaks down the superoxide radical.

Question 29

What are the obligate aerobes bacteria ?

Answer 29

These are just like us : Use glycolysis - Krebs TCA cycle - Electron transport chain with oxygen as the final electron acceptor.
They have all three of “ROS” enzymes.

Question 30

What are the facultative anaerobes ?

Answer 30

Don’t let this name fool you! These bacteria are aerobic.
They have the faculty to be anaerobic - just like the switch to anaerobic glycolysis that our muscles undergo during sprinting.

Question 31

What are the microaerophilic bacteria (also called aerotolerant)?

Answer 31

These bacteria use fermentation and have no electron transport system.
They can tolerate LOW amounts of oxygen, because they have superoxide dismutase (but no catalase).

Question 32

What are the obligate anaerobes?

Answer 32

These guys hate oxygen and have no enzymes to defend against it.

Question 33

Mention 2 obligate intracellular organisms.

Answer 33

Rickettsia and Chlamydia. Steal ATP from the host.

Question 34

Mention some gram negative rods that live in the GI tract (enterics).

Answer 34

1. E. Coli
2. Shigella
3. Salmonella
4. Yersinia
5. Klebsiella
6. Proteus
7. Enterobacter
8. Serratia
9. Vibrio
10. Campylobacter
11. Helicobacter
12. Pseudomonas
13. Bacteroides (anaerobic)

Question 35

Besides enterics, mention some other important gram negative rods.

Answer 35

1. Haemophilus
2. Bordetella
3. Legionella
4. Yersinia
5. Francisella
6. Brucella
7. Pasteurella
8. Gardnerella

Question 36

Mention 3 important gram negative spirals.

Answer 36

Spirochetes :

1. Treponema
2. Borrelia
3. Leptospira

Question 37

Mention some important obligate aerobes.

Answer 37

Gram positive :
1. Nocardia (weakly acid-fast)
2. Bacillus cereus
Gram negative :
1. Neisseria
2. Pseudomonas
3. Bordetella 
4. Legionella
5. Brucella
Acid-fast :
1. Mycobacterium
2. Nocardia

Question 38

Mention some important facultative anaerobes.

Answer 38

Gram positive :
1. Staphylococcus
2. Bacillus anthracis
3. Corynebacterium 
4. Listeria 
5. Actinomyces
Gram negative :
1. Most other gram-negative rods.
No cell wall :
1. Mycoplasma

Question 39

Mention some important microaerophilic bacteria.

Answer 39

Gram positive :
1. Enterococcus
2. Streptococcus (some species are facultative anaerobes)
Gram negative :
1. Spirochetes (treponema, Borrelia, leptospira)
2. Campylobacter

Question 40

Mention the 2 important obligate anaerobes.

Answer 40

Clostridium and Bacteroides.