Flashcards in Bacterial Taxonomy Deck (41):
Why we use the gram stain?
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.
What are the 4 steps to the gram stain?
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.
What color are the gram positive and what color are the gram negative bacteria?
Gram positive : BLUE
Gram negative : RED
What is the layer outside the bacterial cytoplasmic membrane?
The peptidoglycan layer or cell wall.
What are the main components of the peptidoglycan layer?
Repeating disaccharides with 4 amino acids in a side chain extending from each disaccharide.
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?
A transpeptidase located in the inner cytoplasmic membrane.
Target of penicillin : for this reason it is also called penicillin binding protein.
What is the main difference between the bacterial cytoplasmic membrane and the one of animals?
The bacterial cytoplasmic membrane has no cholesterol or other sterols.
What is the important polysaccharide present in the gram positive cell wall?
As what does teichoic acid act?
As an antigenic determinant, so it is important for serologic identification of many gram-positive species.
What are the 3 layer of gram negative cell envelope? (Not including the periplasmic space)
1. Cytoplasmic membrane
2. Peptidoglycan layer
3. A unique outer cell membrane
Where is the periplasmic space in gram negative bacteria located and what are its components?
It is located between the cytoplasmic membrane and an extremely thin peptidoglycan layer.
It is filled with a gel that contains proteins and enzymes.
Does the thin peptidoglycan layer in gram negative bacteria contain teichoic acid?
No, instead it contains a small helical lipoprotein called MUREIN lipoprotein.
What is the importance of the MUREIN lipoprotein?
It is important because it originates from the peptidoglycan layer and extends outward to bind the unique third outer membrane.
Although the outer membrane of gram negative bacteria is composed of a classic phospholipid bilayer, what makes it unique?
The outermost portion of the bilayer contains lipopolysaccharide (LPS).
What are the components of the LPS?
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
What is lipid A?
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).
What are the porin proteins?
Embedded in the gram negative outer membrane are porin proteins, which allow passage of nutrients - unique to gram negative bacteria.
Mention the 6 major differences between gram positive and gram negative bacteria.
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
What are the 4 major shapes of bacteria?
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).
What are the names of the more complex patterns that the different shaped bacteria organize into?
1. Pairs (diplococci)
4. Single bacteria with flagella
What are the major 7 gram-positive bacteria?
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)
What gram negative bacteria are cocci?
Only two groups - both diplococci :
What is the only gram negative spiral shaped organism?
The spirochetes (e.g. Treponema Pallidum).
What are the mycobacteria?
Weakly gram positive (stain better with acid-fast stain) - tuberculosis and leprosy.
Describe briefly the spirochetes.
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.
What is a mycoplasma?
They do not have a cell wall.
They only have a simple cell membrane, so they are neither gram positive nor gram negative.
What is the action of erythromycin and tetracycline?
Both inhibit protein synthesis preferentially at the bacterial ribosomal subunits.
Erythromycin : works at 50S subunit.
Tetracycline : works at 30S subunit.
What are the properties for dividing the bacteria into groups based on their metabolic effects?
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.
What are the three enzymes that bacteria possess to break down thee oxygen radical products?
1. Catalase - breaks down hydrogen peroxide.
2. Peroxidase - also breaks down hydrogen peroxide.
3. Superoxide dismutase - breaks down the superoxide radical.
What are the obligate aerobes bacteria ?
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.
What are the facultative anaerobes ?
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.
What are the microaerophilic bacteria (also called aerotolerant)?
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).
What are the obligate anaerobes?
These guys hate oxygen and have no enzymes to defend against it.
Mention 2 obligate intracellular organisms.
Rickettsia and Chlamydia. Steal ATP from the host.
Mention some gram negative rods that live in the GI tract (enterics).
1. E. Coli
13. Bacteroides (anaerobic)
Besides enterics, mention some other important gram negative rods.
Mention 3 important gram negative spirals.
Mention some important obligate aerobes.
Gram positive :
1. Nocardia (weakly acid-fast)
2. Bacillus cereus
Gram negative :
Mention some important facultative anaerobes.
Gram positive :
2. Bacillus anthracis
Gram negative :
1. Most other gram-negative rods.
No cell wall :
Mention some important microaerophilic bacteria.
Gram positive :
2. Streptococcus (some species are facultative anaerobes)
Gram negative :
1. Spirochetes (treponema, Borrelia, leptospira)