Exam 4: Topic 19

Question 1

Compare and contrast the genera Streptococcus and Staphylococcus in terms of: shape morphological arrangements, gram stain, antimicrobial drugs resistance, ability to survive outside the host, diagnostic tools (MSA and catalase test), and virulence factors.

Answer 1

shape
• Streptococcus: chained-sphere; sometimes diplo
• Staphylococcus: clustered-sphere
 morphological arrangements
• Streptococcus: pairs or chains
• Staphylococcus:
 Gram stain
• Streptococcus: gram +
• Staphylococcus: gram +
 Antimicrobial drug resistance
• Streptococcus: still very sensitive
• Staphylococcus: many drug resistant strains, particularly in hospital acquired forms of staph
 Ability to survive outside of the host (hardiness)
• Staphylococcus: localized–tends to be skin. Systemic can be fatal.
• Streptococcus: tends to be localized (though localized infections can be fatal).
 diagnostic tools (MSA and catalase tests)
• MSA: is a selective and differential medium that can be used to isolate staphylococcus species, which can tolerate high salt concentrations, and differentiate them from streptococcus species
• Catalase test:

	1. Flood a broth culture or agar slant with a few drops of 3% hydrogen peroxide
	2. Look for immediate bubbling, which indicates a positive reaction. 
	3. If no bubbles appear within 20 seconds, the reaction is negative. 
	○ Streptococcus: catalase negative
	○ Staphylococcus: catalase positive  virulence factors (emphasis on evading immune system or secreting exotoxins).
• Streptococcus: evades immune system. 
• Staphylococcus: secrete exotoxins

Question 2

Describe the three basic hemolytic patterns.

Answer 2

• BAG (complete, partial, none)
• Alpha:
○ Is able to partially oxidized the hemoglobin and change the color to a greenish color. Sometimes referred to as partial hemolysis, but that doesn’t mean it actually lyses the red blood cells much.
○ Streptococcus pneumoniae
○ Streptococcus mutans
• Beta:
Beta hemolysis: actually secretes hemolysins as exotoxin. Lyses red blood cells and degrades hemoglobin.
• gamma:
No affect on the red blood cells.

Question 3

What type of hemolytic pattern lyses the red blood cells completely?

Answer 3

• Beta
• Beta hemolysis: actually secretes hemolysins as exotoxin (virulence factor). Lyses red blood cells and degrades hemoglobin.
• Although these hemolysins lyse red blood cells in vitro, it does not appear that red blood cells are the intended recipient in vivo.
• Intended recipients:
○ Immune cells (neutrophils and macrophages)
○ Muscle fibers (and possibly also red blood cells when starved for iron).

Question 4

Describe, in detail, the chemical structure of a hemolysin and its mechanism of action

Answer 4

• Group of pore-forming toxins secreted by B-hemolytic pathogenic Streptococcus and Staphylococcus
• Hemolysins are composed of 7 subunits (and therefore show quaternary structure). Here is one subunit, showing the “hook” (in red) that wedges itself into a membrane. 6 others following until you get a complete pore.

Question 5

What nutrients are bacteria looking for when lysing red blood cells?

Answer 5

• When lysing red blood cells, bacteria are primarily looking for iron, which is abundant within the hemoglobin molecule inside red blood cells; this iron is released when the cell is lysed, making it readily available for the bacteria to utilize as a nutrient

Question 6

In the body, name two additional cellular targets for hemolysins.

Answer 6

• Erythrocytes, leukocytes, monocytes, macrophages, lymphocytes, endothelial, epithelial, and renal epithelial cells.

Question 7

Name three pathogenic bacteria that exhibit the beta hemolysis pattern

Answer 7

• Group B strep
• Streptococcus pyogenes
Staphylococcus aureus

Question 8

Name two species of Staphylococcus. Which species is generally regarded as a
pathogen?

Answer 8

• Staphylococcus aureus (pathogen)
• Staphylococcus epidermidis

Question 9

List TWO diagnostic tests that separate S. aureus from S. epidermidis (a common microflora resident).

Answer 9

• MSA:
○ Aureus: yellow agar
○ Epidermidis: pink agar
• Coagulase:
○ Aureus: positive
○ Epidermidis: negative

Question 10

What is the reservoir for S. aureus?

Answer 10

• Humans; primary location is skin for epidermidis.
• Aureus: heart, skin, blood, bone, brain, lungs, ears, sinuses, muscle.

Question 11

Describe the following virulence factors of S. aureus

Answer 11

• coagulase (tandem): bacteria create a clot around themselves as a protective barrier which isolates them away from neutrophils.
• staphylokinase (tandem): clot buster, dissolves fibrin. The replicated bacteria cells escape in larger numbers.
• hemolysins: lyses RBCs; also lyse muscle cells and immune cells.

Question 12

S. aureus infections are common after surgery in hospitals. (Hence the term “Staph infections”). What is the name for an infection acquired in the hospital?

Answer 12

• Nosocomial infection–did not have infection before admitted into hospital and acquired inside hospital.
Community acquired staph: acquired outside of hospital.

Question 13

Describe Scalded Skin Syndrome and Toxic Shock Syndrome in detail. Can all S. aureus strains cause these diseases?

Answer 13

• Scalded Skin Syndrome:
○ Usually a systemic infection commonly found in newborns. Bacteria enter umbilical stump, since there is still access to blood. Secretes toxin that cause epidermis to peel away from dermis.
• Toxic Shock Syndrome:
○ Superantigens drives immune system crazy!
○ TSS toxin (exotoxin) cause over secretion of histamin, which causes shock and results in very low blood pressure. Blood vessels dilate–systemic vasodilation–very low blood pressure–shock cannot get blood to organs–organ failure then death.

Question 14

What is Group B streptococcus? Why is it clinically significant?

Answer 14

Type of Streptococcus that is found in colon and vaginally canal in about 1/3 of woman.

Question 15

In what organ is Group B strep carried, and why is it screened for in 3rd trimester pregnancies?

Answer 15

• To know if the patient is out of the 1/3 of woman identified as carriers.
• When the baby is coming down the vaginal canal, group B strep can enter the nasal cavity and mouth, entering the mucous membranes and infecting the baby.
Can be deadly for baby causing pneumonia, then sepsis, then meningitis.

Question 16

What is sepsis? What is septic shock?

Answer 16

• Sepsis occurs when chemicals released in the bloodstream to fight an infection trigger inflammation throughout the body. This can cause a cascade of changes that damage multiple organ systems, leading them to fail, sometimes even resulting in death.
• Septic shock is a life-threatening condition caused by a severe localized or system-wide infection that requires immediate medical attention

Question 17

Describe (in detail), the role of S. pyogenes in causing childbed fever in past European maternity wards. How do these events relate to the advent of handwashing?

Answer 17

• any bacterial infections of the female reproductive tract following childbirth or miscarriage. Cause from doctors doing rotations and not cleaning hands after performing autopsy.
• S.pyogenes has mechanisms that help it suppress the immune response and survive.