bio318L exam 2: lab#8

What is *Staphylococcus*?

* gram positive cocci (in clumps like grapes)
* all members are highly resistant to **drying, extremes in pH,** and **high temperatures**
* increase in **community acquired** cases, with infections associated with schools, prisons, and shared athletic equipment

*S. epidermis*

common cause of hospital-acquired infections in immunocompromised patients

*S. saprophyticus*

an occasional cause of UTIs

*S. aureus*

common cause of hospital-acquired skin infections

* normal flora of humans: nasal membranes & skin
* opportunistic infections

Latex Agglutination Test

rapid identification for *S. aureus*

* **Protein A** is a virulence factor found on the cell wall of only *S. aureus*
* antibodies in the test solution bind antigens on Protein A, resulting in a positive result of **clumping**

Latex Agglutination Test Results

positive = *S. aureus*

* clumping occurs because the antibodies in the test solution bind with the antigens found on Protein A which is located on the cell wall of only *S. aureus*

negative = not *S. aureus*

* no clumping occurs due to the absence of Protein A

Catalase Test

key test in differentiating Gram (+) cocci

* substrate is **hydrogen peroxide**
* distinguishes Staph from Strep

Catalase Test Results

positive = Staph

* bubbling will occur due to the presence of catalase
* the catalase reacts with the hydrogen peroxide, resulting in oxygen to be released as seen by the bubbling

negative = Strep

* no bubbling occurs due to the absence of catalase and thus no release of oxygen

Mannitol Salt Agar

Selective & differential media for *Staphylococcus*

* contains **7.5% salt** to inhibit the growth of most bacteria
* contains **phenol-red dye** to detect **mannitol fermentation**
* highly selective for *S. aureus* and some strains of *S. saprophyticus*

Mannitol Salt Agar Test Results

positive = *S. aureus* or *S. saprophyticus*

* a color change from red to **yellow** indicates the fermentation of mannitol, which *S. aureus* and some strains of *S. saprophyticus* are able to do

negative = *S. epidermis*

* no color change indicates the inability to ferment mannitol

Blood Agar Plates

Tests for hemolysis ability

* contains Tripticase Soy Agar (TSA) and **5% sheeps blood**
* beta, alpha, and gamma

Blood Agar Plates Results

**beta** = *S. aureus*

* indicates **complete** breakdown of red blood cells
* if the plate is held up to a light, you **will** be able to see through it

**partial** = none

* indicates **partial** breakdown of red blood cells
* seen by a green tint around the growth

**gamma** = *S. saprophyticus* & *S. epidermis*

* indicates **no breakdown** of red blood cells
* if the plate is held up to a light, you will **not** be able to see through it

Novobiocin Susceptibility

Distinguishes coagulase-negative Staph species

* does not apply to *S. aureus*

Novobiocin Susceptibility Test Results

positive for coag-negative = *S. epidermis*

* seen by a zone of inhibition

negative for coag-negative = *S. saprophyticus*

* seen by no zone of inhibition

What is *Streptococcus*?

* gram positive cocci (in a chain-like pattern)
* highly environmentally sensitive
* normal human **commensal**
* mouth, skin, intestine, and upper respiratory tract

*Strep. mutans*

most common cause of dental plaque

*Strep. pneumoniae*

most common cause of bacterial pneumonia

*Strep. pyogenes*

common cause of throat infections; “Strep throat”

* responsible for **pharyngitis** and systemic infection like scarlet fever
* able to completely lyse RBC

4 *Streptococcus* strains identified in lab

* *S. pyogenes*
* *S. agalactiae*
* *S. equi*
* *E. faecalis*

CAMP Test

Identification for **Group B** or ***S. agalactiae***

Bacitracin Susceptibility Test

Identification for **Group A** or ***S. pyogenes***

Bacitracin Susceptibility Test Results

positive = *S. pyogenes* or Group A

* seen by a zone of inhibition around the disk

negative = not *S. pyogenes* or Group A

* seen by no zone of inhibition around the disk

SXT Sensitivity Test

Identification for **Group C** or **S. equi**

* enzyme inhibitor (blocks folic acid synthesis)

SXT Sensitivity Test Result

positive = *S. equi* or Group C

* seen by a zone of inhibition around the disk

negative = not *S. equi* or Group C

* seen by no zone of inhibition around the disk

Optochin Sensitivity Test

Identification for *S. pneumoniae*

* was not performed in lab
* a zone of inhibition indicates a positive result for *S. pneumoniae*

Bile Esculin Media

Identification for **Group D** or ***E. faecalis***

* bile salts inhibit non-Group D species
* esculin hydrolysis forms black precipitate

Bile Esculin Media Results

positive = *E. faecalis* or Group D

* seen by the formation of black precipitate
* bacteria is able to hydrolyze esculin

negative = not *E. faecalis* or Group D

* seen by no change in the test tube
* bacteria is unable to hydrolyze esculin