lab 4-4: mannitol salt agar

purpose?

for you to differentiate between pathogenic from non pathogenic staphylococcus and also differentiate them from non halophiles

what would be the consequence of omitting the NaCl from the agar?

the lab would lose its selective characteristics. the salt is necessary to eliminate non halophiles and leave behind the halophiles we are looking for (staphylococcus)

why would this be a good clinical test? why are we having you swab your nose?

this would be a good clinical test as it screens for the carriers of staphylococcus aureus, which is the pathogenic species of staphylococci. Screening for carriers allow for proper treatment with antibiotics as preventative measures

why does this experiment specifically test for staphylococci?

this experiment is specific to staphylococci as they are halophiles. This means they can thrive in high salt conditions. the staphylococci can also ferment mannitol, allowing the display of good growth

halophile vs extreme halophile

• halophile thrive in salty environments (2-15%)

• extreme halophiles require a high salt environment for growth (15-25%)

which diseases are associated with staphylococcus areus?

• scalded skin syndrome

• cellulitis

• impetigo

• toxic shock syndrome

what is mannitol?

mannitol is a sugar used in this lab to promote fermentation done by S. aureus only. this is added to incorporate a differential component as the fermentation would result in acid, indicated by yellow through the phenol red.

what ph indicator was added?

phenol red

• yellow=acid

• neutraol = red

• pink = basic

can gram negative organisms grow on this plate?

no because the high salt concentration dehydrates gram negative bacteria.

staphylococcus epidermidis qualities

• cause of nosocomial infections

• - mannitol fermentation

• - coagulase enzyme

Staphylococcus aureus qualitites

• pathogen cause of skin related infections, scalded skin, food poisoning, etc

• + mannitol fermentation

• + coagulase enzyme

• + several exotoxins

similarities between s. aureus and s. epidermidis

• part of normal flora

• gram +

• facultative halophile

you obtain your results and observe a yellow halo. What other biochemical test can you perform to confirm that you have staph aureus.

We can utilize the coagulase test to confirm s. aureus. S. aureus produces the enzyme coagulase to protect itself from host defenses. This creates blood clots that surround the pathogen. WE inoculate the culture in rabbit plasma, which contains clotting proteins. if the plasma solidifies, then this confirms the use of coagulase

list all the ingredients that make MSA differential and selective

• selective: salt allows for an environment for halophiles to thrive and non halophiles dont grow

• differential: mannitol is the sugar present that is fermentable by s. aureus only. Phenol red highlights the fermentation of mannitol

describe the appearance of staphylococcus epidermidis on this plate and why it appears this way.

S. epidermidis shows growth on the plate as it is a facultative halophile, but it remains the color of the plate as it is not able to ferment the mannitol. It can show a slight hue from the peptone usage

which media was used as a negative control? why?

the nutrient agar was used as a negative control. NA is a general purpose media as it is not selective nor differential. all organisms can grow, so this sets a baseline for us to ensure all our cultures are able to show results