microbio practical 2

“selection”

there are dominant traits, and they can help one species outcompete the other

what does it mean when a trait is selected for?

that means the trait is advantageous

“differential”

based on the way something presents itself/ how it grows, you can tell the difference

EMB

eosin-methylene blue

what 2 dyes make up EMB?

eosin and methylene blue

Is EMB selective? If so, how?

yes; selective towards gram negative bacteria because gram positive bacteria usually cant grow in the presence of dyes

Is EMB differential? If so, how?

yes; differentiates the ability to ferment lactose

If a bacteria can ferment lactose, what does that mean about the pH in the media?

lower pH

what does a purple blackish color mean in an EMB plate?

lactose fermenter, dropped pH

what bacteria shows up with a green sheen in EMB and why?

E.coli; grows so well/quickly on lactose, drops pH, drops so fast that MB precipitates, demonstrates a green sheen

what does it mean if a bacteria grew on EMB but didn’t change color?

Since it grew, that must mean the bacteria is gram negative, but if it didn’t change color that means it wasn’t able to ferment lactose

Is MacConkeys (MAC) media selective?

yes; selective from gram negative bacteria because bile salts dont cross the outer membrane well but do a number on the cytoplasmic membrane

Is MacConkeys (MAC) media differential?

yes; neutral red is the differential reagent that turns red when a bacteria can ferment lactose and drops the pH

at what pH’s is neutral red in MAC the color red?

6.8 and below, it is colorless above

what does e.coli do in MAC?

turns hot pink, also turns the media around it hot pink too

is Mannitol Salt Agar (MSA) selective?

yes; selective based on salt tolerance, as it can affect its ability to grow

which genus of bacteria is known to be salt tolerant?

staphylococcus

what is the differential reagent in MSA and at what pH does it change?

phenol red; turns yellow below 6.8, indicates the ability to utilize mannitol

is Phenyl ethyl alcohol (PEA) selective?

yes; it dissolves outer membranes, but since gram positive bacteria don’t have one, they aren’t affected

is PEA differential?

no

what is the inoculum effect?

if bacteria is growing on media, there is a possibility that they’re not growing on PEA but on the cells that died (inoculum). There will be less growth further down the line

What does the oxidase test used to determine?

Whether the bacterial species in question contains the respiratory enzyme cytochrome oxidase.

what colors can the dye be in an oxidase test?

In its reduced form, the dye is clear. If the enzyme is present, it will turn purple.

What is a positive result for the oxidase test?

Turned purple within 60 seconds

What is the catalase test used to determine?

The absence or presence of the catalase enzyme

What is happening during the catalase test?

If the catalase enzyme is present, it will speed up the reaction of turning hydrogen peroxide into water and oxygen, detoxifying it before it has the chance to damage the cell.

What does a positive result look like for the catalase test?

bubbles form within 30-40 seconds

What is the urease test used to determine?

whether the bacterial species in question has the ability to hydrolyze the nitrogen compound urea (enzyme)

What is produced when urea is broken down?

ammonia

what does a positive result look like in the urease test?

pink solution