Lab Practical Practice

blood agar plates

enriched, differential medium

beta hemolytic bacteria

produce enzymes that break down red blood cells, and secrete them into the agar they’re plated on. secrete enzymes that degrade blood cells into the medium. the degraded blood loses its red color and the medium clears. can be recognized by the areas of clearing of the medium surrounding the bacterial colonies. This is often best observed by holding the plate up to the light.

alpha hemolytic bacteria

can break down red blood cells only partially and turn the BAP medium surrounding the colonies green rather than clearing it completely

gamma hemolytic bacteria

doesn’t posses hemolytic enzymes and does not destroy red blood cells. BAP surrounding gamma hemolytic bacteria will remain red

MaConkey agar (MAC)

selective and differential media that contains bile salts and crystal violet that inhibit Gram-positive organisms from growing but allows growth of Gram-negative organisms. contains lactose and neutral red (pH color indicator)

what color do Gram-negative lactose fermenters turn on MAC

pink

what color do lactose nonfermenters turn on MAC

not pink, clear/yellow

what is a SIM sulfur positive result

black precipitate forming near the bottom of the tube

what is a SIM sulfur negative result

no black precipitate forms

what is an indole positive result

red ring forming on top of the agar

what is an indole negative result

red ring doesnt form

what is a motility positive result

fuzziness around the stabline

what is a motility negative result

stabline appears clear