Microbiology Culture Plates for UTI

In a laboratory setting, the urine is commonly cultured on 3 different types of plates:

Blood agar plate (BAP)

BAP is an agar plate enriched, differential media that contains mammalian blood, used to differentiate fastidious organisms and detect hemolytic activity. The three types of hemolytic activity include:

Alpha (α):

Partial lysis of RBCs (cell membrane remains) resulting in a green or brown discoloration around colony from the conversion of hemoglobin to methemoglobin

Beta (β):

Lysis and complete digestion of RBCs surrounding the colony (I.E. Streptococcus hemolyticum)

Gamma (γ):

AKA non-hemolytic is the term referred to as the lack of hemolytic activity.

MacConkey agar (MAC)

MAC is a selective media plate that selects for Gram-negative bacteria. In addition to being Gram–selective, they are also indicator media as the colonies will turn “red” if the bacteria are able to ferment lactose. The most common UTI pathogens that are Gram-negative lactose fermenters are E. coli, Enterobacter, and Klebsiella.

E. coli colonies appear and smooth, round, and bright pink colonies on MAC because they are lactose fermentors (image below)

HurBi plate

The HurBi plate is divided into two sides. These plates are both selective and differential.

One side has selective media to detect Gram+ bacteria and the other side for Gram – bacteria. In addition to the two different types of media, the media is enriched by chromographic substrates that are cleaved by enzymes specific to each bacteria resulting in the generation of unique identifying colored colonies.

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