PPT – COLONY MORPHOLOGY FOR BACTERIAL IDENTIFICATION

Initial Observation and Interpretation of Cultures

•Colonial morphology is usually observed after 18–24 hours of incubation on different culture media.

•Clinical specimens like sputum and wound swabs are commonly plated on BAP, CHOC, and MAC for comparison.

•BAP and CHOC support both fastidious and nonfastidious organisms, but highly fastidious do not grow on BAP (Haemophilus and N. gonorrhoeae)

•CHOC agar provides the extra nutrients required for those highly fastidious organisms.

•Therefore, growth on CHOC but not on BAP or MAC suggests Haemophilus spp. (bacillus) or N. gonorrhoeae (diplococci).

Clockwise from the top—CHOC, BAP, and MAC—large gram-negative rods (enterics) grow on all plates, appearing larger and mucoid on BAP and CHOC, while smaller grayish-brown colonies are fastidious Haemophilus growing on CHOC (arrow), which are not growing on BAP or MAC.

•While MAC agar supports most gram-negative rods, esp. Enterobacteriaceae, but inhibits gram-positive and some fastidious gram-negatives (e.g., Haemophilus, Neisseria).

•Therefore, growth on BAP and CHOC but not MAC suggests a gram-positive or fastidious gram-negative organism.

•Gram-negative rods often appear similar on BAP and CHOC, producing large gray or mucoid colonies; hemolysis is seen only on BAP

•MAC is ideal for characterizing gram-negative rods, differentiating lactose fermenters (pink) from nonfermenters (colorless).

•Some enteric pathogens (e.g., Escherichia/Citrobacter) produce dry pink colonies with a pink bile salt halo.

•Others (e.g., Klebsiella/Enterobacter) form large, mucoid pink colonies, sometimes with cream-colored centers; these features aid presumptive identification.

•A, Lactose-fermenting, gram-negative rods producing pink colonies on MAC.

•B, Nonlactose-fermenting, gram-negative rods producing colorless colonies on MAC.

A, Lactose-fermenting Escherichia/Citrobacter-like organisms growing on MAC. Notice the dry appearance of the colony and the pink precipitate of bile salts extending beyond the periphery of the colonies. B, Close-up of dry, flat Escherichia/Citrobacter-like lactose fermenters growing on MAC.