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|Appearance of Colonies |Microorganisms | |Red, surrounded by reddish precipitation zones, diameter1- 2 mm |Lactose-positive Enterobacteriaceae: coliform bacteria, E. coli | |Pink, pin-point colonies |Enterococci, possibly Klebsiella | |Colorless |Lactose-negative Enterobacteriaceae |
E. coli: dark-blue to violet colonies (Salmon-GAL and X-glucuronide reaction).
Total coliforms: salmon to red colonies (Salmon-GAL reaction) and dark-blue to violet colonies (E. coli).
Other Gram-negatives: colorless colonies, except for some organisms which possess ÃŸ-D-glucuronidase activity. These colonies appear light-blue to turquoise.
In order to confirm E. coli, coat the dark-blue to violet colonies with a drop of KOVACS’ indole reagent. If the reagent turns to a cherry-red color after some seconds, a positive indole formation confirms the presence of E. coli.
E. coli in Macconkey Agar
|Appearance of Colonies |Microorganisms | |Colorless, translucent |Salmonella, Shigella, others | |Large,
red, surrounded by turbid zones |Escherichia coli | |Large, pink, mucoid |Enterobecter, Klebsiella | |Very small, opaque, isolated colonies |Enterococci, Staphylococci, and others |
S. aureus in baird-parker agar
|Appearance of colonies |Microorganisms | |Black, shiny, convex colonies 1-5 m in diameter with a narrow, white edge surrounded by a clear zone 2-5 mm |Staphylococcus aureus | |wide. Opaque rings within the clear zones only appear after 48 hours of incubation | | |Black, shiny, irregular shape. Opaque zone develop around the colonies after 24 hours. |Staphylococcus epidermidis | |Growth sometimes: | | |Very small, brown to black, no clear zones. |Micrococci | |Dark brown, dull, clear zones sometimes appear after 48 hours. |Bacillus species | |White, no clear zones. |Yeasts |
P. aeruginosa in cetrimide agar
Salmonella in Macconkey Agar