4-5: macconkey agar test

purpose of the macconkey agar test

to isolate and differentiate some enteric intestinal pathogens or coliforms from the enterobaceriaceae family

list 2 selective ingredients in this media

bile and crystal violet dye are added to inhibit the growth of gram positive organisms

list 2 differential ingredients in this media.

1. lactose to distinguish which bacteria go through lactose fermentation. a positive lactose fermentation makes this a positive coliform test

2. neutral red dye provides a color to the bacteria. Pink color distinguishes a positive result.

explain what a probable coliform would look like on your plate for the macconkey agar test?

growth for this would indicate a gram - bacteria, as it was able to withstand the detergents (bile and crystal violet dye). along with this, the coliform would produce a pink hue on the media indicating a positive lactose fermentation test through the production of acid. the neutral red dye highlights the acid as pink.

what other two tests could you perform to confirm if your probable coliform is in fact a coliform?

1. BGLB test which inhibit the growth of noncoliform bacteria

2. eosin methylene blue also isolates bacterial growth

the ph indicator in this agar plate is?

neutral pink dye that turns bright pink in the presence of acid

What will a gram positive organism look like on the macconkey agar?

a gram positive would not grow on the agar as the detergents (bile and crystle violet dye) would eliminate them.

waht sugar is added to this agar and why?

lactose is added to the agar since coliforms are known to go through lactose fermentation

what is the gram reaction and morphology of organisms that belong to the enterobacteriaceae family?

they are gram negative bacteria and are bacilli shaped.

what can you conclude about an organism that has good growth and produces it colorless?

the organism would be gram negative as it was able to grow through this selective media, yet it is not a coliform. The organism did not contain a color as it did not go through lactic acid fermentation. Therefore it can not be considered a coliform, as fermentation is necessary for the distinction.

Would it be problematic if the bile salt and crystal violet were omitted from the agar?

the media would lose its selective properties. this means the gram positive organisms would not be filtered out. the neutral red dye indicator would assist with identifying efforts through the pink color hue present. The issue with this is that positive coliform tests require gram negative growth result and a color change. therefore these are needed.

coliform characteristics

• gram negative

• bacilli

• positive motility

• reside in gi track

• lactose fermentation