Module 7

Question 1

What are the characteristics of media for Enterobacteriaceae isolation?

Answer 1

Differential properties indicate pathogens- sugars and pH indicators, H2S detection

Selection- bile salts inhibit normal flora

Question 2

What are the characteristics of group 1 selective media and what types are included?

Answer 2

Grow all Enterobacteriaceae but inhibits others

MAC

EMB

DCA

Question 3

What are the ingredients in MAC? What do colonies look like?

Answer 3

Selective ingredients- bile salts and crystal violet inhibit gram pos and most others

Differential ingredients- lactose and neutral red (lactose fermenters vs nonlactose fermenters)

LF- pink red, bile salt ppt, nonpathogens, may be late

NLF- colourless, enteric pathogens

Question 4

What are the characteristics of group 2 selective media and what types are included?

Answer 4

Moderately selective for Salmonella and Shigella

XLD

HE, HEK

SS

Salmonella chrom agar

Question 5

What are the ingredients of XLD and what do colonies look like?

Answer 5

Selective ingredients- sodium deoxycholate (bile salt)

Differential ingredients- xylose, lactose, sucrose and phenol red

NF Enterobacteriaceae ferments all, yellow colonies

Shigella ferments none, red/clear colonies

Salmonella ferments xylose but acid is neutralized by decarboxylation of lysine, red colonies, produces H2S, black centre

Question 6

What are the ingredients of HE, HEK and what do colonies look like?

Answer 6

Selective ingredient- bile salts

Differential ingredients- lactose, sucrose, salicin, bromothymol blue and acid fuchsin

NF Enterobacteriaceae ferment one or more, yellow colonies surrounded by orange-salmon zones, occasional black centre

Salmonella and Shigella don’t ferment, blue-green colonies with or without a black centre

Question 7

What colours of colonies does Salmonella chrom agar give?

Answer 7

Salmonella- mauve

NF Enterobacteriaceae- blue

Question 8

What are the ingredients of SS agar and what do colonies look like?

Answer 8

Selective ingredients- brilliant green (inhibits NF Escherichia and some Shigella), bile salts, sodium citrate

Differential ingredients- lactose and neutral red

NF Enterobacteriaceae ferment lactose, pink colonies with occasional black centre

Salmonella and Shigella don’t ferment, colourless colonies with or without black centre

Question 9

What are the characteristics of group 3 selective media and what types are included?

Answer 9

Highly selective for Salmonella isolation

Bismuth sulfite

Wilson and Blair’s

Brilliant green

Selective ingredients- brilliant green and bismuth sulfite inhibit most organisms

Possible enteric pathogens- black colonies without metallic sheen

Question 10

What are the characteristics of group 4 selective media and what types are included?

Answer 10

Enrichment media

Selenite broth

Tetrathionate broth

Gram neg broth

Enhance Salmonella growth

Question 11

What are the ingredients of selenite broth, how do organisms grow?

Answer 11

Selective ingredient- sodium selenite

NF ferment lactose, produce acid for selenite action

12hrs- Salmonella

> 12hrs- NF starts to grow

Question 12

What are the ingredients of tetrathionate?

Answer 12

Selective ingredients- calcium carbonate, bile salts, sodium thiosulfate

Allows Salmonella and Shigella to outgrow NF

Have to add iodine solution right before use

Question 13

What are the characteristics of gram neg broth?

Answer 13

Selective ingredients- deoxycholate and citrate

Allows most Shigella and more NF to grow

Question 14

What media is used to isolate E. coli 0157:H7?

Answer 14

Sorbitol-MAC

Chrom agar

Question 15

What are the objectives for Enterobacteriaceae isolation media?

Answer 15

Indication of colonies that are possible pathogens

Reduced normal flora growth