LAB 8 - Enterobacteriaceae

Question 1

aerogenic vs anaerogenic

Answer 1

organisms that ferment glucose producing acid and gas vs acid only

Question 2

these organisms cause GI distress due to toxins

Answer 2

B. cereus
S. aureus
C. perfringens

Question 3

Wilson Blair agar

Answer 3

inhibits GPOS

Question 4

what is the general rule for inoculation of media and biochemical tests?

Answer 4

least selective (most enriched) to most selective
liquid to semi solid to solid

Question 5

E. coli on SMAC

Answer 5

white

Question 6

Yersinia on HEK

Answer 6

tgn/yel

Question 7

E. coli O157:H7 on STEC chromagar

Answer 7

mauve

Question 8

Yersinia on CIN

Answer 8

bulls eye

Question 9

spot indole

Answer 9

p-dimethylamino-cinnanaldehyde
highly sensitive but will not work on pigmented organisms

Question 10

Kovac’s indole vs spot indole

Answer 10

Kovac’s = paradimethylaminobenzaldehyde; cherry red pr pink
spot indole = blue/green

tests for tryptophanase

Question 11

what is the purpose of aerosporin media?

Answer 11

has polymyxin B (antibiotic for GNBs)
inhibits GNB; selective for GPS

Question 12

phenylethyl alcohol agar (PEA)

Answer 12

• The active ingredient, phenylethyl alcohol, inhibits or markedly reduces the growth of Gram-negative organisms by interfering with DNA synthesis
• supports growth of GPCs and anaerobes

Question 13

colistin nalidixic acid

Answer 13

Colistin, a polypeptide antibiotic of the polymyxin group, and nalidixic acid, a first-generation quinolone, are primarily active against gram-negative bacteria thereby making Columbia CNA Agar a good medium for the selective isolation of gram-positive cocci.

Question 14

decarboxylase test

Answer 14

• after the fermentation of glucose in the medium, acids are produced which lower the pH = purple to yellow
• if the organism produces decarboxylase, decarboxylation, or hydrolysis of the amino acid occurs in response to the acid pH = alkaline end products (amies) = purple
• Decarboxylation is an anaerobic reaction

Question 15

pH indicators of decarboxylase test

Answer 15

bromcresol purple and cresol red

Question 16

indicator for TSI

Answer 16

phenol red

Question 17

when is susceptibility testing performed on enteric pathogens?

Answer 17

self-limiting so usually don’t
- unless immunocompromised, hospitalized pts, prolonged infections
- Shigella and Vibrio = automatic testing due to severity

Question 18

serological methods for ID of Shigella, Salmonella, and E. coli O157:H7

Answer 18

• O (somatic antigen) - heat-stabile antigen located in cell wall, resistant to alcohol
• H (flagellar) - heat-labile; resists formalin
• K (capsular) - heat-labile polysaccharide; K1 of E. coli, Vi of S. Typhi; may mask O antigen

Question 19

Serologic grouping of Shigella sp. is based on the ____ antigen

Answer 19

O antigen
- if agg fails = suspension is heated to remove capsular antigen and is retested with O antisera

Question 20

Shigella serotypes

Answer 20

A = S. dysenteriae
B = S. flexneri
C = S. boydii
D = S. sonnei

Question 21

primary intestinal pathogens

Answer 21

• Salmonella, Shigella, Yersinia enterocolitica, E. coli O157:H7 = true pathogens
• NOT commensal flora of GIq

Question 22

oxidase test principle

Answer 22

• tetramethyl-p-phenylenediamine (TMPD) interacts with the ETC at level of cytochrome c
• produces dark purple indophenol
• found only in bacteria whose electron transport systems contain cytochrome c oxidase

Question 23

what can inhibit oxidase activity?

Answer 23

carb fermentation or components of selective media

Question 24

why must TSI reactions be read within the 18-24 hr inc period?

Answer 24

acid reactions may revert to alkaline due to oxidative decarboxylation; glucose fermenters may falsely appear to be non-fermenters

Question 25

detection of hydrogen sulfide in TSI

Answer 25

due to sodium thiosulfate and ferrous ammonium sulfate

Question 26

TSI ingredients

Answer 26

pH-sensitive dye (phenol red), 1% lactose, 1% sucrose, 0.1% glucose, as well as sodium thiosulfate and ferrous sulfate or ferrous ammonium sulfate

Question 27

citrate utilization

Answer 27

- certain organisms can use citrate as their sole source of carbon - pos = growth -> blue due to alkaline end products produced (bromothymol blue) - useful for differentiation of lactose fermenters - Kleb and Enterobacter utilize citrate but E. coli does not

Question 28

Malonate utilization

Answer 28

- utilize malonate as a source of carbon? - end products = acetate and CO2 - alkaline pH so bromthymol blue changes from green to blue (alkaline) in a positive test

Question 29

Phenylalanine deaminase test

Answer 29

- test for ability to deaminate phenylalanine to phenyl pyruvic acid by adding ferric chloride as an indicator - Proteus-Providence (+); rest of colifs (-) - green is pos

Question 30

urease

Answer 30

- splits urea into CO2 and ammonia - phenol red turns deep cerise due to alkalinity

Question 31

Orthonitrophenyl-beta-galactopyranoside (ONPG) test

Answer 31

- orthonitrophenyl-beta-galactopyranoside - detects beta-galactosidase (cleaves lactose into glucose & galactose) - also depends on PERMEASE ( transports lactose into cells) - yellow is pos

Question 32

Lactose fermenter (ONPG pos)

Answer 32

both permease enzyme and beta galactosidase E. coli, Kleb spp., Enterobacter spp

Question 33

Late lactose fermenter (ONPG pos)

Answer 33

only beta galactosidase Citrobacter spp, Arizona sp

Question 34

Y. enterocolitica motility

Answer 34

+ in RT - in vivo

Question 35

MAC ingredients

Answer 35

Peptone Lactose monohydrate Bile salts (and CV inhibit GPC) Sodium chloride Neutral red Crystal Violet

Question 36

SMAC

Answer 36

E. coli is pink on MAC and pk/ng on SMAC whereas E. coli O157:H7 is pk on MAc and wt on SMAC