other microbes: Staphylococcus aureus, Bacillus cereus, Enterococcus faecalis, Clostridium perfreingens, yeast and moulds

Question 1

Staphylococcus aureus

- enterotoxin

Answer 1

• Enterotoxin A-F: heat resistant polypeptide (121*C, 20min)
• requirements for the production: Temp 10-45*C, water activity > 0,92
• infective dose: < 0,1 ug toxin –> 10^5 microbe/gram food

Question 2

Staphylococcus aureus

- culturing media

Answer 2

Giolitti-cantoni broth, Baird-Parker agar,

Question 3

Staphylococcus aureus

- Giolitti-cantoni broth

Answer 3

• composition: mannitol, LiCl, potassium tellurite

- reaction: potassium tellurite is reduced to metallic tellurium by S.aureus, therefore black compounds is developed

Question 4

Staphylococcus aureus

- Baird-parker agar

Answer 4

• composition: mannitol, LiCl, potassium tellurite, egg yolk emulsion
• reaction: colonies of Staph. aureus appear as black and shiny whith a fine white rim, surrounded by a clear zone after 18-24hr incubation

Question 5

Staphylococcus aureus

- confirmation methods

Answer 5

• gram staining, coagulase test, thermostable deoxyribonuclease test

Question 6

Staphylococcus aureus

- coagulase test

Answer 6

• staph aureus is coagulase +
• the coagulation test is positive in case of S. aureus 99%, while for S. hyicus and S. delphi it is sometimes positive.
• in case of S. epidermidis the test is negative (no coagluation)

Question 7

Staphylococcus aureus

- Thermostable deoxyribonuclease test

Answer 7

evaluation:
- S. aureus has thermostable DNase enzyme in 100%. it is susceptible to traces of the enzyme (0.005 ug/ml) so the tst can be done from the heat treated food itself, result can be obtained in 4-5hr.
- Positive reaction: pink ring (S.aureus is positve)

Question 8

Bacillus cereus

- characteristics

Answer 8

• glucose fermentation without gas production
• catalase, lecitinase, phospholipase positive
• nitrate –> nitrite reduction
• b-hemolysis

Question 9

Bacillus cereus

- MYP agar

Answer 9

MYP agar (mannitol, yolk, polymyxin)

• composition: meat extract, peptone, mannitol, phenol red, egg yolk, polymyxin-B
• evaluation: B.cereus form large, pink color colonies around percipitation zone –> 1day, 37*C

Question 10

Bacillus cereus

- confirmation

Answer 10

double zone b-hemolysis is een on sheep blood agar –> 1day, 30*C

Question 11

Enterococcus faecalis

- characteristics

Answer 11

• indicator of fecal contamination and insufficient heat treatment
• most resistant to disinfectants (except for spores=
• glucose fermentation without gas production
• aerotolerant anaerobic

Question 12

Enterococcus faecalis

- sherman criteria

Answer 12

• can survive: 60*C for 30min (can survive low pasturization)
• can grow in 6,5% salt concentration, pH 9.6, 10-45*C, milk with 0,1% methylene blue, blood agar with 40% bile

Question 13

Enterococcus faecalis

- media to use

Answer 13

• Slanetz-Bartley agar, Na-azide broth/Raj broth, Litsky-Mallmann broth

Question 14

Enterococcus faecalis

- Slanetz-Bartley agar

Answer 14

• composition: triphenyl tetrazolium chloride, sodium azide

- evaluation: enterococcus reduces the TTC to red formazan, so the colonies are reddish brown

Question 15

Enterococcus faecalis

- Na-azide broth, Raj broth

Answer 15

it contains Na-azide for the selectivity and bromothymol blue as indicator

Question 16

Enterococcus faecalis

- Litsky-Mallman broth

Answer 16

it contains Na-azide for the selectivity and ethylviolet as indicator

Question 17

Enterococcus faecalis

- confirmation

Answer 17

• gram staining, catalase test, heat resistance examination

Question 18

Enterococcus faecalis

- catalase test

Answer 18

• it is catalase negative

- inoculation into meat broth

Question 19

Enterococcus faecalis

- heat resistance examination

Answer 19

• inoculation into meat broth

- evaluation: if the broth is turbid and the microbe is catalase negative, the presence of enterococcus is confirmed

Question 20

Clostridium perfringens

- culturing

Answer 20

• sulphite-cycloserine agar (SC),

Question 21

Clostridium perfringens

- confirmation

Answer 21

fluid thioglycolate medium, Lactose-sulphite broth

Question 22

Clostridium perfringens

- Sulphite-cycloserine agar (SC)

Answer 22

• inhibitor: cycloserine
• reaction: black colonies are formed by FeS production (sulphite reduction)
• cycloserine inhibits other microbes

Question 23

Clostridium perfringens

- fluid thioglycolate medium

Answer 23

• composition: L-cystin, sodium thioglycolate, resazurine
• reaction: sodium thioglycolate decreases the redox potential (ensures anaerobic conditions), resazurine controls the anaerobic conditions. the agar inhibits the oxygenation of the broth

Question 24

Clostridium perfringens

- lactose-sulphite broth

Answer 24

• composition: lactose, disodium disulphote, ferric ammonium citrate
• reaction: black iron sulfide precipitate at the bottom of the tube

Question 25

Yeast and moulds

- media

Answer 25

Lowered pH media:
- malt extract agar, beer agar, potato-glucose agar

Antibiotic content group:

• yeast extract-dextrose-chloramphenicol-agar
• oxytetracyclin-glucose-yeast extract (OGY)- agar
• dichloran-rose bengal-chloramphenicol (DRBC) agar