National Standard of the People’s Republic of China
GB 4789.10-2010
Replace GB/T 4789.10-2008
National food safety standard
Food microbiological examination: Staphylococcus aureus
Issued on 26-03-2010
Implemented on 01-06-2010
I s s u e d b y Ministry of Health
of the People’s Republic of China
GB 4789.10
2010
Preface
This standard replaces GB/T 4789.10-2008 Microbiological examination of food hygiene- Detection of
Staphylococcus aureus and GB/T4789.37-2008 Microbiological examination of food hygiene- Enumeration of
Staphylococcus aureus.
The main revised content of this standard to GB/T 4789.10-2008 and GB/T 4789.37-2008 are as
follows:
- The Chinese and English name of the standard has been changed.
- The scope has been amended.
- Procedure of sample preparation has been standardized.
- Explanation of Coefficient 1.1 in the Calculation Fomula has been added.
- The name of Trypticase soy broth has been standardized and changed to 10% sodium chloride
Trypticase soy broth.
- Method 2 - Baird-Parker enumeration of Staphylococcus aureus and Method 3 –MPN
enumeration of Staphylococcus aureus have been added.
Appendix A, Appendix B and Appendix C of this standad are normative appendixes.
This standard replaces all previous standard as follows:
-
GB 4789.10-1984, GB 4789.10-1994, GB/T 4789.10-2003, GB/T 4789.10-2008.
-
GB/T 4789.37-2008.
GB 4789.10
2010
National food safety standard
Food microbiological examination: Staphylococcus aureus
1. Scope
This standard stipulates the examination method for staphylococcus aureus in foods.
Method 1 in this standard applies to qualitative examination of staphylococcus aureus in foods.
Method 2 applies to enumeration of staphylococcus aureus in foods relatively containing more
Staphylococcus aureus. Method 3 applies to enumeration of staphylococcus aureus in foods
relatively containing fewer Staphylococcus aureus but more other microorganism.
2. Apparatus and Materials
In addition to the conventional apparatus for sterilization and culture in microbiological laboratory,
other apparatus and materials are as follows:
2.1 Constant temperature incubator: 36 1
2.2 Refrigerator: 2~5
2.3 Constant temperature water bath: 37~65
2.4 Balance weighing to 0.1g
2.5 Homogenizer
2.6 Oscillator
2.7 Sterile pipette: 1ml with graduation of 0.01ml, 10 ml with graduation of 0.1ml, or micropipette and
pipette tips
2.8 Sterile conical flask: 100 ml and 500ml
2.9 Sterile petri dish: diameter 90mm
2.10 Injector: 0.5mL
2.11 pH meter or pH colorimetric tubes or precise pH test paper
3. Culture medium and Reagent
3.1 10% sodium chloride trypticase soy broth: See A.1 in Appendix A.
3.2 7.5% sodium chloride broth: See A.2 in Appendix A.
3.3 Plasma agar plate: See A.3 in Appendix A.
3.4 Baird-Parker agar plate: See A.4 in Appendix A.
3.5 Brain heart infusion broth (BHI): See A.5 in Appendix A.
3.6 Rabbit plasma: See A.6 in Appendix A.
GB/T 4789.10-2010
3.7 Diluent: Phosphate buffer solution: See A.7 in Appendix A.
3.8 Nutrient agar small slant: See Chapter A.8 in Appendix A.
3.9 Gram stain solution: See Chapter A.9 in Appendix A.
3.10 Sterile normal saline: See Chapter A.10 in Appendix A.
Method 1 Qualitative examination of staphylococcus aureus
4. Examination Procedures
Refer to Figure 1 for the examination procedures of staphylococcus aureus.
Testing sample
25g(mL)+225mL 7.5% sodium chloride broth or 10% sodium chloride trypticase soy broth , homogenization
36 1
18~24h
Baird-Parker agar plate, Plasma agar plate
36 1
Smear staining
Plasma plate for 18~24h
Baird-Parker plate for 18~24h or 45~48h
Observation of hemolysis
BHI broth or Nutrient agar small slant
36 1
18~24h
Plasma coagulase test
Report
Figure 1 Examination procedures of staphylococcus aureus
5. Operating Procedures
5.1 Treatment of sample
Take 25g sample into an sterile homogenization cup containing 225mL 7.5% sodium chloride broth or
GB 4789.10
2010
10% sodium chloride trypticase soy broth, homogenize at 8000r/min~10000 r/min for 1~2min, or take
sample into a sterile homogenization bag containing 225mL 7.5% sodium chloride broth or 10%
sodium chloride trypticase soy broth, and flap with a rattling-type homogenizer for 1~2min. For liquid
sample, take 25mL sample into a sterile conical flask containing 225mL 7.5% sodium chloride broth
or 10% sodium chloride trypticase soy broth (proper amount of sterile glass beads may be put into the
flask in advance), shake and mix it homogeneously.
5.2 Enrichment and Isolation incubation
5.2.1 Incubate the above homogeneous sample solution at 36 1
for 18~24h. Staphylococcus
aureus becomes turbid growth in 7.5% sodium chloride broth, it also becomes turbid growth in10%
LST if sample is serious polluted.
5.2.2 Respectively streak inoculate above cultures on Baird-Parker agar plate and blood plate,
incubate at 36 1
for 18~24h or 45~ 48h.
5.2.3 When staphylococcus aureus are on the Baird-Parker agar plate, diameters of colonies are
2mm~3mm and the color is gray to black with a border of light color. The colonies are surrounded by
a turbid zone, which have transparent rings on their outer layer. When contacting the colony with
inoculating needle, the hardness is similar to butter to gum. Non-fat soluble colonies may occur
occasionally, but they have no turbid zones or transparent rings. Comparing with the typical colony,
the colony isolated from long-kept frozen or dry foods will generate lighter black, and maybe rougher
and drier appearance. Colonies forming on the blood plate are large, round, smooth, salient, humid,
in golden yellow (sometimes in white), complete transparent hemolytic circles surround the colonies.
Take above mentioned colonies for microscopic examination of Gram Stain and plasma coagulase
test.
5.3 Identification
5.3.1 Staining and microscopic examination: Staphylococcus aureus are gram-positive coccus,
arranged in grape shape. They have no spores and capsules with diameter of 0.5 ~1 m.
5.3.2 Plasma coagulase test: Pick one or more suspicious colonies from Baird-Parker agar plate or
blood plate, inoculate into 5mL BHI and nutrient agar small slant respectively, incubate at 36 1 for
18~24h.
Take 0.5mL fresh-prepared rabbit plasma into a small test tube, add 0.2~0.3mL BHI culture,
oscillate and shake the tube, put it into incubator or water bath at 36 1 , observe every half -hour
within 6h. If coagulation occurs (clots when the tube is tilted or inverted) or coagulation volume is
larger than 50% original volume, it is judged as positive. Meanwhile, broth culture for plasma
coagulase test, containing both positive and negative staphylococcus strains, is used as control.
Commercial reagent can also be used for plasma coagulase test according to manual.
If result is suspicious, select colonies from nutrient agar small slant into 5mL BHI, incubate at 36
1 for 18~24h and repeat.
5.4 Detection for staphylococcal enterotoxin
To identify if suspected food poisoning sample or staphylococcus aureus strains may generate
staphylococcal enterotoxin, should refer to Appendix B to examine staphylococcal enterotoxin.
6. Results and Report
6.1 Result determination: STAPHYLOCOCCUS aureus can be determined if meet 5.2.3, 5.3.
6.2 Result report:: Staphylococcus aureus is detected or not detected In 25g (or 25mL) sample.
GB/T 4789.10-2010
Method 2 Baird-Parker enumeration of staphylococcus aureus
7. Examination procedures
Refer to Figure 2 for the enumeration procedures of Staphylococcus aureus.
Testing sample
25g(mL) sample +225mL diluent, homogenization
Decimal serial dilutions
Choose 2~3 consecutive homogenous sample solutions with appropriate dilution;
inoculate on Baird-Parker agar plate
36 1
45~48h
Enumeration and plasma coagulase test
Report
Figure 2 Baird-Parker enumeration procedures of staphylococcus aureus
8. Operating Procedures
8.1 Dilution of sample
8.1.1 Solid and semi-solid sample: Take 25g sample into an sterile homogenization cup containing
225mL phosphate buffer or normal saline, homogenize at 8000r/min~10000 r/min for 1~2min, or take
sample into a sterile homogenization bag containing 225mL diluent, and flap with a rattling-type
homogenizer for 1~2min, to get homogeneous sample solution of 1: 10.
8.1.2 Liquid sample: Use sterile pipette to take 25mL sample into a sterile conical flask containing
225mL phosphate buffer or normal saline (proper amount of sterile glass beads should be put into the
flask in advance), mix well to get homogeneous sample solution of 1: 10.
8.1.3 Use 1mL sterile pipette or micropipette to take 1mL homogeneous sample solution of 1:10, feed
it into a sterile tube containing 9mL diluent along the tube wall ( note: pipette or pipette tip do not
touch the diluent), shake or repeatedly insufflate and flap with 1mL sterile pipette, mix well to get
GB 4789.10
2010
homogeneous sample solution of 1:100.
8.1.4 Make decimal serial dilutions of homogeneous sample solution according 8.1.3, change a new
1mL sterile pipette or pipette tip once per diluted.
8.2 incubation of sample
Choose 2~3 consecutive homogenous sample solutions with appropriate dilution (the initial liquid
sample may be chosen for liquid product) according the evaluation of pollution to sample. When
making decimal serial dilutions, take 1mL homogenous sample solution and respectively inoculate
0.3mL, 0.3ml, 0.4ml of the solution on 3 Baird-Parker agar plates, then smear all over the plate with
L-shaped rod noticing do not touch the edge. If there are beats of water on Baird-Parker agar plates,
dry them in incubator at 25~50
till the beads of water disappear before use.
8.3 Incubation
8.3.1.1 Generally keep the plates still for 10min after smear. If the sample solution can not be
absorbed easily, inoculate the plates in incubator at 36 1
for 1h, invert the plates after the solution
is absorbed. Incubate at 36 1
for 45~48h.
8.4 Enumeration of typical colonies and confirmation
8.4.1 When staphylococcus aureus are on the Baird-Parker agar plate, diameters of colonies are
2mm~3mm and the color is gray to black with a border of light color. The colonies are surrounded by
a turbid zone, which have transparent rings on their outer layer. When contacting the colony with
inoculating needle, the hardness is similar to butter to gum. Non-fat soluble colonies may occur
occasionally, but they have no turbid zones or transparent rings. Comparing with the typical colony,
the colony isolated from long-kept frozen or dry foods will generate lighter black, and maybe rougher
and drier appearance.
8.4.2 Choose plates with typical staphylococcus aureus colonies and with total colonies count of 3
plates for same dilution between 20~200CFU.
a) If only one dilution plates’ count is between 20~200CFU and typical colonies can be found on them,
then enumerate typical colonies on these dilution plates.
b) If the lowest dilution plates’ count is less than 20CFU and typical colonies can be found on them,
then enumerate typical colonies on these dilution plates.
c) If a certain dilution plates’ count is greater than 200CFU and typical colonies can be found on them,
but typical colonies can not be found on next dilution plates, then enumerate typical colonies on these
dilution plates.
d) If a certain dilution plates’ count is greater than 200CFU and typical colonies can be found on them,
and typical colonies can be found on next dilution plates but the count is not between 20~200CFU,
then enumerate typical colonies on these dilution plates.
Calculate as formula (1) for all above.
e) If two consecutive dilution plates’ count is between 20~200CFU, then calculate as formula (2).
8.4.3 Optionally choose 5 colonies from typical ones (choose all if less than 5), do plasma coagulase
test respectively.
9. Calculation
Formula (1):
GB/T 4789.10-2010
In the formula:
T
count of staphylococcus aureus colonies in sample
A
total count of typical colonies for a certain dilution
C
count of colonies positive in plasma coagulase test for a certain dilution
D
count of colonies used for plasma coagulase test for a certain dilution
d
dilution factor
Formula (2):
In the formula:
T
count of staphylococcus aureus colonies in sample
A1
total count of typical colonies for the first dilution (low dilution multiple)
A2
total count of typical colonies for the second dilution (high dilution multiple)
B1
count of colonies positive in plasma coagulase test for the first dilution (low dilution multiple)
B2
count of colonies positive in plasma coagulase test for the second dilution (high dilution
multiple)
C1
count of colonies used for plasma coagulase test for the first dilution (low dilution multiple)
C2
count of colonies used for plasma coagulase test for the second dilution (high dilution
multiple)
1.1
calculation coefficient
d
dilution factor (the first dilution)
10. Results and Report
According to the count of typical staphylococcus aureus colonies on Baird-Parker agar plate and
result calculated as formulas in 9, report count of staphylococcus aureus per g (mL) sample,
expressed as CFU/g(mL). If T=0, report as “<1 X the lowest dilution multiple”.
Method 3 MPN Enumeration of staphylococcus aureus
11. Examination procedures
Refer to Figure 2 for MPN enumeration procedures of staphylococcus aureus.
Testing sample
25g(mL) sample +225mL diluent, homogenization
Decimal serial dilutions
Choose 3 homogenous sample solutions with appropriate dilutions; respectively
take 1mL to 3 tubes containing 10% sodium chloride trypticase soy broth
GB 4789.10
36 1
2010
45~48h
inoculate on Baird-Parker agar plate
36 1
45~48h
Plasma coagulase test
Check MPN table
Report the result
Figure 3 MPN enumeration procedures of Staphylococcus aureus
12. Operating Procedures
12.1 Dilution of sample
According to 8.1
12.2 Inoculation and incubation
12.2.1 Choose 3 homogenous sample solutions with appropriate dilution (the initial liquid sample may
be chosen for liquid product) according the evaluation of pollution to sample. When making decimal
serial dilutions, respectively take 1mL homogenous sample solution into 3 tubes containing 10%
sodium chloride trypticase soy broth for each dilution and incubate at 36 1
for 45~48h.
12.2.2 Use incubating loop to pick one loop from the tubes containing colonies, inoculate on
Baird-Parker agar plate and incubate at 36 1
for 45~48h.
12.3 Confirmation of typical colonies
12.3.1 See 8.4.1
12.3.2 Pick at least one colony from typical colonies to BHI broth and nutrient agar slant, incubate at
36 1
for 18~24h. Do plasma coagulase test according to 5.3.2.
13. Results and Report
Calculate the amount of the corresponding tubes that the plasma coagulase test of the colony is
positive. Check MPN retrieval table (see appendix C), report the most probable number of
staphylococcus aureus per g (mL) sample, expressed as MPN/g (mL).
GB/T 4789.10-2010
Appendix A
(Normative Appendix)
Culture Medium and Reagent
A.1
10% sodium chloride Trypticase soy broth
A.1.1 Composition
Trypticase (or tryptone)
17.0g
Phytone (or soya peptone)
3.0g
NaCl
100.0g
K2HPO3
2.5g
Sodium pyruvate
10.0g
Glucose
2.5g
Distilled water
1000mL
pH 7.3 0.2
A.1.2 preparation method
Mix and heat above compositions, slowly agitate and dissolve, adjust pH, separately put 225mL
into each bottle and conduct autoclave sterilization at 121
for 15min.
A.2 7.5% NaCl broth
A.2.1 Composition
Peptone
Beef extract
NaCl
Distilled water
pH 7.4
10.0g
5.0g
75g
1000mL
A.2.2 Preparation method
Heat and dissolve the above compositions, adjust pH, separately put 225mL into each bottle and
conduct autoclave sterilization at 121 for 15min.
A.3 Blood agar plate
A.3.1 Composition
Agar of soybean powder (pH 7.4~7.6)
De-fiber sheep blood (or rabbit blood)
100mL
5~10mL
A.3.2 Preparation method
Heat and dissolved agar, then cool it to 50
operation, shake well and pour on the plate.
A.4 Baird-Parker agar plate
A.4.1 Composition
Tryptone
Beef extract
10.0g
5.0g
, add de-fiber sheep blood (or rabbit blood) via aseptic
GB 4789.10
Yeast extract
Sodium pyruvate
Glycine
2010
1.0g
10.0g
12.0g
5.0g
LiCl⋅6H2O
Agar
20.0g
Distilled water
950mL
pH 7.0 0.2
A.4.2 Formulation method for bacteria enrichment
Mix 50mL 30% yolk saline and 10mL 1% potassium tellurite solution, which has been sterilized and
filtered, store the mixture in the refrigerator.
A.4.3 Preparation method
Add all the components into the distilled water, heat and boil heat to completely dissolving, adjust
pH, put every 95mL into each bottle, conduct autoclave sterilization at 121
for 15min. For use,
heat and melt the agar, cool it to 50 , add 5mL yolkpotassium tellurite solution as bacteria
enrichment, which has been preheated to 50
, in each 95mL, shake and pour it on the plate.
Culture medium shall be dense opaque. It can’t be stored in the refrigerator for more than 48h before
use.
A.5
BHI broth
A.5.1 Composition
Tryptone
10.0g
NaCl
5.0g
Na2HPO4 12H2O
2.5g
Glucose
2.0g
Beef heart infusion
500mL
pH 7.4 0.2
A.5.2 Preparation method
Heat and dissolve, adjust pH, put into 16mmX160mm test tube, take 5mL broth into each bottle and
conduct autoclave sterilization at 121 for 15min.
A.6 Rabbit plasma
Take 3.8g sodium citrate, add 100mL distilled water, dissolve and filtrate for bottling, conduct
autoclave sterilization at 121
for 15min.
Preparation of rabbit plasma: Take 3.8% sodium citrate, add 4 proportions of rabbit blood (100%),
mix well and settle (or conduct centrifugation at 3000r/min for 30min) to reduce blood cells and obtain
plasma.
A.7 Phosphate buffer solution
A.7.1 Composition
KH2PO4
34.0g
Distilled water
500mL
pH 7.2
A.7.2 preparation method
Stock solution: Weigh 34.0g KH2PO4, dissolve it in 500mL distilled water, use 175mL 1mol/L NaOH
solution to adjust pH to 7.2, dilute it to 1000mL with distilled water and then store in refrigerator.
Diluent: Take 1.25mL stock solution, dilute it to 1000mL with distilled water, separately put into the
suitable containers and conduct autoclave sterilization at 121
for 15min.
A.8 Nutrient agar small slant
A.8.1 Composition
GB/T 4789.10-2010
Peptone
Beef extract
NaCl
Agar
Distilled water
pH 7.2~7.4
10.0g
3.0g
5.0g
15.0~20.0g
1000mL
A.8.2 Preparation method
Dissolve all the components into the distilled water except agar, add 2mL 15% NaOH for
dissolution, correct pH to 7.2~7.4, add agar, heat and boil to melt the agar, separately put into
13mmX130mm tubes and conduct autoclave sterilization at 121 for 15min.
A.9 Gram stain solution
A.9.1 Crystal violet stain solution
A.9.1.1 Composition
Crystal violet
1.0g
95% ethanol
20.0mL
1% aqueous solution of diammonium oxalate
80.0mL
A.9.1.2 Preparation method
Completely dissolve crystal violet in ethanol and then mix it with diammonium oxalate solution.
A.9.2 Gram iodine solution
A.9.2.1 Composition
1.0g
I2
KI
2.0g
Distilled water
300mL
A.9.2.2 Preparation method
Firstly mix I2 with KI well, add a little distilled water, shake well and then add distilled water to
300mL after complete dissolution.
A.9.3 Hansa yellow counterstain solution
A.9.3.1 Composition
Hansa yellow
0.25g
95% ethanol
10.0mL
Distilled water
90.0mL
A.9.3.2 Preparation method
Dissolve Hansa yellow in the ethanol and then dilute it with distilled water.
A.9.4 Staining method
a) Fix the smear on the flame, drip crystal violet stain solution, stain it for 1min and wash with distilled
water.
b) Drip Gram iodine solution, react for 1min and wash with water.
c) Drip 95% ethanol, decolorize for about 15~30s until the staining solution is washed (avoid
excessive decolorization) and wash with water.
d) Drip hansa yellow counterstain solution, counter for 1min, wash with distilled water, dry and
conduct microscopic examination.
A.10 Sterile normal saline
A.10.1 Composition
NaCl
8.5mL
Distilled water
1000mL
GB 4789.10
A.10.2 Preparation method
Weigh 8.5g NaCl, dissolve it in 1000 mL distilled water, autoclave at 121
for 15min
2010
GB/T 4789.10-2010
Appendix B
(Normative Appendix)
Inspection Method for Staphylococcal Enterotoxin
B.1 Reagents and materials
Except additional provision, All reagents should be analytically pure, the testing water be conformed
to the provision of GB/T 6682 Grade 1 Water.
B.1.1 ELISA assay kit for A, B, C, D, E type of staphylococcal enterotoxin
B.1.2 pH test paper: range 3.5~8.0, precision 0.1.
B.1.3 0.25mol/L, pH 8.0 Tris buffer solution: Dissolve 121.1g Tris in 800mL de-ionized water, add
42mL concentrated HCl after cooling to ambient temperature, adjust pH to 8.0.
B.1.4 Phosphate buffer solution with pH 7.4: Take 0.55g NaH2PO4·H2O (or 0.62g NaH2PO4· 2H2O),
2.85g Na2HPO4·2H2O (or 5.73g Na2HPO4·12H2O), 8.7g NaCl to 1000mL distilled water, mix well.
B.1.5 n-heptane
B.1.6 10% sodium hypochlorite solution
B.1.7 Enterotoxin toxin-producing culture medium
B.1.7.1 Composition
Peptone
20.0g
Pancreatic digest of casein
200mg (amino acid)
NaCl
5.0g
K2HPO4
1.0g
1.0g
KH2PO4
CaCl2
0.1g
0.2g
MgSO4
Nicotinic acid
0.01g
Distilled water
1000mL
pH7.2 7.4
B1.7.2 Preparation method
Mix all the components in the water and adjust pH to 7.2~7.4, conduct autoclave sterilization at 121
for 30min.
B.1.8 Nutrient agar
B.1.8.1 Composition
Peptone
10.0g
Beef extract
3.0g
NaCl
5.0g
Agar
15.0~20.0g
Distilled water
1000mL
B.1.8.2 Preparation method
Dissolve all the components except agar into the distilled water, add 2mL 15% NaOH for
dissolution, correct PH to 7.2~7.4, add agar, heat and boil to melt the agar, separately put into bottles
and conduct autoclave sterilization at 121
for 15min.
B.2 Instrument and Appartus
B.2.1 balance weighing to 0.01g
B.2.2 homogenizer
B.2.3 Centrifuge: 3000 g 5000 g
GB 4789.10
2010
B.2.4 Centrifuge tube: 50mL
B.2.5 Filter: aperture of membrane 0.2 m
B.2.6 Trace sampler: 20~200 L, 200~1000 L
B.2.7 Muti-channel trace sampler: 50~300 L
B.2.8 Microplate Washer (Optional)
B.2.9 Microplate Reader, wavelength 450nm
B.3 Instrument and Appartus
This method can be completed by using ELISA assay kit for A,B,C,D,E type of staphylococcal
enterotoxin. This method is based on the reaction of enzyme-linked immunosorbent assay (ELISA).
A~E holes of each microporous article of the 96-microtiter-plate were coated with A, B, C, D, E type of
staphylococcal enterotoxin antibody. H hole is for the positive quality control and has been coated
with mixed type of staphylococcal enterotoxin antibodies. F and G holes are for negative controls,
coated with antibodies of non-immunized animals. If the samples contain staphylococcal enterotoxin,
the dissociative staphylococcal enterotoxin combines with the specific antibody coated on the
micropores to form antigen-antibody complex and the remaining compositions are washed off during
washing the plate. The antigen-antibody complex (two anti-) combines with peroxidase markers and
non-binding enzyme markers are washed off during washing. Add enzyme substrate and color
reagent and incubate, the enzyme Catalytic substrates on the enzyme markers decompose and
change the colorless reagent to be blue. Color can turns to yellow from blue by adding reaction
stopping solution and indicates the end of enzyme reaction. Measure the absorbance value of
micropore solution with 450 nm wavelength microplate reader. Staphylococcal enterotoxin in samples
is proportional to the absorbance.
B.4 Inspection procedures
B.4.1 Method for detection of staphylococcal enterotoxin in the culture of isolated strains.
Inoculate tested strains on nutrient agar slant (tube 18mm*180mm) and incubate at 37
for 24h,
wash the lawn with saline and put it into 60mL toxin-producing culture medium, one bottle for each
strain, incubate at 37
for 48h with oscillation of 100 times/min, take the liquid and centrifugate at
8000r/min for 20min, heat at 100
for 10min, take upper clear liquid and dilute, take 100 L diluted
sample solution and tested.
B.4.2 Method for extraction and detection of staphylococcal enterotoxin from food sample
B.4.2.1 Milk and milk powder
Dissolve 25g milk powder into 125mL, 0.25M, pH8.0 Tris buffer solution and mix, same to liquid milk,
prepare according to the following procedures. Centrifugate at 15 , 3500g for10min, receive the
skim milk by removing the surface layer of fat, dilute with distilled water(1:20), take 100 L diluted
sample solution and tested.
B.4.2.2 Foods containing fat not exceeding 40%
Weigh 10g sample and mince, add 15mL pH4.7 PBS solution and homogenize, shake for 15min.
Centrifugate at 15 , 3500g for 10min, removing the surface layer of fat if necessary. Take upper
clear liquid and filter to degerming, take 100 L filtrate to be tested.
B.4.2.3 Foods containing fat exceeding 40%
Weigh 10g sample and mince, add 15mL pH4.7 PBS solution and homogenize, shake for 15min.
Centrifugate at 15 , 3500g for 10min, take 5.0mL upper suspension to another Centrifuge tube, add
5ml n-heptane, mix well for 10min, Centrifugate at 15 , 3500g for 5min, remove all the above
organic phase ( layer of n-heptane), note not remain any of n-heptane, filter the aqueous phase of the
GB/T 4789.10-2010
lower part to degerming, take 100 L filtrate to be tested.
B.4.2.4 Other foods are treated appropriately according to the above methods of food treatment.
B.4.3 Inspection
B.4.3.1 All operations should be under ambient temperature (20~25 ), Temperature of all reagents
in ELISA assay kit for A,B,C,D,E type of staphylococcal enterotoxin should rise to ambient room
before use. Change new pipette tips when sucking up different reagents and sample solution during
determination. Immerse the used pipette tips and exhausted liquid in 10% sodium hypochlorite
solution overnight.
B.4.3.2 Put required numbers of microporous articles into the framework (one sample needs one
microporous article), Add sample solutions into A~G holes of microporous article, 100 L for each
hole, Add 100 L into H hole, flap the microporous article gently to mix it homogeneously. Sealed with
adhesive paper to prevent solution evaporation, incubate at ambient temperature for 1h.
B.4.3.3 Dump the liquid in the holes into container containing 10% sodium hypochlorite solution and
flap some times on absorbent paper to ensure no liquid remained in holes. Use multi-channel sampler
to add 250 L eluant in each hole, dump again and flap on absorbent paper to dry, repeat 4 times of
above washing plate operation. This step also can be completed by Microplate Washer.
B.4.3.4 Add 100 L enzyme labelled antibody in each hole, flap the microporous article gently to mix it
homogeneously, incubate at ambient temperature for 1h.
B.4.3.5 Repeat washing plate procedure 4.3.3.
B.4.3.6 Add 50 L TMB substrate and color former into each micro-hole, flap gently to mix it
homogeneously, incubate at ambient temperature and in dark, light-avoiding place for 1h.
B.4.3.7 Add 100 L 2mol/L Sulfuric acid stopping solution, lap gently to mix it homogeneously, use
microplate reader to measure OD value of each hole under 450nm in 30min.
B.4.4 Calculation of the result and expression
B.4.4.1 Quality control
Calculated result of OD value for positive quality control should greater than 0.5, OD value for
negative quality control should less than 0.3, test results should not be approved if the results do not
meet the above requirements simultaneously. Exclude the interference of endogenous peroxidase for
positive results.
B.4.4.2 Calculation of critical value
F hole and G hole of each microporous article are for negative quality control, the average value of
the OD values for the two holes plus 0.15 is for critical value.
Sample: Negative quality control 1=0.08
Negative quality control 2=0.10
Average value=0.09
Critical value-0.09+0.15=0.24
B.4.4.3 Result expression
Judge sample holes for which value are less than OD value to be negative, expressed as not detect a
certain type of staphylococcal enterotoxin in sample. Judge sample holes for which value are greater
than or equal to OD value to be positive, expressed as detect a certain type of staphylococcal
enterotoxin in sample.
B.5 Biosafety
GB 4789.10
2010
Because the existence of other potentially infectious material in sample can not be eliminated, Do
treat the rejected materials in strict accordance with GB19489.
Appendix C
(Normative Appendix)
Most probable number (MPN) retrieval table of staphylococcus aureus
C.1 Most probable number (MPN) retrieval table of staphylococcus aureus
See Table C.1 to retrieve most probable number (MPN) of staphylococcus aureus in 1g (mL) sample
Table C.1 Most probable number (MPN) retrieval table of staphylococcus aureus
Amount of positive
tubes
MPN
95%
confidence
interval
Amount of negative
tubes
Lower
limit
Upper
limit
0.10
0.01
0.001
<3.0
-
9.5
2
2
0
1
3.0
0.15
9.6
2
2
1
0
3.0
0.15
11
2
0
1
1
6.1
1.2
18
0
2
0
6.2
1.2
0
3
0
9.4
1
0
0
1
0
1
MPN
95%
confidence
interval
Lower
limit
Upper
limi
21
4.5
42
1
28
8.7
94
2
2
35
8.7
94
2
3
0
29
8.7
94
18
2
3
1
36
8.7
94
3.6
38
3
0
0
23
4.6
94
3.6
0.17
18
3
0
1
38
8.7
110
1
7.2
1.3
18
3
0
2
64
17
180
0
2
11
3.6
38
3
1
0
43
9
180
1
1
0
7.4
1.3
20
3
1
1
75
17
200
1
1
1
11
3.6
38
3
1
2
120
37
420
1
2
0
11
3.6
42
3
1
3
160
40
420
1
2
1
15
4.5
42
3
2
0
93
18
420
1
3
0
16
4.5
42
3
2
1
150
37
420
2
0
0
9.2
1.4
38
3
2
2
210
40
430
2
0
1
14
3.6
42
3
2
3
290
90
1000
2
0
2
20
4.5
42
3
3
0
240
42
1000
2
1
0
15
3.7
42
3
3
1
460
90
2000
2
1
1
20
4.5
42
3
3
2
1100
180
4100
2
1
2
27
8.7
94
3
3
3
>1100
420
-
0.10
0.01
0.001
0
0
0
0
0
0
Note1: 3 dilutions [0.1 g(mL) 0.01 g(mL) and 0.001 g(mL)] are adopted in this table, inoculate 3
tubes for each dilution.
Note2: If the tested-sample quantities are changed to 1g (mL) 0.1 g(mL) and 0.01 g(mL),
numbers listed in this table should be decreased for 10 times, If the tested-sample quantities are
changed to 0.01 g(mL) 0.001 g(mL) and 0.0001 g(mL), numbers listed in this table should be
increased for 10 times, and so on with others.
GB/T 4789.10-2010
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