BS 4285-3.11:1985 pdf free download.Microbiological examination for dairy purposes Part 3: Methods for detection and/or enumeration of specific groups of microorganisms — Section 3.11: Detection and enumeration of faecal streptococci.
1 Scope
This Section of BS 4285 describes a method for the detection and enumeration of faecal streptococci in milk and dairy products.
NOTE The titles of the publications referred to in BS 4285-3.11 are listed on the inside back cover.
2 Definition
For the purposes of this Section of BS 4285, the following definition applies:
faecal streptococci
those bacteria that form characteristic colonies when grown on the selective medium under the conditions specified
3 Principle
A test portion or series of decimal dilutions is mixed with a selective culture medium in Petri dishes and incubated. The numbers of characteristic colonies on selected plates are counted and the number of faecal streptococci per millilitre or per gram of the original sample are calculated.
4 Diluents and culture medium
4.1 Diluents
Use diluents described in BS 4285-1.2.
4.2 Streptococcus agar
4.2.1 Composition. The composition of the streptococcus agar shall be as follows:
proteose peptone 10 g
yeast extract 10 g
sodium chloride 5 g
sodium glycerophosphate 10 g
maltose 20 g
lactose I g
sodium azide 0.4 g
bromocresol purple
(1 % solution) 1.5 mL
agar 20 g or according to supplier’s
instructions
water 1 000 mL
4.2.2 Preparation. Add the ingredients to the water and heat to boiling to dissolve. Adjust the pH so that it will be 7.2 after sterilizing. Dispense in flasks in convenient volumes and sterilize for 10 mm at 121± 1 °C.Cooltoabout60°Candadd 1 mLofa filter-sterilized 1 °A) aqueous solution of triphenyltetrazolium chloride per 100 mU of sterile medium. Mix to obtain uniform distribution. Cool to 45 °C and prepare the plates.
NOTE It is recommended that. a dehydrated complete medium (also known as KF streptococcus agar) be used following the manufacturer’s instructions.
WARNING. Sodium azide is toxic and great care is required in the preparation of the medium. It also forms explosive compounds with metals and before disposal should be rendered safe by addition of excess sodium nitrite.
5 Apparatus
NOTE For details of apparatus including its preparation and
sterilization, see BS 4285.1.2.
5.1 Usual microbiological laboratory apparatus.
5.2 Incubator, at 37 ± 1 DC.
5.3 Waterbath, at 45 ± 1 °C.
5.4 Petri dishes.
5.5 Total delivery pipettes, of 1 mL nominal
capacity.
5.6 Colony counting equipment.
6 Sampling
Take the laboratory sample in accordance with
BS 4285-1.1.
7 Preparation of the test sample
Prepare the test sample from the laboratory sample
in accordance with BS 4285-1.1.
8 Procedure
8.1 Test portion and dilutions Measure the test portion and prepare serial decimal dilutions from the test sample (see BS 4285-1.1), making as many dilutions as are necessary to produce acceptable counts at two successive dilutions.
8.2 Inoculation of Petri dishes Inoculate the Petri dishes as described in
BS 4285-2.1.
8.3 Addition of medium to Petri dishes Add 10 mU to 12 mU of the medium (4.2.2) at 45 ± I °C to each inoculated dish, mix and allow to set as described in BS 4285-2.1.
8.4 Incubation of the plates
Invert the plates and incubate at 37 ± 1 °C for 48 h (see also BS 4285-1.3).
8.5 Counting of colonies
Count only red or pink colonies, following the
procedure given in BS 4285-2.1.
9 Calculation and expression of results
Calculate the number of organisms and express the results as described in BS 4285-2.1.
10 Test report
The test report shall give the number and date of BS 4285-3.11, i.e. BS 4285-3.11:1985 and state the results obtained. It shall also state any operating conditions not specified in BS 4285-3.11 or regarded as optional, as well as any circumstance that may have influenced the result.
The report shall include all details required for complete identification of the sample.