SCHEDULE 2Testing Methods
PART 1METHOD FOR THE ISOLATION OF CLOSTRIDIUM PERFRINGENS
Time of testing1
Tests shall be begun on receipt of the sample or on the first working day which allows this method to be completed. If the test is not begun on the day of receipt the sample shall be stored in a refrigerator at between 2°C and 8°C until required. If the sample has been refrigerated it shall be removed from the refrigerator and stored at room temperature for at least one hour before the test is started.
Samples2
Tests shall be carried out using two 10 gram portions of each sample submitted for testing. Each 10 gram sample shall be placed aseptically in a sterile container containing 90 ml Clostridium perfringens diluent consisting of 0.1% peptone and 0.8% sodium chloride at a pH of 7 and mixed thoroughly until the sample is evenly suspended.
Inoculations
3
For each portion of the sample 1 ml of solution shall be transferred to a sterile 90 mm petri dish (in duplicate), to which 15 ml of Shahidi – Ferguson agar (SF agar)31 at a temperature of 47°C±1°C shall be added and immediately gently mixed by swirling the dish with 5 clockwise and 5 anticlockwise circular movements.
4
Once the agar has set, each agar plate shall be overlaid with a further 10 ml SF agar at a temperature of 47°C±1°C. Once the overlay has set and with the plate lids uppermost the plates shall be incubated anaerobically at 37°C±1°C for 20 hours±2 hours.
Samples with colonies of Clostridium perfringens
5
After incubation, each set of duplicate plates shall be examined for colonies characteristic of Clostridium perfringens (black). The sample provisionally fails if any colonies characteristic of Clostridium perfringens are present, in which case the following procedure shall be followed to establish whether or not the colonies are Clostridium perfringens.
6
In the case of each plate, 10 characteristic colonies of Clostridium perfringens shall be subcultured on to a further SF agar plate. If there are less than 10 colonies on the plate, all characteristic colonies shall be subcultured on to the further plate. The plates shall be incubated anaerobically at 37°C±1°C for 20 hours±2 hours.
7
If the surface area of the plates is overgrown and it is not possible to select well isolated characteristic colonies, 10 suspect colonies shall be subcultured on to duplicate SF agar plates and incubated anaerobically at 37°C±1°C for 20 hours±2 hours.
8
One characteristic colony from each plate shall be subcultured on to SF agar and incubated anaerobically at 37°C±1°C for 20 hours±2 hours.
Subcultured colonies9
After incubation each plate shall be examined for colonies characteristic of Clostridium perfringens. All colonies characteristic of Clostridium perfringens shall be–
a
stab inoculated into motility nitrate medium32; and
EXAMINATION OF SUBCULTURES
Motility10
The motility nitrate medium shall be examined for the type of growth along the stab line. If there is evidence of diffuse growth out into the medium away from the stab line, the bacteria shall be considered to be motile.
Reduction of nitrate to nitrite11
After examination of the motility nitrate medium, 0.2 ml to 0.5 ml of nitrite detection reagent shall be added to it. The formation of a red colour confirms that the bacteria have reduced nitrate to nitrite. Cultures that show a faint reaction (i.e. a pink colour) should be discounted. If no red colour is formed within 15 minutes, a small amount of zinc dust shall be added and the plate allowed to stand for 15 minutes. If a red colour is formed after the addition of zinc dust no reduction of nitrate to nitrite has taken place.
Production of gas and acid from lactose and liquefaction of gelatin
12
The lactose gelatin medium shall be examined for the presence of small gas bubbles in the medium.
13
The lactose gelatin medium shall be examined for colour. A yellow colour indicates fermentation of lactose.
14
The lactose gelatin medium shall be chilled for one hour at between 2°C and 8°C and then checked to see if the gelatin has liquefied. If the medium has solidified it shall be re-incubated anaerobically for a further 18 to 24 hours, the medium chilled for a further one hour at between 2°C and 8°C and again checked to see if the gelatin has liquefied.
15
The presence of Clostridium perfringens shall be determined on the basis of the results from paragraphs 10 to 14. Bacteria which produce black colonies on SF agar, are non-motile, reduce nitrate to nitrite, produce gas and acid from lactose and liquefy gelatin within 48 hours shall be considered to be Clostridium perfringens.
Control Tests
16
Control tests shall be carried out each day that a test is initiated using–
a
Clostridium perfringens no more than seven days old at the time of use;
b
Escherichia coliNCTC 1041835 or equivalent not more than seven days old at the time of use; and
c
rendered animal protein which is free of Clostridium perfringens.
17
10 gram portions of the rendered animal protein shall be placed aseptically in each of two sterile containers containing 90 ml Buffered Peptone Water (BPW)36 and mixed thoroughly until the samples are evenly suspended.
18
One colony of Clostridium perfringens shall be placed in 10 ml BPW and mixed to form an even suspension. 0.1 ml of the suspension shall be added to the suspension in the preceding paragraph. This shall be repeated for Escherichia coli.
19
These are then treated and examined in the same way as test samples. If no typical colonies are formed then that day’s testing shall be invalid and shall be repeated.