HYGIENIC MANUFACTURE AND PRESERVATION 749 1.37 Selective enrichment media for the isolation of pathogens 1.371 Staphylococcus aureus (a) Salt Meat Broth 37 ø Sub-culture to Nutrient Agar at 24 h. Carry out slide and coagulase test from Nutrient Agar (see 1.88 below). (b) Baird-Parker Medium (Oxoid code CM275) 37 ø For further details see 5.3 Ref. (13) (Appendix E). 1.372 Pseudomonas aeruginosa (Ps. pyocyanea) 37 ø Plate on Cetrimide Agar from broth at 24 h. 1.373 Salmonellae Selenite F broth 37 ø 43 ø Subculture on brilliant green agar (Bacto) after 24 and 48 h. Incubate at 37 ø and examine the plates for colonies with Salmonella characteristics and identify by standard biochemical and serological methods [see Selected Bibliography. Appendix E, 5.3, Refs. (4, t3)• 1.38 Staphylococcal coagulase test This test is used to differentiate pathogenic Staph. aureus from non- pathogenic staphylococci. 1.381 Test reagent Human or rabbit plasma. Dried rabbit plasma, reconstituted and diluted 1:5 in isotonic saline is satisfactory. 1.382 Slide test Use an 18-24 h Nutrient Agar Culture (not a broth culture). Mark a clean slide into two sections. Place a loopful of water (not saline) on each section and emulsify a colony or small amount of culture in each drop until a homogeneous suspension is obtained. If no clumping occurs in 10-20 s, add and mix a loopful of neat reconstituted plasma. Avoid excess plasma as this may give false positives. The second suspension serves as a control. A positive result is indicated by visible clumping within 10 s. Delayed clumping does not constitute a positive result. 1.383 Tube test Use an 18-24 h plain Nutrient Broth Culture. Place 0.5 ml of diluted plasma in each of two small test tubes. To one tube add 0.5 ml of an 18-24 h

750 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS broth culture. Incubate both tubes at 37 ø and examine after 1 h and at intervals up to 24 h. A positive result is indicated by a definite clot formation. Coagulation usually takes place within 1-4 h. Granularity or ropiness is regarded as doubtful and the test should be repeated. Negative tubes will be clear or only faintly cloudy with no coagulation. If the plasma has been stored in a refrigerator it may be sufficiently cold to delay coagulation it is advisable to allow the plasma to attain room temperature before use. The slide test detects 'bound' coagulase which acts on fibrinogen directly the tube test detects 'free' coagulase which acts on fibrinogen in conjunc- tion with other factors in the plasma. Either or both coagulases may be present. The slide test is a valuable presumptive test but negative results must be confirmed by a tube test. Known coagulase-positive and negative strains must always be tested in parallel and, with the tube tests, an uninoculated control must also be set up. Controls: Positive - Staphylococcus aureus (NCIB 9518, FDA 209, ATCC 6538 or NCTC 8532) Negative -S. epidermidis (NCTC 7291 or 4276). 1.384 Additional screening test The ability of coagulase-positive staphylococci to split deoxyribonucleic acid (DNA) provides the basis for a diagnostic laboratory test for the identification of potentially pathogenic organisms. Test Inoculate a plate of DNase Agar (Oxoid CM321) as follows: Use an 18-24 h Nutrient Broth Culture. Place 1 drop onto the surface of the agar so that a thick plaque of growth is evident after 18 h incubation. Flood the plate with N/1 HC1 and look for clearing around the colonies (DNase positive). 1.4 Inactivators suitable for counting procedures in the presence of common antimicrobials and preservatives Special difficulties arise when the product being tested is inhibitory to bacterial growth or when it contains a bacteriostatic agent. In order to obtain a true bacterial count, the bacteriostatic effect must be overcome