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SELENITE CYSTINE BROTH BASE

Code: CM0699

An enrichment medium for the isolation of salmonellae from faeces and food products.

Directions
Dissolve 4g of sodium biselenite LP0121 in 1 litre of distilled water and then add 19g of Selenite Cystine Broth Base. Warm to dissolve and dispense into containers. Sterilise by placing in free flowing steam for 15 minutes. DO NOT AUTOCLAVE.

Robertson¹ reported miscarriages and possible tetragenic effects on pregnant laboratory workers which may have been caused by the ingestion of sodium biselenite. To minimise any possible risk of teratogenicity to laboratory workers the sodium biselenite is not included in the dry powder but should be prepared separately as a solution to which the Selenite Cystine Broth Base is added.

Description
Selenite Cystine Broth Base is modified from the formula of Leifson² with added cystine³. This addition has given favourable results in many studies⁴. Liefson² suggested that it is best to tube the medium to a depth of 2 inches (50mm) or more.

The effect of the cystine may be due to its reducing abilities which will lower the toxicity of selenite to micro-organisms and/or the extra organic sulphur provided may have a sparing effect on the critical sulphur components of the bacteria, again reducing the selective effect of the selenite.
Selenite Cystine Broth is used for enrichment culture of salmonellae from faeces, foodstuffs and other materials. The formulation corresponds to that recommended by the AOAC⁵ for detection of Salmonella in foodstuffs. It is included among the standard methods media of the American Public Health Association^6,7. It also complies with the requirements of the United States Pharmacopoeia⁸.

Technique
The proportion of sample in the enrichment broth should not exceed 10-20% (1 or 2g in 10-15ml). Solid material is added to the normal strength broth. Liquid samples are mixed with double strength medium in the ratio of 1:1. Incubate for 12-24 hours at 35-37°C. Some workers have recommended that 43°C be used^9,10.

Subculture to any combination of greater and lesser inhibitory, selective agars for the Enterobacteriaceae.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared medium at 2-8°C away from light.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Light straw coloured solution

Quality control

   Inhibited growth: <100 cfu when subcultured on TSA
* This organism is available as a Culti-Loop®

Precautions
Observe the precautionary comments made about sodium biselenite in Selenite Broth Base CM0395.
Discard the prepared medium if large amounts of reduced selenite can be seen as a red precipitate in the bottom of the bottle.
Do not incubate longer than 24 hours because the inhibitory effect of selenite is reduced after 6-12 hours incubation¹¹.
Take subcultures of broth from the upper third of the broth column which should be at least 5 cm in depth.

References
1. Robertson D.S.F. (1970) Lancet i 518-519
2. Leifson E. (1936) Am. J. Hyg. 24(2) 423-432.
3. North W. R and Bartram M. T. (1953) Appl. Microbiol. l. 130-134.
4. Fricker C. R. (1987) J. Appl. Bact. 63. 99-116.
5. Association of Official Analytical Chemists (1998) Bacteriological Analytic Manual. 5th Edn. AOAC. Washington DC.
6. American Public Health Association (2001) Compendium of Methods for the Microbiological Examination of Foods. APHA Inc. Washington DC.
7. American Public Health Association (1978) Standard Methods for the Examination of Dairy Products. 14th Edn. APHA Inc. Washington DC.
8. United States Pharmacopoeia USP 28 2005 Microbial Test Limits.
9. Harvey R. W. S. and Scott T. (1953) Mon. Bull. Min. Hlth & PHLS. 12. 149-150.
10. Harvey R. W. S. and Price T. H. (1979) J. Appl. Bact. 46. 27-56.
11. Chattopadhyay W. and Pilford J. N. (1976) Med. Lab. Sci. 33. 191-194.