Thermo Scientific™ Crossley Milk Medium (Dehydrated)

Description

Carry out routine examination of canned food samples for anaerobes with Thermo Scientific™ Oxoid™ Crossley Milk Medium (Dehydrated). The medium gives rapid growth without the use of special anaerobic apparatus while allowing the detected bacteria to be provisionally identified by their reactions upon the medium.

Canned foods are more often prone to flat sour spoilage due to contamination by either mesophilic or thermophilic anaerobic spore formers.
Crossley milk medium is recommended, in the second edition of Tanner’s `The Microbiology of Foods’¹, for the examination of meat, meat products, and canned foods for sporing anaerobes.

Organism	Diagnostic reaction	Organism indicated
1	Neutral or alkaline pH (purple colour), gas production,soft curd followed by rapid digestion of casein, often to clear brown liquid, formation of black sediment accompanied by typical foul odor	Cl. putrificum Cl. sporogenes Cl. flabelliferum Cl oedematiens Cl. histolyticum
2	No initial change of pH, formation of soft curd within 2-3 days, slight gas formation. Complete digestion later with alkaline reaction, no odor	Cl. centrosporogenes
3	Slight acidity (pale yellow colour), formation of soft curd, and whey. Slight gas production	Cl. sphenoides
4	Acid (bright yellow colour), formation of firm clot and gas. Bleaching of the indicator may sometimes occur	Cl. butyricum
5	(a) Acid, formation of ’stormy’ clot. (b) Acid and ’stormy’ clot but with less gas and cloudy whey	(a) Cl. perfingens (b) Usually Cl. tertium
6	Strong alkaline pH, with peptonization commencing at the surface and spreading downwards. Digestion not complete, no blackening, no odor, no gas production	B. subtilis B. vulgaris
7	Acid and clot, or slightly acid only. Peptonization in some cases	B. cereus B. coagulans B. silvaticus and various cocci (more detailed tests required)

Crossley² modified his medium, by the addition of 20% (w/v) of autoclaved meat or fish paste, so that it was suitable for the examination of vegetable and dairy products.
The medium recommended by Jepsen³ in `Meat Hygiene’ published by the World Health Organization, is Crossley Milk Medium modified by the addition of 20% (w/v) of cooked fish; it is suitable for the examination of meat products for clostridia, and gives diagnostic reactions essentially similar to those outlined above. Riemann⁴ modified Crossley Milk Medium by the addition of 0.08% of cysteine hydrochloride, before autoclaving, or by the addition of 1mL of a sterile 10% sodium thioglycollate solution just before use.

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