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Dehydrated Culture Media

SORBITOL MacCONKEY AGAR (SMAC) with BCIG

Code: CM0981

A selective and differential medium for the detection of Escherichia coli O157 incorporating the chromogen 5-bromo-4-chloro-3-indolyl-b -D-glucuronide (BCIG)

Typical Formula*

gm/litre

Peptone

20.0

Sorbitol

10.0

Bile Salts No. 3

1.5

Sodium chloride

5.0

Neutral red

0.03

Crystal violet

0.001

Agar

15.0

5-bromo-4-chloro-3-indolyl-b-D-glucuronide (BCIG)

0.1

pH 7.1 ± 0.2 @ 25°C

* Adjusted as required to meet performance standards 

Directions
Suspend 51.6g of SMAC with BCIG in 1 litre of distilled water. Mix well and sterilise by autoclaving at 121°C for 15 minutes. Pour into sterile Petri dishes.

Description
SMAC with BCIG combines two different screening mechanisms for the detection of Escherichia coli O157, the failure to ferment sorbitol and the absence of b -glucuronidase activity. In a study with artificially contaminated meat samples, SMAC with BCIG reduced the number of false suspect Escherichia coli O157 colonies by 36%1.

The formulation includes 5-bromo-4-chloro-3-indolyl- b-D-glucuronide (BCIG) as a substrate for b-glucuronidase, an enzyme which is not usually present in Escherichia coli O157 strains. The non-sorbitol-fermenting and b-glucuronidase-negative Escherichia coli O157 will appear as straw coloured colonies. Organisms with b-glucuronidase activity will cleave the substrate and release indoxyl (or halogenated indoxyl) which is rapidly oxidised to the insoluble indigo or its analogue, leading to a distinct blue-green colouration of the colonies2.

Enterohaemorrhagic strains of Escherichia coli and in particular the serotype O157:H7 have presented an increasing concern among food and clinical microbiologists since their role as a causative agent in two major food related outbreaks of haemorrhagic colitis was established in 1982. Although a variety of serotypes are known to produce verotoxins a three-year study in England and Wales showed that 99% of isolates from patients suffering diarrhoea, bloody diarrhoea or haemolytic uraemic syndrome (HUS) belonged to serogroup O1571.

The intestinal tract of ruminants is the prime reservoir of Escherichia coli O157 and other enterohaemorrhagic Escherichia coli (EHEC) strains, therefore meats derived from cattle, sheep, goat and deer can be expected to be contaminated. Foods implicated in human illness related to Escherichia coli O157 include meats, dairy products, vegetables, salads, apple juice and water3.

Technique
Inoculate the plates with a suspension of the food, faeces, etc. to produce separated colonies. Incubate for 24 hours at 35-37°C. Examine the plates for straw coloured colonies which are sorbitol-negative and glucuronidase-negative organisms. Confirm suspected Escherichia coli O157 with the Escherichia coli O157 Latex Test DR0620 or Dryspot E. coli O157 DR0120.

E. coli O157 do not usually ferment sorbitol and are glucuronidase-negative, so the colonies will appear colourless. Most other Escherichia coli serogroups are glucuronidase-positive and sorbitol-fermenting and will therefore appear as blue to purple coloured colonies.
When high numbers of Gram-negative organisms are expected to be present, the medium may be made more selective by the addition of Cefixime-Tellurite (CT) Selective Supplement SR0172.

SMAC with BCIG is a USDA/FSIS recommended medium for the presumptive identification of Escherichia coli O157:H7/NM..

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 plates at 2-8°C.

Appearance
Dehydrated medium: Straw/pink coloured, free-flowing powder
Prepared medium: Dark red coloured gel

Quality control 

Positive control: 

Escherichia coli O157:H7 (glucuronidase-negative) NCTC 12900

Good growth: colourless colonies

Negative control:

 
Escherichia coli (glucuronidase-positive) ATCC® 25922 *Good growth: blue/purple coloured colonies

Enterococcus faecalis ATCC® 19433*

Inhibited

References
1. Thomas, A., Cheasty, T., Frost, J. A., Chart, H., Smith, H. R. and Rowe, B. (1996) Epidemiol Infect. 117, 1-10.
2. Desmarchelier, P. M. and Grau, F. H. (1997) Escherichia coli. In: Foodborne Microorganisms of Public Health Significance. 5th Edition. pp.231-264. A. D. Hocking (Ed.). AIFST (NSW Branch) Food Microbiology Group, Australia.
3. Okrend, A. J. G., Rose, B. E. and Lattuada, C. P. (1990) J. Food Prot. 53, 941-943.

 
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