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Other products used in the isolation of Salmonellae:
A highly selective medium for isolation and identification of Salmonellae from clinical, environmental and food samples.
|Ferric ammonium citrate|
Final pH 7.4 ± 0.2 @ 25°C
XLT-4 SELECTIVE SUPPLEMENT
Supplement available in 100 ml (SR0237C)
Suspend 59g of XLT-4 Agar Base in 1 litre of distilled water, add 4.6ml of XLT-4 Selective Supplement and bring the medium to the boil.
Do not overheat, do not autoclave.
Cool to approximately 50°C and pour into sterile Petri dishes. It is not advised to hold the medium at 50°C for longer than 1 hour as this may cause the medium to precipitate.
XLT-4 (Xylose Lactose Tergitol™ 4) Agar is a highly selective plating medium used for isolation and identification of salmonellae from clinical, environmental and food samples according to Miller and Tate1.
The peptones and yeast extract in this formulation provide a source of amino nitrogen, along with essential nutrients and vitamins; this ensures optimal growth of salmonellae. The selective agent is Tergitol™ 4 (also known as Niaproof 4 or sodium tetradecylsulfate) which is an anionic surfactant. This largely inhibits or reduces the growth of unwanted background flora. The background colour of the plate is red, due to the inclusion of the phenol red, which will change colour due to pH changes from fermentation and decarboxylation reactions.
Differentiation on this medium is facilitated by the fermentation of xylose, lactose and sucrose as well as the decarboxylation of lysine. Salmonellae appear as black or red colonies with a black centre due to their ability to reduce thiosulphate to hydrogen sulphide, which causes the colony to blacken (as there are ferric ions present). Escherichia coli grow as yellow colonies as they are able to ferment the lactose in the formulation, causing a pH drop which shifts the colour of the phenol indicator from red to yellow. Other organisms, such as shigellae, do not ferment the lactose, or reduce the thiosulphate, so appear as clear red colonies with no blackening.
1. The sample to be tested should first be enriched using suitable media and incubation conditions.
2. Using a microbiological loop, subculture from the enrichment broth onto XLT-4 Agar.
3. Incubate plates aerobically at 35-37°C and examine for growth at 18-24 hours and 48 hours.
XLT-4 Agar contains Tergitol™ 4 which acts to inhibit many of the background flora organisms which can grow and cause problems with differentiation on other salmonellae plating media. This enhanced selectivity allows a significant increase in the recovery of salmonellae, while reducing false-positive suspect colonies. XLT-4 Agar has been shown to save time and the costs of further identification of false presumptive positive colonies when compared to other salmonellae plating media2.
Dehydrated medium: straw coloured, free-flowing powder
Prepared medium: clear red gel
Storage conditions and Shelf life
The dehydrated medium should be stored at 10-30°C and used before the expiry date on the label.
Prepared medium may be stored for up to 2 weeks at 2-8°C.
Salmonella enteritidis ATCC®13076 *
|Good growth: black or, red with black center|
|Escherichia coli ATCC®25922 *||Reduced growth, yellow|
|Enterococcus feacalis ATCC®29212 *||No growth|
1. Miller, R.G., Tate, C.R. (1990) The Maryland Poultryman, April: 2-7
2. Dusch, H. and Altwegg, C. J. of Clin. Micro. Apr. 1995, p.802-804. Vol 33, No.4