New culture medium improves differentitation of O157 from other strains of E coli and allows laboratories to avoid the difficulties of making their own CR-SMAC
Oxoid CR-SMAC (Cefixime and Rhamnose supplemented Sorbitol MacConkey agar) has been shown to improve the differentiation of E coli O157 from non-O157 E coli strains and other organisms, when compared to non-supplemented SMAC, and eliminates the problems associated with making this medium in-house.
Although some laboratories make CR-SMAC simply by adding rhamnose and cefixime to SMAC, this procedure is fraught with difficulties. The high sugar content can weaken the gel strength of the medium and may make it susceptible to over-heating.
In addition, the cefixime supplement is acidic, causing a change in pH that may affect the performance of the medium.
Oxoid dehydrated CR-SMAC has been formulated to overcome these problems and is now available as part of the standard Oxoid range.
Until recently, CT-SMAC (Cefixime Tellurite Sorbitol MacConkey agar) has been used alongside non-SMAC for the detection of E coli O157. However, this method may miss some tellurite-sensitive strains of E coli O157 on the CT-SMAC plate and the heavy growth of background organisms may conceal some E coli O157 strains on the non-selective SMAC plate.
As an alternative to SMAC, Oxoid CR-SMAC improves the detection of E coli O157 strains by differentiating more non-O157 E coli strains from the target organism, while inhibiting overgrowth by background organisms, in particular Proteus spp and allowing tellurite sensitive strains to grow.
In studies performed at Norwich (MAFF) and London (CPHL), CR-SMAC has been shown to achieve a high level of recovery of E coli O157 when used in conjunction with CT-SMAC. Oxoid CR-SMAC is made using CR-SMAC Agar Base (CM1005) and Cefixime Supplement (SR0191E).
Suspected E coli O157 colonies can be confirmed using the Oxoid E coli O157 Latex Test (DR0620M) or Oxoid Dryspot E coli O157 (DR0120M).
