Conventional MacConkey agar differentiates lactose-fermenting bacteria from non-lactose fermenters, whereas Sorbitol MacConkey Agar (SMAC) replaces lactose with sorbitol to distinguish organisms based on their ability to ferment this carbohydrate. Sorbitol fermentation is an important characteristic used to differentiate E. coli O157:H7 from most non-O157 E. coli strains. Typically, E. coli O157:H7 does not ferment sorbitol and produces colorless colonies on SMAC, while most other E. coli strains ferment sorbitol and produce pink colonies. This distinction supports the presumptive identification of E. coli O157:H7 during laboratory testing.

When an organism ferments sorbitol, acid production lowers the pH of the medium, causing the pH indicator to change color and resulting in pink-to-red colonies. Most E. coli O157:H7 strains do not ferment sorbitol within the standard 18–24-hour incubation period and therefore produce colorless colonies. In contrast, most nonpathogenic E. coli strains ferment sorbitol, producing pink-to-red colonies. This difference in sorbitol fermentation helps distinguish presumptive E. coli O157:H7 colonies from sorbitol-fermenting E. coli strains.

This difference makes SMAC useful for screening presumptive O157 colonies. However, it is not a definitive identification method. Other organisms may also produce colorless colonies, and some O157 strains can ferment sorbitol. Colony appearance should therefore be interpreted as a presumptive finding rather than proof of serotype or pathogenicity.

HiMedia's MacConkey Sorbitol Agar Base (M298I) is formulated for the detection of E. coli O157. Its product documentation describes colorless colonies for the specified O157 control strain and pink-red colonies for sorbitol-fermenting organisms under the stated conditions.

TC-SMAC combines the differential principle of SMAC with cefixime and potassium tellurite. These selective agents help inhibit competing microorganisms, improving the visibility of presumptive O157 colonies in samples with mixed microbial populations.

The FDA BAM Chapter 4A describes typical O157 colonies on TC-SMAC as colorless or neutral gray, often with a smoky center and a diameter of approximately 1–2 mm after incubation at 37°C ± 1°C for 18–24 hours. Sorbitol-fermenting bacteria, including most other E. coli, typically appear pink to red.

For laboratories preparing this medium from HiMedia products, the MacConkey Sorbitol Agar Base (M298I) can be supplemented with the specified TC Selective Supplement (FD147), following the manufacturer's instructions and the applicable procedure.

TC-SMAC improves selectivity, but it does not eliminate every competing organism or overcome the limitations of sorbitol-based differentiation. A second isolation medium and further screening remain important where required by the method.

Chromogenic media contain substrates that produce colored compounds when acted on by specific microbial enzymes. The resulting color pattern provides an additional visual characteristic that can help distinguish the target organism from background flora.

The reaction is formulation-specific. Different chromogenic O157 media may produce different colony colors, so a color described for one commercial medium should not automatically be applied to another.

HiCrome® EC O157 Agar, Modified (M1574A) | HiCrome® EC0157: H7 Selective Agar base Modified (M1575A):

This media is recommended for the isolation and differentiation of E. coli O157 from food and environmental samples.

According to the product’s technical documentation, the medium contains sorbitol and a chromogenic mixture that enables differentiation based on characteristic colony colors. Under the specified test conditions, E. coli O157:H7 produces dark purple-to-magenta colonies, whereas the reference E. coli strain ATCC 25922 produces bluish-green colonies. These distinct reactions help differentiate presumptive E. coli O157:H7 from other E. coli strains. For enhanced selectivity, HiCrome® EC O157:H7 Selective Agar Base, Modified (M1575A) is designed to suppress the growth of non-O157 E. coli strains while allowing E. coli O157:H7 to produce characteristic dark purple-to-magenta colonies, facilitating its presumptive identification.

The product data sheet also notes that further biochemical and serological testing is necessary for confirmation. Laboratories should consult the current technical documentation for preparation, incubation and interpretation requirements.

HiCrome® MacConkey Sorbitol Agar Base (M1340):

The HiCrome® MacConkey Sorbitol Agar Base (M1340) is another option for the selective isolation of E. coli O157. It retains sorbitol-based differentiation and incorporates a β-D-glucuronidase indicator system.

According to HiMedia's technical data, the specified O157 strain produces colorless colonies, while the reference E. coli strain may produce blue-green colonies. The medium can also be supplemented with TC Selective Supplement (FD147) when indicated by the selected procedure.

Although the name includes “HiCrome,” M1340 should not be interpreted as having the same colony-color principle as M1574A. Check the relevant technical data sheet before interpreting colony appearance or choosing a medium for a validated workflow.

FDA BAM Chapter 4A describes a cultural isolation procedure for overnight enrichment samples that are probable positive by real-time PCR, as well as samples that have not undergone that screening. The procedure uses TC-SMAC and one of the chromogenic agars specified by the method, including Rainbow Agar O157 or R&F E. coli O157 agar.

Under the stated FDA BAM conditions, plates are incubated at 37°C ± 1°C for 18–24 hours. Typical colonies are then selected for presumptive isolate screening and further confirmation.

This distinction is important: FDA BAM specifies particular chromogenic media for its procedure. HiMedia's M1574A, M1575A and M1340 are relevant product options for laboratories evaluating O157 media, but they should not be assumed to be direct substitutes for the chromogenic media named in FDA BAM without appropriate method validation and regulatory review.

Using two differential approaches can improve the opportunity to recover and recognize presumptive target colonies. Nevertheless, media selection, colony picking, serological or molecular screening and final confirmation must follow the applicable method.

SMAC is useful for presumptive screening, but its limitations should be understood:

• Sorbitol-fermenting O157 strains: These may appear pink or red rather than colorless, making a conventional SMAC screening pattern less reliable.
• Non-O157 STEC: Many non-O157 STEC strains ferment sorbitol and cannot be distinguished from other sorbitol-fermenting E. coli by this reaction alone.
• Pathogenicity: Neither colorless colonies on Sorbitol MacConkey Agar nor characteristic colony colors on chromogenic media confirm the presence of Shiga toxin genes or establish the pathogenicity of an E. coli strain. Further confirmatory testing is required for accurate identification.
• Species and serotype identity: Colony appearance must be followed by the screening and confirmation procedures specified in the relevant method.

Non-O157 STEC investigations require appropriate Shiga toxin gene screening and follow-up isolation and confirmation procedures. Levine EMB agar may support the isolation and differentiation of E. coli in certain workflows, but it is not specific for O157 and cannot independently establish STEC status.