Recent developments in culture media have given rise to the use of chromogenic substrates as a means of differentiating bacteria. This is one such medium and is a selective medium for the isolation and presumptive identification of Listeria monocytogenes from clinical and food samples. The medium is made selective by the inclusion of Lithium chloride, Ceftazidime, Polymixin B, Nalidixic acid (to suppress other bacteria) and Amphotericin B (to suppress yeasts and fungi). With the combination of both the chromogenic substrate and phospholipase C enzyme reactions, it is possible to differentiate L.monocytogenes from other Listeria spp. Users should be aware that some strains of L.ivanovii are capable of producing an opaque halo, highlighting the need to confirm presumptively identified colonies.

Mineral Modified Glutamate Medium with 1.5% Agar
A modification of Mineral Modified Glutamate Medium where 15g/Litre of Agar has been added to create a plating medium suitable for the isolation of E.coli from food samples. Using this medium during the resuscitation stage allows for the recovery of E.coli from frozen, dried, heat-processed or low pH foods.

Brazier’s CCEY Agar with 1% Lysed Horse Blood – Blood, Cycloserine, Cefoxitin, 4% Egg Yolk Emulsion
Based on Fastidious Anaerobe Agar, Cholic Acid and p-Hydroxyphenylacetic Acid are added to enhance the isolation and differentiation of Clostridium difficile from clinical specimens. Cholic Acid promotes spore germination and p- Hydroxyphenylacetic Acid enhances production of p-cresol, a distinctive metabolite of Clostridium difficile. The medium is made selective by the inclusion of Cefoxitin and D-Cycloserine and Egg Yolk emulsion is added to differentiate Clostridium difficile from the Lecithinase producing clostridia. Lysed Blood is also added which optimises the colony fluorescence when cultures are examined under UV light.

Approved by the National Committee for Clinical Laboratory Standards (NCCLS) in USA this medium is approved for use in antimicrobial sensitivity testing by the disc diffusion method and is recommended particularly for use with the Bauer-Kirby Technique It is low in Thymine and Thymidine and is therefore suitable for use in testing Sulphonamides & Trimethoprim and controlled to ensure correct zone sizes with Tetracyline and Aminoglycoside antibiotics. It can be considered as an alternative to Iso-Sensitest Agar. This particular formulation has an additional 2% Sodium Chloride added to the medium making it suitable for the detection of resistance to Methicillin in staphylococci and it is included in the recommendations of British Society for Antimicrobial Chemotherapy (BSAC) for this purpose. It is not however recommended for testing of organisms requiring a CO2 enriched environment due to the pH effect on the medium. If incubation in a CO2 enriched environment is essential control organisms should be included to confirm that results have not been altered.

Colorex™ mSuperCARBA™ is a selective chromogenic medium that has been developed for the detection and isolation of carbapenemase producing Enterobacteriaceae (CPE). Nosocomial infections due to CPE are particularly difficult due to the limited treatment options. Therefore, this medium is designed to simplify the detection of CPE carriers and to allow for improved monitoring of high risk patient groups. The distinctive colonial colouration of the various species can reduce the need for additional follow up testing allowing many positive results to be issued within 24 hours of receipt of the sample. All presumptive positive colonies should be confirmed for carbapenemase production.

Typical colour reactions are as follows:

Escherichia coli – Red/Pink colonies;

Klebsiella spp., Enterobacter spp., & Citrobacter spp. – Metallic blue colonies;

Other Gram –ve CPE – Colourless colonies;

Carbapenem sensitive bacterial species, Gram +ve bacterial species & yeasts – Inhibited.

Chromogenic medium for detection of Clostridium difficile.

Clostridium difficile (C.difficile) is the leading cause of nosocomial infectious diarrheoa in adults. These infections occur mostly in patients who have both medical care and antibiotic treatment and have become more frequent and more difficult to treat in the last years due to the emergence of highly toxigenic C.difficile strains.

Although PCR has become the leading C.difficile detection technique, culture is essential for strain typing and antimicrobial susceptibility testing. CHROMagarTM C.difficile is a new fluorogenic culture medium, extremely sensitive and selective, especially designed to simplify and speed up (24h) the culture of C.difficile.

About a quarter of pregnant women in the UK are estimated to carry Streptococcus agalactiae. As a result of this, babies become colonized with Streptococcus agalactiae (GBS) during labour and birth; the vast majority are unaffected by this colonization, however, a small percentage become infected with conditions such as eye infections, pneumonia, septicaemia or meningitis. Colorex™ StrepB Agar is a chromogenic media that presumptively identifies Streptococcus agalactiae (mauve/red colonies) after 18-24 hours incubation in aerobic conditions. Enterococci are differentiated by the formation of blue colonies; other organisms are inhibited or colourless.

NB: Some strains of Group A, C & G streptococci may also produce mauve colonies. Therefore, final identification may require additional testing.

Colorex™ MRSA is a chromogenic medium for the selective isolation of Methicillin-Resistant Staphylococcus aureus (MRSA). The medium can be used for the routine screening of clinical specimens for MRSA from a variety of sampling sites such as the nose, throat and groin. The medium incorporates a nutritious peptone base medium and a number of selective agents to inhibit most Gram-negative and Gram-positive bacteria as well as yeasts and moulds. The chromogenic detection of specific enzyme activity leads to the formation of pink/mauve colonies indicating MRSA (including low level resistant and hetero-resistant strains) following incubation at 37°C for 18-24 hours. Other organisms, if present are indicated by blue or colourless colonies. Any presumptive isolates must be confirmed using serological and/or biochemical techniques available to the laboratory. The use of this chromogenic medium does not diminish the requirement for conventional antimicrobial susceptibility tests for the confirmation of methicillin resistance.

Limitations:

1. S.aureus strains that possess a low MIC to the selective agent present in the medium but are mec A negative may form colonies on the medium.

2. Some MRSA strains may form typical colonies surrounded by a matte halo. The formation of the halo serves no diagnostic function.

3. Certain methicillin-resistant coagulase-negative staphylococci (CNS) may produce characteristic colonies. In some cases differentiation may be achieved by examination of the colour of these colonies, as they may be considerably darker in colour (bluish purple to a very dark pink/magenta).

4. Certain bacterial species other than staphylococci may produce colonies with a characteristic colour.

Vancomycin-resistant Enterococcus (VRE) infections are especially aggressive and have been associated with mortality rates approaching 60% to 70%. They are now the second-leading cause of nosocomial infections in the U.S., and their prevalence is increasing worldwide. Resistance to vancomycin has the potential to be transferred from bacteria to bacteria. Cross-resistance is mediated by plasmids and transposons, which may transfer the genes associated with resistance to other much more aggressive pathogens, such as staphylococci and streptococci. Three principal types of vancomycin resistance are found in Enterococcus spp.; VanA, VanB and VanC genotypes. VanA and VanB types account for most significant infections in clinical settings, involving E.faecium and E.faecalis. VanC resistance is a low-level intrinsic resistance found in other Enterococcus spp. The Colorex™ VRE media is another chromogenic media in the Colorex™ range, enabling presumptive identification of vancomycin resistant Enterococci by the formation of mauve/pink coloured colonies (for VanA and VanB genotypes) and blue coloured colonies (for VanC genotypes) after 18-24 hours incubation.

This is a chromogenic medium for the isolation and presumptive identification of Staphylococcus aureus. Mauve colonies indicate Staph aureus following incubation (18 – 24 hours) at 37°C, other organisms, if not inhibited, are indicated by blue or colourless colonies. Studies have suggested that this media has a specificity and sensitivity of 99.4% and 95.5% respectively (Gaillot et al 2000).