AEROMONAS MEDIUM BASE (RYAN)
Code: CM0833
A selective diagnostic medium for the isolation of Aeromonas hydrophila from clinical and environmental specimens when used with Ampicillin Selective Supplement.
Typical Formula* | gm/litre |
Proteose peptone | 5.0 |
Yeast extract | 3.0 |
L. Lysine monohydrochloride | 3.5 |
L. Arginine monohydrochloride | 2.0 |
Sorbitol | 3.0 |
Inositol | 2.5 |
Lactose | 1.5 |
Xylose | 3.75 |
Bile Salts No.3 | 3.0 |
Sodium thiosulphate | 10.67 |
Sodium chloride | 5.0 |
Ferric ammonium citrate | 0.8 |
Bromothymol blue | 0.04 |
Thymol blue | 0.04 |
Agar | 12.5 |
Final pH 8.0 + 0.2 @ 25°C | |
* Adjusted as required to meet performance standards
AMPICILLIN SELECTIVE SUPPLEMENT
Code: SR0136
Vial contents (each vial is sufficient for 500 ml of medium) | per vial | per litre |
Ampicillin | 2.5mg | 5.0mg |
Directions
Suspend 29.5g in 500ml of distilled water. Bring gently to the boil. DO NOT AUTOCLAVE. Cool to 50°C and aseptically add one vial of Ampicillin Selective Supplement SR0136 reconsituted as directed. Mix well and pour plates.
Description
Ryan1modified the formulation of XLD Medium so that it would support the growth of Aeromonas spp and Plesiomonas spp as well as the usual Enterobacteriaceae. It could therefore be used as a universal medium in the investigation of enteric disease. However, to improve its performance in the isolation of aeromonads, the addition of ampicillin at 5 mg/l is recommended. The effectiveness of ampicillin as a selective agent for Aeromonas spp has been reported by several workers 2,3,4,5,6. The value of Aeromonas Medium Base (Ryan) is that the recommended level of ampicillin is well below that which can cause inhibition of some strains of aeromonad7.
The utility of Aeromonas Medium (Ryan) and its superiority over some other formulae for detection of Aeromonas spp. in tap water, bottled water and foods including meat, poultry, fish and seafoods has been reported8,9,10. Aeromonas Medium (Ryan) is specified by the MAFF/DHS Steering Group on the Microbiological Safety of Food for detection and enumeration of Aeromonas hydrophila in clinical specimens11. Aeromonas spp occur widely in soil and water, where they cause diseases in fish and amphibians. They also occur in untreated and chlorinated drinking water, raw foods and raw milk11,12 . It is considered that the major cause of gastrointestinal infections by Aeromonas spp12,13 is from ingesting infected water14,15.
The role of these organisms in gastrointestinal disease is still subject to debate but a rapidly expanding body of literature suggests that Aeromonas spp can cause a wide spectrum of enteric symptoms in adults as well as children5,16 It would therefore be a useful diagnostic aid to include this selective medium when investigating diarrhoeal disease.
Aeromonas Medium Base has been developed to improve the enumeration and isolation of Aeromonas spp from clinical and environmental specimens.
Technique
1. Prepare the medium according to directions and pour into sterile dishes. The prepared medium may be stored at 2-8°C up to 5 days.
2. Inoculate the plates with a suspension of food, faeces etc., diluted to form single colonies on the inoculated plate.
3. Incubate the plates aerobically at 30-35°C for 24 hours. If further incubation is required hold at room temperature (22-25°C).
4. Examine the plates for the presence of dark green, opaque colonies with darker centres. Confirm the identity with biochemical tests.
The typical colonial appearance of Aeromonas isolates on this medium is as follows:
Aeromonas species: | Dark green, opaque with darker centre, diameter 0.5-1.5mm. |
| Pseudomonas species: | Blue/grey translucent, diameter from pinpoint to 0.25mm. |
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 of medium 2-8°C.
Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Green coloured gel
Quality control
Positive control: | Expected results |
Aeromonas hydrophila ATCC® 7966* | Good growth; opaque green colonies with dark centres |
Negative control: | |
| Escherichia coli ATCC® 11775* | No growth |