DEXTROSE TRYPTONE AGAR

Code: CM0075

For the detection and enumeration of `flat-sour’ thermophiles and mesophiles in food products. Acid producing organisms such as `flat-sour’ thermophiles form yellow colonies surrounded by a yellow zone.

* Adjusted as required to meet performance standards

Directions
Suspend 27g in 1 litre of distilled water and bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.

Description
A bacteriologically controlled medium for the detection and enumeration of thermophilic and mesophilic organisms in food products, etc.
Dextrose Tryptone Agar, evolved as the result of several years research by Williams¹ is most suitable for the cultivation and enumeration of the thermophilic bacteria causing `flat-sour’ spoilage of canned food. Its use for routine cultural purposes is recommended by Cameron² and the Association of Official Analytical Chemists³. Dextrose Tryptone Agar is also recommended by:
1. Tanner⁴ for the examination of canned food, sugar, and starch for thermophilic bacteria of the Bacillus stearothermophilus type (i.e. `flat-sour’ spoilage bacteria).
2. The American Public Health Association⁵ for the enumeration of mesophilic and thermophilic aerobic bacteria in sweetening agents used in frozen dairy foods.
3. The National Canners Association⁶ for determination of the total plate and `flat-sour’ count of thermophilic bacteria spores in ingredients, such as sugar and starch.
4. The American Public Health Association⁷ for the enumeration of mesophilic organisms and `flat-sour’ spores in sugars, starches and other complex carbohydrates; and for the enumeration of `flat-sour’ thermophiles in cereals and cereal products, dehydrated fruits and vegetables, and spices.
5. Baumgartner and Hersom ⁸ for the examination of low and medium-acid canned food (above pH 4.5) for `flat-sour’ thermophiles, mesophilic aerobes, and facultative anaerobes.

Bashford ⁹ reported that the addition of 0.5-1% of meat extract greatly improves the medium.

Townsend et al. (National Canners Association¹⁰) showed that some batches of bromo-cresol purple are more inhibitory than others but this variability is overcome in the Oxoid medium by stringent biological control.

Technique
The instructions given below are included only as an indication of the mode of use of Dextrose Tryptone Agar, and will vary according to the original sample and the exact purpose of the investigation. For more exact details of technique it is advisable to consult one of the standard manuals mentioned in the references.
Enumeration of Mesophiles - into each of 5 Petri dishes, pipette dilutions of the sample to be tested. Cover and mix the inoculum with sterile Dextrose Tryptone Agar and incubate for 72 hours at 32°C. Count the total number of colonies, with separate totals for acid producing (yellow halo) and non-acid producing colonies.
Enumeration of `flat-sour’ Thermophiles - inoculate as above and incubate for 48 hours at 55°C. `Flat-sour’ colonies (e.g. Bacillus stearothermophilus) are typically round, 2-5mm in diameter, with an opaque centre, and surrounded by a yellow zone in contrast with the purple medium.

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 agar plates at 2-8°C.

Appearance
Dehydrated Medium: Mauve coloured, free-flowing powder.
Prepared medium: Purple coloured gel

Quality control

* This organism is available as a Culti-Loop®

Precautions
Incubation at 55°C must be carried out under humid conditions e.g. wrapped dishes or in a high humidity environment.

References
1. Williams O. B. (1936) Food Res. 1(3) 217-221.
2. Cameron E. J. (1936) J. Assoc. Official Agr. Chem. 19. 433-438.
3. Association of Official Analytical Chemists (1978) Bacteriological Analytical Manual 5th Edn. AOAC Washington DC.
4. Tanner F. W. (1944) `The Microbiology of Foods’ 2nd ed., Garrard Press, Champaers pp. 762-763; 1127-1128.
5. American Public Health Association (1972) Standard Methods for the Examination of Dairy Products. 13th Edn. APHA. Washington DC.
6. National Canners Association (1968) Laboratory Manual for Food Canners and Processors. Vol.1. p 13.
7. American Public Health Association (1992) Compendium of Methods for the Microbiological Examination of Foods. APHA. Washington DC.
8. Baumgartner J. G. and Hersom A. C. (1956) `Canned Foods’ 4th ed., Churchill Ltd., London, pp. 229-230 and 247.
9. Bashford T. E. (1948) Personal Communication.
10. National Canners Association (1954) `A Laboratory Manual for the Canning Industry’ 1st ed., National Canners Association, Washington.