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BAIRD-PARKER AGAR BASE

Code: CM0275

a selective and diagnostic medium for the isolation and enumeration of Staphylococcus aureus in foods

* Adjusted as required to meet performance standards

Directions
Suspend 63g in one litre of distilled water and boil to dissolve the medium and sterilise by autoclaving at 121°C for 15 minutes. Cool to 50°C and aseptically add 50ml of Egg Yolk Tellurite Emulsion (SR0054). Alternatively, 50ml of Egg Yolk Emulsion (SR0047) and 3ml of Potassium Tellurite 3.5% (SR0030) can be used. Mix well before pouring into sterile Petri Dishes.

Description
Baird-Parker¹ developed this medium from the tellurite-glycine formulation of Zebovitz et al.² and improved its reliability in isolating Staphylococcus aureus from foods. Baird-Parker added sodium pyruvate, to protect damaged cells and aid their recovery² and egg yolk emulsion as a diagnostic agent. It is now widely recommended by national and international bodies for the isolation of Staphylococcus aureus ⁴.

The selective agents glycine, lithium and tellurite have been carefully balanced to suppress the growth of most bacteria present in foods, without inhibiting Staphylococcus aureus.

Egg yolk emulsion makes the medium yellow and opaque. Staphylococcus aureus reduces tellurite to form grey-black shiny colonies and then produces clear zones around the colonies by proteolytic action. This clear zone with typical grey-black colony is diagnostic for Staphylococcus aureus (see Table 1 for typical reactions). On further incubation, most strains of Staphylococcus aureus form opaque haloes around the colonies. This is probably due to the action of a lipase. Not all strains of Staphylococcus aureus produce both reactions. Some strains of Staphylococcus saprophyticus produce both clear zones and opaque haloes but experienced workers can distinguish these from Staphylococcus aureus by the longer incubation time required⁵.

Colonies typical of Staphylococcus aureus but without an egg yolk reaction should also be tested for coagulase production⁶. Egg yolk reaction negative strains of Staphylococcus aureus may occur in some foods, especially cheese. Smith and Baird-Parker⁷ found that the addition of 50µg of sulphametazine per ml of medium suppressed the growth and swarming of Proteus species. Small numbers of Staphylococcus aureus could then be recovered from specimens containing mixed Proteus strains.

Baird-Parker and Davenport⁸ showed that the recovery of damaged staphylococci was greater on Baird-Parker medium than on other recovery media tested. Broeke⁹ and de Waart et al.¹⁰ found Baird-Parker medium valuable in ecological studies on foods incriminated in staphyloenterotoxicosis. Of the 522 strains of Staphylococcus aureus tested, 97.5% isolated from human and food origins developed characteristically and quantitatively on Baird-Parker medium.

Table 1: Colony characteristics of typical organisms on Baird-Parker Egg Yolk Tellurite Medium

Technique

1. Dry the surface of agar plates for a minimal period of time prior to use.
2. With a glass spatula, spread 0.1ml aliquots of food dilutions made up in Buffered Peptone Water on the agar surface until it is dry. Up to 0.5ml may be used on larger dishes.
3. Incubate the inverted dishes at 35°C. Examine after 24 hours and look for typical colonies of Staphylococcus aureus. Re-incubate negative cultures for a further 24 hours.

Quantitative results
Count the Staphylococcus aureus like colonies and test them for coagulase reaction.
Report Staphylococcus aureus results per gram of food.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Prepared plates of medium are best used freshly prepared¹¹.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Straw coloured gel

Quality control

Includes testing in accordance with ISO 11133:2014¹².

This organism is available as a Culti-Loop®

Precautions
Regard all suspicious colonies as Staphylococcus aureus regardless of negative reactions in the medium and carry out further tests.
Colonies of some contaminating organisms growing in close proximity to the coagulase positive colonies may partially digest the coagulase halo reaction.

References
1. Baird-Parker A. C. (1962) J. Appl. Bact. 25. 12-19.
2. Zebovitz E., Evans J. B. and Niven C. F. (1955) J. Bact. 70. 686-689.
3. Baird-Parker A. C. (1963) J. Gen. Microbiol. 30. 409-413.
4. Chopin A., Malcolm S., Jarvis G., Asperger H., Beckers H. J., Bertona A. M., Cominazzini C., Carini S., Lodi R., Hahn G., Heeschen W., Jans J. A., Jervis D., I., Lanier J. M., O’Connor F., Rea M., Rossi J., Seligmann R., Tesone S., Waes G., Mocquot G. and Pivnick H. (1985) ICMSF Methods studies XV. J. Food Protect. 48. 21-27.
5. Shaw S., Scott M. and Cowan T. (1957) J. Gen. Microbiol. 5. 1010-1023.
6. Devries L. A. and Hajek V. (1960) J. Appl. Bact. 49. 1-11.
7. Smith B. A. and Baird-Parker A. C. (1964) J. Appl.Bact. 27. 78-82.
8. Baird-Parker A. C. and Davenport E. (1965) J. Appl.Bact. 28. 390-402.
9. Broeke R. Ten (1967) Antonie van Leeuwenhoek 33. 220-236.
10. Waart J., de Mossel D. A. A., Broeke R. Ten and Moosdijk A. van de (1968) J. Appl. Bact. 31. 276-285.
11. Holbrook R., Anderson J. M. and Baird-Parker A. C. (1969) J. Appl. Bact. 32. 187-191.
12. ISO 11133:2014 Microbiology of food, animal feed and water - Preparation, production, storage and performance testing of culture media