Preston Agar Base

Principle And Interpretation

This selective medium was described by Bolton and Robertson (1) for isolation of Campylobacter species and is recommended by APHA (2). Isolation of Campylobacter species on selective agar medium is made both with or without selective broth enrichment. Direct plating without enrichment is adequate for fresh faecal samples, faecal contents or intestinal specimens as high numbers of the organisms may be anticipated. For the food samples enrichment is required.

Peptic digest of animal tissue and beef extract in the medium provide nitrogen, vitamins and minerals necessary to support bacterial growth. Sodium chloride provides essential ions.

Preston Selective Supplement (FD042) contains antibacterial and antifungal agents. Polymyxin B is active only against gram-negative bacteria and Proteus species are sometimes resistant. Trimethoprim usually inhibits Proteus species as well as other gram-negative bacteria. Rifampicin is also active against gram-negative organisms. Cycloheximide acts as antifungal agent.

Direct plating without enrichment is adequate for fresh faecal samples, fecal contents or intestinal specimens as high numbers of the organisms may be anticipated. For food sample s enrichment is required.

The Campylobacter species grow well in microaerobic conditions i.e. in 5% O2 at 42°C in about 48 hours. Addition of about 4 drops of glycerol to a filter paper kept within the jar/container will hamper confluent and swarming growth of Campylobacter (3). On Preston Agar Base thermotolerant Campylobacter species tend to produce moist, grey, flat spreading growth, which tends to coalesce. Occasionally some contaminating organisms may grow on this medium but they are usually restricted to the area of primary inoculum. These include Pseudomonas species, more resistant coliforms, Streptococcus species and yeasts.