Evaluation of culture techniques for isolation of Pseudomonas pseudomallei from soil

Abstract

Three selective enrichment broths and four selective agar media were evaluated for their ability to support the growth of Pseudomonas pseudomallei both at 35°C and at ambient temperature (range, 20 to 32°C; mean, 25°C). Colony counts of 50 strains of P. pseudomallei and recovery studies with 1 soil strain in 60 simulated soil samples demonstrated that enrichment with Trypticase soy broth incorporating 5 mg of crystal violet per liter and 20 mg of colistin per liter (CVCB) and subculture to Ashdown medium supported the growth of all 50 strains and produced the highest recovery rates with the greatest suppression of other soil flora. An enrichment broth of MacConkey broth (purple) incorporating 10 mg of crystal violet per liter, 5 mg of bromcresol purple per liter, 25 mg of gentamicin per liter, and 650 mg of streptomycin per liter showed greater suppression of soil bacteria than CVCB, but it failed to support the growth of three strains of P. pseudomallei. Recovery rates were essentially the same irrespective of whether the soil samples were incubated at 35°C or at ambient temperature, provided cultures were incubated in protected shade for an extended period. This is an important feature for field work in large-scale epidemiological surveys in which resources are limited.