for growth and plasmid stabilization of the bacteria generally the cultures are incubated at 5 u00b0 c to 25 u00b0 c most preferably 25 u00b0 c for about 17 to 18 hours the media useful for growth of the y enterocolitica stock cultures is any nutrient media suitable to support growth and plasmid stability of the bacteria cells for optimum growth and plasmid stability the preferred media is bhi broth the ph of the nutrient media suitable for growth of the organism is about neutrality i e ph 6 7 to 7 2 in accordance with the binding assay of the invention stock cultures of y enterocolitica are plated on a nutrient rich media and incubated at a time and temperature sufficient to express plasmid genes in the virulent p strains which bind to crystal violet dye the media may be any rich medium containing agar preferably bhi agar colonies are preferably incubated at near 33 u00b0 c to near 40 u00b0 c for about 24 to 30 hours for optimum results colonies are incubated at about 35 u00b0 c to 37 u00b0 c for about 24 to 30 hours the cv dye solution useful in the invention assay consists of crystal violet dye dissolved in water preferably distilled water the aqueous dye solution may be used in any concentration sufficient to allow the virulent p colonies of the bacteria to bind the dye but insufficient to allow the medium plates or avirulent p.