<P> Procedure: First, add the alpha - naphthol; then, add the potassium hydroxide . A reversal in the order of the reagents being added may result in a weak - positive or false - negative reaction . </P> <P> VP is one of the four tests of the IMViC series, which tests for evidence of an enteric bacterium . The other three tests include: the indole test (I), the methyl red test (M), and the citrate test (C). </P> <P> VP positive organisms include Enterobacter, Klebsiella, Serratia marcescens, Hafnia alvei, Vibrio cholera biotype eltor, and Vibrio alginolyticus . </P> <P> VP negative organisms include Citrobacter sp., Shigella, Yersinia, Edwardsiella, Salmonella, Vibrio furnissii, Vibrio fluvialis, Vibrio vulnificus, and Vibrio parahaemolyticus . </P>

When is the vp test used as a confirmatory test