Indole-positive Enterobacteriaceae colonies
may render adjacent indole-negative colonies
falsely positive (6). Indole-positive K. oxytoca
and E. coli were shown to confer falsely positive
results onto neighboring indole-negative S.
marcescens, E. cloacae, K. pneumoniae, and P.
mirabilis. The falsely positive characteristic was
lost when the colony was transferred in pure
culture to an uninoculated plate and retested
after 24 h. Indole-negative strains were rendered
falsely positive if they grew within 5 mm of the
positive strain. The red reaction of P. alcalifaciens
with the DMACA reagent was unchanged
when the organism was grown next to E. coli or
K. oxytoca.