Chlamydiae have
amino acid requirements for growth in tissue culture as defined by those
amino acids whose individual omission from the growth medium prevents chlamydial multiplication. We have tested the hypothesis that this inhibition of growth arises as a result of antagonism between particular
amino acids such that inhibition occurs when the concentration of one
amino acid is reduced in the presence of the antagonist
amino acid at high concentration. Using the Chlamydia psittaci strain guinea pig
inclusion conjunctivitis (GPIC), in the presence of
cycloheximide, the requirement for
valine was abrogated by the simultaneous omission of
isoleucine, that for
phenylalanine by simultaneous omission of
tryptophan and that for
leucine by simultaneous omission of
isoleucine plus
valine. The antagonism shown between
leucine and
isoleucine plus
valine appears to be unique among bacteria. In the absence of
cycloheximide, GPIC had an additional need for
tryptophan,
tyrosine and
isoleucine; these
amino acid requirements were shown for both infected McCoy, HeLa and BHK cells. The results are consistent with a mechanism for regulation of parasite growth which depends on the balance of
amino acid concentrations in the extracellular environment.