Gorini, Luigi (Harvard Medical School, Boston, Mass.). Effect of l-cystine on initiation of anaerobic growth of Escherichia coli and Aerobacter aerogenes. J. Bacteriol. 82:305-312. 1961.-Under anaerobic conditions Escherichia coli and Aerobacter aerogenes, inoculated in a mineral-citrate-glucose medium at densities up to 10(6) bacteria per ml, exhibit a long lag, or fail to initiate growth at all. Growth is initiated rapidly if the medium is supplemented with various SH or SS compounds. Of these the most active is l-cystine, which is fully effective at 1 to 2 mum. In a heavily seeded semisolid medium without cystine, turbidity rapidly appears at the aerobic surface and then slowly extends throughout the anaerobic region of the culture. This finding implies that a sufficiently dense anaerobically growing culture creates conditions in the medium which eliminate the requirement for cystine. The nature of this effect on the medium has not been determined, but certain possibilities (pH, pCO(2)) have been eliminated. The anaerobic cystine requirement becomes more pronounced in the presence of Cu(++), at concentrations far lower than those required for inhibition under aerobic conditions. While it is possible that cystine is acting by complexing the toxic metal ion, it seems more likely that l-cystine is an essential metabolite, poorly produced under anaerobic conditions, and that the marked toxicity of Cu(++) under anaerobic conditions depends on its complexing of cystine.