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The gram-negative oral and systemic pathogen Aggregatibacter (Actinobacillus) actinomycetemcomitans produces a leukotoxin (LtxA) that is a member of the RTX (repeats in toxin) family of secreted bacterial toxins. We have recently shown that LtxA has the ability to lyse erythrocytes, which results in a beta-hemolytic phenotype on Columbia blood agar. To determine if LtxA is regulated by iron, we examined beta-hemolysis under iron-rich and iron-limiting conditions. Beta-hemolysis was suppressed in the presence of FeCl 3 . In contrast, strong beta-hemolysis occurred in the presence of the iron chelator deferoxamine. We found that secretion of LtxA was completely inhibited by free iron, but expression of ltxA was not regulated by iron. Free chromium, cobalt, and magnesium did not affect LtxA secretion. Other LtxA-associated genes were not regulated by iron. Thus, iron appears to play an important role in the regulation of LtxA secretion in A. actinomycetemcomitans in a manner independent of gene regulation.

Yersinia enterocolitica synthesized an exocellular antigen common to the serotypes associated with enterocolitis but absent from other serotypes or from other Yersinia species. Both virulent Ca2+-dependent and avirulent Ca2+-independent isogenic pairs derived from the enterocolitis-associated serotypes synthesized the common antigen. Requirements for the synthesis of this common antigen were (i) the presence of metabolizable sugars and (ii) growth on a solid medium at 37°C. The antigen was identified as a 24,000-dalton protein loosely associated with the cell surface but absent from either the cell envelope or the cytoplasmic fraction.