Membrane Morphology and Leukotoxin Secretion Are Associated with a Novel Membrane Protein of Aggregatibacter actinomycetemcomitans

Gram-negative bacteria display either a flat or an irregular outer membrane. The periodontal pathogenAggregatibacter (Actinobacillus )actinomycetemcomitans has an irregular outer membrane. We have identified a gene that is associated with the biogenesis of this morphology. The gene is part of a three-gene operon and codes for a 141-kDa protein designated morphogenesis protein C (MorC), which is conserved in several gram-negative bacteria includingHaemophilus influenzae andPasteurella multocida . Insertional inactivation of this gene resulted in the conversion of an irregularly shaped membrane to a flat membrane. Associated with this morphological change were the autoaggregation of the bacteria during planktonic growth and a concomitant increase in the surface hydrophobicity of the bacterium. The absence of MorC also resulted in the loss of the secretion of leukotoxin but not theltxA transcription. Our findings suggest that MorC is critical for membrane morphology and leukotoxin secretion inA. actinomycetemcomitans .