Human Serum-Specific Activation of Alternative Sigma Factors, the Stress Responders in Aggregatibacter actinomycetemcomitans

Abstract

Aggregatibacter actinomycetemcomitans, a known pathogen causing periodontal disease and infective endocarditis, is a survivor in the periodontal pocket and blood stream; both environments contain serum as a nutrient source. To screen for unknown virulence factors associated with this microorganism, A. actinomycetemcomitans was grown in serum-based media to simulate its in vivo environment. Different strains of A. actinomycetemcomitans showed distinct growth phenotypes only in the presence of human serum, and they were grouped into high- and low-responder groups. High-responders comprised mainly serotype c strains, and showed an unusual growth phenomenon, featuring a second, rapid increase in turbidity after 9-h incubation that reached a final optical density 2- to 7-fold higher than low-responders. Upon further investigation, the second increase in turbidity was not caused by cell multiplication, but by cell death. Whole transcriptomic analysis via RNA-seq identified 35 genes that were up-regulated by human serum, but not horse serum, in high-responders but not in low-responders, including prominently an alternative sigma factor rpoE (σE). A lacZ reporter construct driven by the 132-bp rpoE promoter sequence of A. actinomycetemcomitans responded dramatically to human serum within 90 min of incubation only when the construct was carried by a high responder strain. The rpoE promoter is 100% identical among high- and low-responder strains. Proteomic investigation showed potential interactions between human serum protein, e.g. apolipoprotein A1 (ApoA1) and A. actinomycetemcomitans. The data clearly indicated a different activation process for rpoE in high- versus low-responder strains. This differential human serum-specific activation of rpoE, a putative extra-cytoplasmic stress responder and global regulator, suggests distinct in vivo adaptations among different strains of A. actinomycetemcomitans.

Aggregatibacter actinomycetemcomitans serotypes associate with periodontal and coronary artery disease status

“…Whole‐genome sequencing of 31 A. actinomycetemcomitans strains revealed greater similarity between serotypes “b” and “c”, and on the other hand, among serotypes “a”, “d”, “e” and “f” (Kittichotirat, Bumgarner, & Chen, ). It has been reported that A. actinomycetemcomitans strains grow differentially in the presence of human serum (Tang‐Siegel et al., ), which supports association of specific serotypes with different clinical features.…”

Effect of periodontal treatment on Aggregatibacter actinomycetemcomitans colonization and serum IgG levels against A. actinomycetemcomitans serotypes and Omp29 of aggressive periodontitis patients

“…Furthermore, the growth of serotype c strains, but not the others, is highly stimulated by human serum, indicating differences in gene regulation among strains of this species (Tang-Siegel et al, 2016). On the other hand, A. actinomycetemcomitans serotype a was less able to stimulate the secretion of IL-6 by gingival fibroblasts and more efficient to induce IL-8 production than the JP2 clone (serotype b) (Dongari-Bagtzoglou & Ebersole, 1996).…”

“…Moreover, high toxic activity of A. actinomycetemcomitans in epithelial cells susceptible to the cytolethal distending toxin was reported in strains belonging to serotypes b and c (Kawamoto et al, ). Furthermore, the growth of serotype c strains, but not the others, is highly stimulated by human serum, indicating differences in gene regulation among strains of this species (Tang‐Siegel et al, ). On the other hand, A. actinomycetemcomitans serotype a was less able to stimulate the secretion of IL‐6 by gingival fibroblasts and more efficient to induce IL‐8 production than the JP2 clone (serotype b ) (Dongari‐Bagtzoglou & Ebersole, ).…”

Outer membrane vesicle-mediated serum protection inAggregatibacter actinomycetemcomitans

“…M. catarrhalis, N. gonorrhoeae, P. gingivalis, and V. cholerae [30][31][32][33]. Based on previous studies with A. actinomycetemcomitans [17,21,51], we routinely determined the serum survival in 50% NHS. A. actinomycetemcomitans strains of all serotypes are ubiquitously serum resistant [17,19], and due to the apparent unavailability of a natural serum-susceptible strain, a strain D7SS ompA1 ompA2 double mutant was selected for the present experimental studies.…”