Periodontitis Culprit Aggregatibacter actinomycetemcomitans (IMAGE) Georgia Institute of Technology Caption Bacteria behind acute periodontitis. Aggregatibacter actinomycetemcomitans most often lives peacefully in the mouth until circumstances lead it to become infectious. It forms flower-like colonies, here under a microscope sporting a colorful stain added by a researcher. Aa is gram-negative. Credit Derren Ready (2012) CIL:38942, CIL. Dataset. https://doi.org/doi:10.7295/W9CIL38942 Creative Commons license Usage Restrictions See credit, CC License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.