A dentist has found an ulcer on the oral mucosa of a 7-year-old girl. The ulcer is 1.5 cm in diameter, it has uneven edges and a gray floor. After the ulcer scrape was stained using the Ziel-Nielsen technique, thin ruby-red bacilli were detected in the slide. The bacilli are isolated or arranged in chaotic clusters. This pathogen is characteristic of the following discase: