A 40-year-old male presents with fever for 3 days accompanied by abdominal pain and diarrhea, passing stool over 20 times per day with scant volume, mucus, and pus/blood. Stool microscopy shows red blood cells (++), white blood cells (++), and mucus (++). A diagnosis of acute bacillary dysentery is made. The typical symptoms of abdominal pain, diarrhea, and pus/blood in stool are caused by: