A 34-year-old woman is seen in the emergency depament. About 5 hours previously, she began to have nausea, vomiting, abdominal cramps, and diarrhea. She then developed double vision, which prompted her to ask her husband to bring her to the emergency depament. When she is asked to go into the examining room, she stumbles. When she tries to answer questions, her voice sounds odd and she seems to mumble. Physical examination demonstrates a dry mouth, drooping eyelids, markedly diminished pupillary light reflex, a flaccid facial expression, and difficulty in opening her mouth. Within an hour, her condition has markedly deteriorated. She can no longer stand and is intubated because of difficulty breathing. Which of the following microbial species is MOST likely to have caused her condition?