What is the causative agent for bloody diarrhea with a very small inoculum?

The correct answer is Shigella, which is known for causing bloody diarrhea, particularly with a very small inoculum. Shigella is highly infectious and can be transmitted by ingesting as few as 10 to 100 organisms. This low infectious dose is a key characteristic of Shigella infections, which leads to the onset of bacillary dysentery, often presenting with bloody and mucoid diarrhea.

In contrast, other bacteria listed typically require a larger inoculum to cause illness. For instance, Escherichia coli may cause diarrhea, but strains that present with bloody diarrhea, such as Enterohemorrhagic E. coli, generally have a higher infectious dose. Salmonella and Campylobacter are also associated with gastroenteritis but do not have the same ability to cause bloody diarrhea with such a small number of organisms. Instead, these pathogens usually present with diarrhea that is not predominantly bloody and require more exposure for infection.

Overall, the key features of Shigella's low infectious dose and its propensity to cause dysentery make it the primary causative agent for bloody diarrhea among the listed choices.