All bacteria that is rod shaped is considered a bacillus. Bacilli bacteria are distinguished not only by their rod-like appearance, but also by being gram positive. Bacilli are typically aerobic organisms, but there are rare cases of anaerobic bacilli as well.
Bacteria can be harmful or helpful to humans. Some are used in antibiotics to help people when they get sick, while others can cause food poisoning. There are two main types of bacilli: bacillales and lactobacillales. Since bacilli are gram positive cells, they have thick cell walls. When exposed to the Gram stain, they will turn and stay a crystal violet color.