Treponema pallidum can be considered a gram-negative bacterium even though its cell envelope differs from other gram-negative bacteria.
You might know T. pallidum because it causes syphilis, a sexually transmitted disease that affects the skin and mucous membranes of the external genitalia, and also sometimes the mouth.
Treponema pallidum is an obligate parasite bacteria, meaning it can't survive outside a living body. To be more specific, outside of a human being's body. They belong to a group of bacteria called spirochetes, which are long and thin, and contain endoflagella, which are a band of protein filaments that coil within the spirochetes, and give them a spiral shape - kind of like a curly fry, but a little less appetizing. The endoflagella also help the spirochetes to move around by spinning or twisting, a bit like a drill that’s slowly boring into a piece of wood.
People that have syphilis can transmit the disease to others, in one of two ways. The first way is called acquired syphilis and that’s when Treponema pallidum enters the body through bodily fluids. That can happen when there are tiny cuts, or breaks in the skin or mucous membranes of the external genitalia or mouth and when there’s sexual contact - including oral, anal, and vaginal sex.
It can also happen when people share contaminated needles, or when they have direct contact with a skin lesion on an infected person, because the lesion is covered in this fluid which is rich in spirochetes. The second way is called congenital syphilis and that’s when a pregnant person has syphilis and Treponema pallidum infects a baby either in the uterus or while the baby exits through the vagina at birth.
In acquired syphilis, there are three stages to the infection. The first stage is called primary syphilis or the early localized stage, and it usually starts 1 to 3 weeks after the T. pallidum lands on the skin or mucous membrane.