Treponema pallidum is the bacterium that causes syphilis, one of the oldest and most consequential sexually transmitted infections in human history. It belongs to a group of bacteria called spirochetes, named for their distinctive corkscrew shape. Despite being discovered over a century ago, this organism remains remarkably difficult to study because it cannot survive outside a human host, and it has evolved sophisticated ways to hide from the immune system.
Shape, Size, and Movement
Treponema pallidum is tiny, even by bacterial standards. Each organism measures 6 to 15 micrometers long but only 0.1 to 0.2 micrometers wide, making it too thin to see under a standard light microscope. Its body is coiled like a corkscrew, and it moves by rapidly spinning along its length while bending, flexing, and snapping. This unusual motion is powered by internal flagella: three originate from each end of the bacterium, wind through the space beneath its outer membrane, and overlap in the middle. That design lets the organism burrow through tissue and mucous membranes with surprising efficiency.
Why It Can’t Be Grown in a Lab
Most disease-causing bacteria can be cultured on plates or in broth, which makes them relatively easy to study. Treponema pallidum is a striking exception. It has a stripped-down genome of only about 1,041 genes, compared to more than 4,400 in a common lab bacterium like E. coli. That minimal toolkit means it lacks the ability to manufacture many essential nutrients on its own, including nucleotides, fatty acids, amino acids, and vitamins. Instead, it scavenges everything it needs from the human body, relying heavily on glucose from the bloodstream as its primary energy source.
This total dependence on a living host is the reason researchers still cannot reliably grow Treponema pallidum in laboratory cultures. It has slowed vaccine development and made the organism harder to study than almost any other common pathogen.
How It Evades the Immune System
Treponema pallidum is sometimes called a “stealth pathogen,” and for good reason. Its outer membrane is unusual: it contains very few surface proteins that the immune system can recognize and target. Most bacteria display molecules on their surface that act like flags, triggering an immune response. Treponema pallidum’s outer membrane is largely bare, presenting few antigenic targets. This allows it to persist in the body for years, even decades, without being cleared. The immune system eventually mounts a response, but the bacterium’s ability to dodge detection early on is what allows syphilis to progress through its characteristic stages.
The Stages of Syphilis
Syphilis unfolds in a predictable sequence if left untreated, with quiet periods between active phases.
The primary stage begins with a single painless sore, called a chancre, at the site where the bacterium entered the body. This sore typically appears within a few weeks of infection and lasts 3 to 6 weeks before healing on its own, whether or not treatment is given. Because the sore is painless and often hidden (inside the mouth, on the genitals, or in the rectum), many people never notice it.
The secondary stage follows, sometimes overlapping with the healing of the primary sore. It produces a body-wide rash that can appear on the palms of the hands and soles of the feet, along with flu-like symptoms such as fever, sore throat, and swollen lymph nodes. These symptoms also resolve without treatment, which can create a false sense of recovery.
The latent stage is a silent period with no visible signs or symptoms. It can last for years. The bacterium is still present in the body and can still be transmitted, particularly during the early latent phase.
Tertiary syphilis develops in some untreated individuals 10 to 30 years after the initial infection. At this point, the disease can damage the heart, blood vessels, brain, and other organs, sometimes fatally. Not everyone with untreated syphilis reaches this stage, but those who do face serious, irreversible harm.
How Syphilis Is Diagnosed
Diagnosing syphilis relies on blood tests rather than direct detection of the bacterium. Two categories of tests are used together. The first type looks for general markers of tissue damage and inflammation caused by the infection. The second type specifically detects antibodies your body produces against Treponema pallidum.
In the traditional approach, the general screening test is done first, and any positive result is confirmed with the more specific test. Many labs now use a “reverse” approach, starting with the specific test. Both methods agree about 99% of the time. The traditional approach can sometimes miss early or late latent infections, while the reverse approach may produce more false positives in populations where syphilis is uncommon.
Treatment and Antibiotic Resistance
Penicillin has been the standard treatment for syphilis since the 1940s, and it remains highly effective. For primary and secondary syphilis, a single injection is typically all that’s needed. Later stages require a longer course of treatment, usually weekly injections over several weeks.
One emerging concern is resistance to a class of antibiotics called macrolides, which are sometimes used as alternatives for people with penicillin allergies. Resistance is driven by specific mutations in the bacterium’s genetic material, and these mutations have appeared independently in multiple lineages of the organism. One resistant strain dates back to at least 1977. This resistance pattern has made macrolides unreliable as substitutes, reinforcing penicillin’s role as the first-line treatment.
Transmission From Mother to Child
Treponema pallidum can cross the placenta during pregnancy, infecting the fetus at any stage of the disease and during any trimester. The risk is highest when the mother has early-stage syphilis, with roughly a 50% chance of transmission. During latent syphilis, the risk drops to around 35%, but it never disappears entirely without treatment.
Congenital syphilis, the term for infection acquired before or during birth, can cause stillbirth, severe birth defects, and long-term developmental problems. Nearly 4,000 cases of congenital syphilis were reported in the United States in 2024, a figure that is 700% higher than it was a decade ago. Routine prenatal screening is the most effective way to catch and treat maternal infection before it reaches the baby.
Current Trends
After years of rising numbers, reported cases of primary and secondary syphilis in the U.S. declined for the second consecutive year in 2024, falling 22% compared to the previous year. That drop is encouraging, but the continued rise in congenital syphilis cases signals that infection is still reaching vulnerable populations who lack access to early prenatal care and testing. Globally, syphilis remains a major public health challenge, particularly in low-resource settings where screening and treatment are less available.