Spirochetes, Rickettsiae, Chlamydiae & Mycoplasma Guide

Spirochetes are corkscrew bacteria too thin for the Gram stain, rickettsiae are obligate intracellular bacteria spread by ticks and lice, chlamydiae are obligate intracellular bacteria with a two-form life cycle, and mycoplasmas are bacteria with no cell wall at all. Together they cause syphilis, typhus, chlamydial infections and atypical pneumonia

This matters because these "atypical" organisms demand atypical thinking: syphilis is diagnosed by blood tests, not culture; rickettsial fevers are treated with doxycycline even in children; chlamydia needs a drug that enters human cells; and mycoplasma laughs at beta-lactams because there is no wall to attack.

Key Concepts

• Treponema pallidum: the syphilis spirochete; seen by dark-ground microscopy from a chancre; stages run from painless chancre to secondary rash to latency to tertiary gummas and neurosyphilis.
• Syphilis serology: non-treponemal tests (VDRL, RPR) screen and their titres track treatment; treponemal tests (TPHA, FTA-ABS) confirm and usually stay positive for life.
• Borrelia: Borrelia burgdorferi causes Lyme disease (erythema migrans rash, then joints, heart and nerves); relapsing-fever Borrelia is louse- or tick-borne.
• Rickettsiae: obligate intracellular, arthropod-borne; Rocky Mountain spotted fever, epidemic typhus and scrub typhus (Orientia tsutsugamushi, with an eschar)
• Chlamydiae: obligate intracellular with infectious elementary bodies and replicating reticulate bodies; C. trachomatis causes trachoma and genital infection, C. pneumoniae causes atypical pneumonia, C. psittaci comes from birds.
• Mycoplasma: no cell wall, so intrinsically resistant to penicillins and cephalosporins; smallest free-living organisms; M. pneumoniae causes walking pneumonia with cold agglutinins; M. genitalium causes urethritis.

How It Works

1. A patient with suspected syphilis is screened with RPR or VDRL, which detect anti-lipid antibodies.
2. A reactive screen is confirmed with a treponemal test such as TPHA, which detects antibodies against the organism itself.
3. The non-treponemal test is reported as a titre (1:2, 1:4, 1:8 and so on), which reflects disease activity.
4. After benzathine penicillin treatment, the titre is repeated at intervals.
5. A fourfold (two-dilution) fall in titre indicates successful treatment; a sustained fourfold rise signals reinfection or failure.

Worked Example

A young man presents with a painless genital ulcer. Step 2: RPR is reactive at a titre of 1:64 and TPHA is positive, confirming early syphilis. Step 3: he receives benzathine penicillin 2.4 million units intramuscularly as a single dose. Step 4: at 6 months the RPR is 1:16. Step 5: from 1:64 to 1:32 is one dilution (twofold), and 1:32 to 1:16 is a second dilution, giving a total fourfold decline, which meets the criterion for treatment success. If the titre had instead risen to 1:128, that fourfold rise would have demanded investigation for reinfection.

Common Misconceptions

• "A negative VDRL rules out syphilis." Non-treponemal tests can be negative in very early, latent or late disease and in the prozone phenomenon;
• "Rickettsial fevers occur only in forests." Scrub typhus and murine typhus strike in villages, farms and urban edges wherever mites, fleas and rodents thrive.
• "Chlamydia always causes obvious symptoms." Most genital chlamydial infections are silent, especially in women, yet they cause infertility, which is why screening and partner treatment matter.

Real-World Applications

• Antenatal screening: RPR testing of every pregnant woman prevents congenital syphilis.
• Fever workup: doxycycline 100 mg twice daily is the drug of choice for all rickettsial fevers, including in children with suspected Rocky Mountain spotted fever.
• STI control: single-dose azithromycin or doxycycline for chlamydia, with partner notification.
• Community pneumonia: adding a macrolide or doxycycline covers Mycoplasma and Chlamydia pneumoniae.

Spirochetes, rickettsiae, chlamydiae and mycoplasmas break the usual rules of staining, culture and penicillin susceptibility. Syphilis is tracked by titres, rickettsiae by vectors and doxycycline, chlamydiae by their intracellular life, and mycoplasmas by their missing wall.