Bacterial causes of STIs

Bacterial sexually transmitted diseases are very important both clinically and historically. Syphilis has done more to shape the face of modern western life than any other disease and much of contemporary medicine evolved in the study of this infection.

After falling in incidence for many years, both gonorrhea and syphilis have been increasing in Canada in recent years. The fall was poorly understood considering that other STDs such as Herpes simplex, Chlamydia trachomatis and Human papilloma virus did not fall in incidence concomittantly.

Neisseria gonorrhoeae

Kidney bean shaped Gram negative cocci arranged in pairs (diplococci). There are many other species of Neisseria that may be found in the normal upper respiratory tract and vagina of humans. N. meningitidis is the only other clinically important species.

Disease is predominantly manifest by purulent urethritis in males and cervicitis or pelvic inflammatory disease (PID) in females. Both sexes can suffer from proctitis, pharyngitis and conjunctivitis and neonates are at risk for opthalmia neonatorum. Disseminated disease is a rare but serious complication that can present as septic polyarthritis.

Diagnosis is made by Gram stain and culture of genital secretions. In males, the finding of Gram negative, intracellular diplococci in urethral secretion is a very sensitive and specific indicator of N. gonorrhoeae infection.

In females, because of the possible presence of commensal Neisseria species, Gram stain findings are not as conclusive.

Culture may be difficult as these organisms are fastidious and require special enriched and selective media for their growth. They are also sensitive to cooling therefore genital specimens should not be refrigerated prior to transport to the laboratory. Fortunately, new Nucleic Acid Amplification tests (NATs) are now available and increase diagnostic sensitivity.

Susceptibility testing is important as resistance to many antimicrobials is increasing rapidly and differentially geographically making travel history important in assessing the likelihood of resistance.

Further Reading

Todar’s Pathogenic Neisseria

Treponema pallidum (Syphilis)

Treponema pallidum is one of many corkscrew shaped, extremely slender (1 mm) organisms referred to as spirochaetes. It has never been cultivated in vitro making culture confirmation of infection impossible, and study of the organism difficult.

Syphilis is relatively rare in Canada at present however incidence has been increasing in several urban centers in recent years. It is still important to be knowledgeable about this classic illness, especially its serologic diagnosis. Many screening tests are done and results can pose diagnostic dilemmas as discussed below.

Clinical Disease

Syphilis has been referred to as the great mimic. It may be responsible for a huge variety of clinical presentations and adult disease is divided into three stages:

Primary Syphilis
A painless chancre appears at the site of inoculation after an incubation period of 3 to 90 days (mean 21 days). Diagnosis is made by special microscopy referred to as “darkfield”, a technique that has higher resolution capacity than standard light microscopy and allows for visualization of the slender spirochaetes. The chancre heals spontaneously in 3 to 6 weeks and is often not recognized or is ignored.

Secondary Syphilis
This is the phase in which spirochaetes disseminate widely though the body and can cause numerous symptoms. Typically a patient presents with a florid rash that includes involvement of the palms and soles with accompanying constitutional signs and symptoms such as lymphadenopathy, fever and malaise.

Tertiary Syphilis
Neurosyphilis which often presents as dementia, cardiovascular syphilis which mainly affects the aorta, and Tabes dorsalis – a degenerative condition of the posterior columns of the spinal cord are the classic manifestations of late syphilis which may occur many years after initial infection. Approximately 1/3 of patients with untreated syphilis will go on to have tertiary syphilis.

Congenital Syphilis
Infection in utero can have catastrophic consequences. Fortunately, therapy during pregnancy can prevent transmission and is the basis for screening serology early in pregnancy. The classic clinical findings include saddle nose, saber shins and Hutchinson’s incisors and diagnosis is confirmed serologically.

Laboratory Diagnosis

T. pallidum cannot be cultured and therefore serology forms the mainstay of diagnosis apart from darkfield microscopy described above. There are two kinds of serology: non-treponemal and treponemal which will be described in some detail below.

Non-treponemal Tests
As the name implies, these tests measure antibodies that are not directed at the organism itself rather antigens of human origin that are liberated by infection. The antigens are ill defined lipids referred to as reagin and the tests as “reaginic” tests. They are inexpensive and useful as screening tests and also for following response to therapy as titres vary with disease activity.

Patients with active early syphilis generally have high titres. If a patient is successfully treated for syphilis their non-treponemal test reactivity usually disappears completely. Unfortunately, false positive reactions are fairly common and therefore Treponemal serology must confirm infection. Examples of non-treponemal tests include VDRL (venereal diseases research laboratory), RPR (rapid plasma reagin) and ART (automated reagin test).

Treponemal tests
These are tests that are no different than any other serologic test for antibodies to a microorganism. Those infected with T. pallidum develop antibodies and they are generally measurable for life. They are much more expensive than non-treponemal tests and are used as confirmatory assays. Examples include MHA (microhemaglutination) and FTA-ABS (Fluorescent treponemal antibody – absorbed).

Chancroid – Haemophilus ducreyi

Chancroid is an uncommon, but not unheard of in North America, painful ulcerative genital disease. It is caused by Haemophilus ducreyi and must be differentiated from syphilis. It is diagnosed by culture of the offending organism on special bacteriologic media. It is much more prevalent in warm climes and has been associated, particularly in Africa, with increased transmission rates of HIV, likely due to blood contact associated with lesions.

Bacterial Vaginosis

This is a common, ill-defined condition that is characterized by smelly vaginal discharge and lack of inflammation. Several organisms have been implicated in the genesis of this syndrome, most notably Gardnerella vaginalis. However, the finding of this organism is not diagnostic and its absence does not exclude this disease and most laboratories no longer culture for it. Diagnosis is based on typical clinical findings, in particular the characteristic odor, an increased pH of vaginal secretions (normally approx. 4.5 – the most acidic part of the body) that can be tested with litmus paper, and characteristic findings on Gram stain (decreased numbers of lactobacilli, increased numbers of other bacterial forms, absence of inflammatory cells). See Lab – Vaginal swab in adults.

Further Reading

CDC – Bacterial Vaginosis