STI Pathogens and Diagnostic Tests
Start from one idea: a diagnostic test only works if it matches how the organism lives. Everything in this chapter is built from that single principle. A bacterium that grows only inside human cells cannot be cultured on a plate. A spirochaete that has never been grown reliably in the laboratory can only be diagnosed indirectly, through the antibody the host makes against it. A virus that hides silently in a nerve ganglion will not be found by any test once the visible lesion has crusted over. The biology of the pathogen dictates the assay, the timing of the assay, and how you must read a negative result. If you understand the biology first, the test choices stop being a list to memorise and become predictable.
From that first principle, two consequences follow that organise the whole chapter. First, STI microbiology is site-specific: a pathogen that infects the columnar cells of the endocervix behaves differently from one that ulcerates squamous skin, one that lives free in vaginal fluid, one that circulates as a systemic viraemia, and one that quietly drives cervical carcinogenesis over years. Second, the specialist must connect five elements in a chain — organism, anatomical site, syndrome, test, and consequence. Naming Chlamydia trachomatis is trivia; knowing that it is an obligate intracellular parasite of columnar epithelium, that it ascends to cause salpingitis, that this is why a vaginal swab can carry it but a culture plate cannot grow it, and that the consequence is tubal infertility and ectopic pregnancy — that is specialist reasoning.
South African practice adds a second layer on top of the biology. Much primary-care STI treatment is syndromic: the patient is treated at the first visit for the likely pathogens of a recognised symptom group, because same-day treatment matters where laboratory access and follow-up are limited. Specialist-level reasoning still requires knowing which pathogen the syndrome is trying to cover, when laboratory testing genuinely improves care, and when a negative test must not falsely reassure. We will build up to that, but only after the underlying microbiology is in place.
How STI Pathogens Live — the Biology That Drives the Test
Before any syndrome or test makes sense, you need to know what these organisms are and how they survive. STI pathogens are not a random list; they fall into a few biological families, and each family's lifestyle predicts exactly which test will and will not work.
Bacteria that cannot live outside a human cell. Chlamydia trachomatis is an obligate intracellular bacterium — it has no independent metabolism for energy and must live inside a host cell to replicate. It does this through a two-form life cycle. The infectious form, the elementary body (EB), is small, metabolically inert and tough enough to survive briefly outside cells; it is taken up into a host cell where it transforms into the larger, metabolically active reticulate body (RB). The RB divides repeatedly inside a membrane-bound inclusion, then the daughters condense back into EBs and are released to infect new cells, the whole cycle taking roughly two days. Chlamydia actively prevents the host cell from fusing lysosomes onto the inclusion, so the cell shelters rather than destroys it. Two facts for the clinic follow directly: chlamydia cannot be grown on artificial media (it needs living cells, which is why culture was abandoned for routine diagnosis in favour of nucleic acid detection), and because it parasitises columnar epithelium, the right specimen is from the endocervix or a site lined by columnar cells, not the squamous vaginal wall.
Bacteria that are simply hard to keep alive. Neisseria gonorrhoeae is a Gram-negative diplococcus that, like chlamydia, infects columnar cells of the cervix and urethra rather than the squamous vagina — it is a parasite of the endocervix, not the vagina. It is fastidious: it dies quickly outside its warm, moist niche, so a swab left in the wrong medium or transported slowly yields a falsely negative culture. This single property explains why gonococcal specimens must be plated at the bedside or transported fast and cold-chain-correct, and why a negative gonococcal culture after poor transport tells you nothing. Importantly, seeing Gram-negative intracellular diplococci on a smear is a clue, not an identification — microscopy can never speciate, so such cocci must never be reported as gonorrhoea until confirmed.
A spirochaete that has never been reliably cultured. Treponema pallidum, the cause of syphilis, cannot be grown on laboratory media and is serologically indistinguishable from the spirochaetes of yaws and pinta. This is why the laboratory can only provide evidence of treponemal infection — past or present — and cannot, by itself, "diagnose syphilis." The practical rule that flows from this biology is unforgiving: if there is genuine doubt about the cause of treponemal antibodies, especially in pregnancy, the patient must be assumed to have active syphilis and treated. The organism evades the immune system in part by coating itself in host proteins such as fibronectin, which is one reason infection can smoulder for years.
Bacteria with no cell wall. The mycoplasmas, including Mycoplasma genitalium, are unique among bacteria in lacking a rigid peptidoglycan cell wall. This is not laboratory trivia: it means cell-wall-active antibiotics such as the penicillins and cephalosporins have no target and are intrinsically useless against them, and it makes the organism difficult to culture, so detection rests on molecular methods. M. genitalium is a recognised cause of persistent urethritis and cervicitis and contributes to PID; its growing resistance to first-line agents is the reason it is increasingly tested for rather than assumed covered by syndromic treatment. The related genital mollicutes Ureaplasma urealyticum and Mycoplasma hominis are common colonisers of sexually active women and have been recovered from chorioamnionitis, though their causal role in disease is less clear.
A protozoan that swims. Trichomonas vaginalis is a flagellated protozoan that lives free in vaginal and urethral fluid. Because it is motile, a fresh wet-mount specimen examined immediately can show the characteristic darting trichomonads — but the parasite stops moving and becomes hard to recognise within minutes, so a delayed slide loses sensitivity. This is why molecular testing now outperforms microscopy for trichomoniasis.
Viruses, which are not cells at all. Viruses consist essentially of a nucleic acid genome inside a protein coat, sometimes with an outer envelope, and they can only replicate inside host cells. They carry either DNA or RNA, never both, and that genome type, together with the envelope, determines their behaviour and the test used:
- Herpes simplex viruses (HSV-1 and HSV-2) are enveloped double-stranded DNA viruses that establish lifelong latency in sensory nerve ganglia and reactivate intermittently. A primary (first-ever) genital infection produces a high viral load shed over a long period and carries a far higher transmission risk than a recurrent episode, where shedding is briefer. This latency-and-reactivation biology is exactly why a lesion swab is most sensitive when the lesion is fresh and vesicular, and why sensitivity collapses as the ulcer crusts and heals.
- Human papillomavirus (HPV) is a non-enveloped DNA virus. Of the many genital types, the low-risk types (such as 6 and 11) cause warts but rarely cancer, while high-risk oncogenic types (such as 16 and 18) drive premalignant and malignant cervical disease. The mechanism is specific and worth knowing: high-risk HPV produces the oncoproteins E6 and E7, which bind and inactivate the host tumour-suppressor proteins p53 and pRB respectively, removing the brakes on the cell cycle and allowing accumulation of the genetic damage that progresses to cancer. The diagnostic goal for HPV is therefore not "cure an infection" but "detect persistent oncogenic infection and the cellular change it causes."
- HIV is a retrovirus — an RNA virus that reverse-transcribes its genome into the host DNA — which is why it cannot be cleared and why its key microbiological marker is the viral load (the amount of circulating virus), not the antibody alone.
- Hepatitis B (HBV) is a DNA virus and hepatitis C (HCV) is an RNA virus; both are bloodborne and sexually transmissible, and the difference between detecting host antibody and detecting viral genome is central to interpreting their tests.