Common Bacteria, Fungi & Parasites Causing Sexually Transmitted Infections

Overview of Sexually Transmitted Infections (STIs)

  • >>More than 30 distinct bacteria, viruses & parasites transmitted via sexual activity

  • >>Daily, 1\text{ million} curable STIs acquired world-wide (age 15\text{–}49)

  • Eight pathogens account for greatest disease burden (WHO 2016)

    • Curable: Syphilis, Gonorrhoea, Chlamydia, Trichomoniasis

    • Incurable: Hepatitis B, Herpes, HIV, HPV

  • WHO 2022 estimates

    • 8\text{ million} adults (15–49 y) acquired syphilis

    • 1.1\text{ million} pregnant women infected → >390\,000 adverse births

Bacterial STIs

• Treponema pallidum (Syphilis)
• Neisseria gonorrhoeae (Gonorrhoea)
• Chlamydia trachomatis (Chlamydia, LGV)
• Haemophilus ducreyi (Chancroid)
• Klebsiella granulomatis (Granuloma inguinale)
• Chlamydia trachomatis serovars L1-L3 (LGV)
• Gardnerella vaginalis (Bacterial vaginosis – linked to sexual activity)

Fungal Conditions Associated With Sex

• Tinea cruris (Jock itch) – Trichophyton rubrum, Epidermophyton floccosum
• Genital Candidiasis – Candida albicans (opportunistic; not strictly STI)

Parasitic Conditions

• Trichomoniasis – Trichomonas vaginalis
• Pediculosis pubis – Pthirus pubis
• Scabies – Sarcoptes scabiei (rarely sexual-only)

Core Laboratory Procedures

  • Dark-field microscopy → Syphilis spirochetes

  • Gram stain → Gonorrhoea, NGU, Chancroid, BV

  • Wet mount → Trichomonas vaginalis

  • 10\% KOH mount → Candidiasis

  • Bubo aspiration/smear → LGV, Chancroid

  • NAATs, serology, culture per organism (details below)

Syphilis (Treponema pallidum ssp. pallidum)

Biology

  • Spirochaete: 6\text{–}15\,\mu m long, 0.25\,\mu m wide; microaerophile; fragile ex vivo

  • Few/no surface proteins; coats itself with host fibronectin → immune evasion

Transmission & Course

  • Sexual, trans-placental, transfusion, direct skin breach

  • Incubation 10\text{–}90 days (avg 21)

  • Untreated progression: Primary → Secondary → Latent (early, late) → Tertiary

  • Treat: single IM benzathine penicillin G (alt: doxycycline, macrolide, tetracycline)

Pathogenesis Highlights

  • “Great imitator”; highly invasive, low toxin production

  • Endoflagella give motility → endothelial adherence

  • Host reaction → endarteritis + plasma-cell infiltrate ⇢ tissue damage

  • Tertiary gumma = type-IV hypersensitivity (sensitized T-cells, macrophages)

  • Outer sheath lacks immunogens; TH1 down-regulated; antibodies vs cardiolipin, etc.

Clinical Phases

  1. Primary – painless chancre; rich in organisms; heals 4–8 wk

  2. Secondary – hematogenous spread → maculopapular rash (palms/soles), condylomata lata, fever, lymphadenopathy; most infectious

  3. Latent – sero-positive w/o signs; early (<1 y) still infectious; late mainly non-infectious but vertical spread possible

  4. Tertiary (yrs 1–30) – cardiovascular (aortitis), neuro-syphilis, gummas

Laboratory Diagnosis

  • Dark-field microscopy (motility); silver stain (Warthin-Starry, Steiner)

  • Serology

    • Nontreponemal: VDRL, RPR (titres; four-fold change significant)

    • Treponemal: FTA-ABS, TPHA/TPPA, AIA

    • Seroconversion ≈ 3\text{–}6 wk; follow-up VDRL at 1,3,6,12 mo post-therapy

Gonorrhoea (Neisseria gonorrhoeae)

Biology & Growth

  • Gram-negative, oxidase+ diplococcus; facultative anaerobe; intracellular in PMNs

  • Requires 5\% CO₂; cultured on Thayer–Martin VPN or chocolate agar

Pathogenesis

  • Type IV pili → initial adherence & antigenic variation (minicassettes)

  • Opa (P.II) proteins → tight binding, invasion, microcolony, phase variation

  • Porin P.I → pore formation, anti-phagocyte

  • LOS → TNFα release, strong neutrophil response (purulent exudate)

  • Sialylation of LOS → serum resistance, complement evasion

  • IgA1 protease, iron uptake from transferrin/lactoferrin

Clinical

  • Men: urethritis (purulent discharge, dysuria); 10\% asymptomatic

  • Women: cervicitis, urethritis; \tfrac{1}{3} asymptomatic; PID, ectopic pregnancy

  • Disseminated Gonococcal Infection (DGI) 1\text{–}2\% → arthritis-dermatitis, septic arthritis (knee)

  • Neonatal transmission → ophthalmia neonatorum

Treatment & Resistance

  • WHO surveillance: high quinolone resistance; rising azithromycin; emerging ESC resistance

  • CDC/MOH: single IM 250–500 mg ceftriaxone + azithro 1 g PO (cover Chlamydia)

Specimen & Diagnosis

  • Site-dependent swabs + first-void urine (NAAT)

  • Gram stain of male urethral pus: intracellular diplococci ★ rapid

  • Culture = gold standard (Thayer–Martin, 35–37^\circ\text{C}, 3\%–5\% CO₂)

  • NAAT (PCR/LCR), DNA probe ↑Sensitivity; blood/synovial culture for DGI

Chlamydia trachomatis

Biology / Life-Cycle

  • Obligate intracellular; biphasic life-cycle

    1. Elementary body (EB) – infectious, metabolically inactive, disulfide-cross-linked wall

    2. Reticulate body (RB) – non-infectious, metabolically active, replicates every \approx 3 h inside inclusion; cycles 7–21 d

  • Cannot synthesise ATP – “energy parasite”

  • Three biovars → serovars:
    • Trachoma (A–C)
    • Genital tract (D–K)
    • LGV (L1–L3)

Clinical Spectrum

  • Genital chlamydia most common bacterial STI; 70\text{–}80\% women, 50\% men asymptomatic

  • Sequelae: PID, infertility, ectopic pregnancy, chronic pelvic pain, neonatal conjunctivitis/ pneumonia, HIV facilitation, cervical cancer link

  • LGV: transient genital ulcer → painful inguinal/femoral nodes (buboes), fistulae; ↑in HIV+ MSM

Diagnosis

  • First-catch urine or self-collected vaginal swab NAAT = sensitivity >95\% / specificity >99\%

  • Culture in McCoy cells (columnar epithelium) only \approx50\% sensitive

  • Cytology: inclusion bodies (Giemsa); serology for LGV

Therapy

  • Uncomplicated GTI: doxycycline 100 mg bid 7 d OR azithro 1 g single

  • LGV: erythro or doxy ≥21 d

Other Bacterial Ulcerative STIs

• Granuloma inguinale (Klebsiella granulomatis)

  • Painless, beefy-red ulcers; Donovan bodies on smear (Giemsa)

  • Treat doxy/azithro ≥3 wk until healed
    • Chancroid (Haemophilus ducreyi)

  • Painful soft chancre, serpiginous, purulent; unilateral tender nodes

  • Gram “school-of-fish” coccobacilli; culture on chocolate; treat azithro single dose or ceftriaxone IM

Fungal & Dermatophyte Conditions

Tinea cruris

  • Dermatophytes T. rubrum, E. floccosum → keratinase invasion of stratum corneum

  • Risk ↑ with moist/tight clothing; fomite spread; topical imidazole/allylamine

Candidiasis (Candida albicans)

  • Dimorphic yeast; commensal → opportunistic in immunosuppressed, diabetic, antibiotics, pregnancy

  • Genital ↓: cottage-cheese discharge; pruritus; balanitis in men

  • Culture: Sabouraud dextrose agar; CHROMagar speciation; treat topical azoles, nystatin (systemic fluconazole prn)

Parasitic STIs

Trichomoniasis

  • Trichomonas vaginalis trophozoites (flagellated) observed in wet mount

  • Frothy, foul discharge; strawberry cervix; treat metronidazole/tinidazole (both partners)

Pediculosis pubis & Scabies

  • Pthirus pubis – pruritic pubic lice; permethrin, malathion, nit combing

  • Sarcoptes scabiei – burrowing mites; intense nocturnal itch (type IV HS); topical permethrin or ivermectin PO

Pelvic Inflammatory Disease (PID) & Related Syndromes

  • Upper GT infection: endometritis, salpingitis, tubo-ovarian abscess, pelvic peritonitis

  • Polymicrobial 30\text{–}40\%; key agents N. gonorrhoeae, C. trachomatis, anaerobes (Bacteroides, Peptococcus), G. vaginalis, streptococci, H. influenzae

  • Endometritis postpartum/abortion – mixed vaginal flora; historically S. pyogenes

  • Epididymitis <35 y: N. gonorrhoeae, C. trachomatis; ≥35 y: Enterobacteriaceae, staphylococci, corynebacteria

Epidemiology & Statistics

  • Incident curable STIs, adults 2020 (WHO):
    • African Region 96\text{ M}
    • Americas 74\text{ M}
    • Europe 23\text{ M}
    • Eastern Mediterranean 36\text{ M}
    • South-East Asia 60\text{ M}
    • Western Pacific 86\text{ M}
    • Global total 374\text{ M}

  • Global 2016 new cases (Rowley):
    • Chlamydia 127\text{ M}
    • Gonorrhoea 87\text{ M}
    • Syphilis 6\text{ M}
    • Trichomoniasis 156\text{ M}

  • USA 2008 total prevalent STIs \approx 110\text{ M}; HPV highest 79\text{ M}

  • Malaysia studies
    • Nordin 2001: Syphilis 50.8\%, Trichomoniasis 19.2\%, Gonorrhoea 8.5\%
    • HKL 2013–17: Genital warts 26.7–45.6\% ; Herpes 11.8–31.5\% ; Syphilis ≈19\%; Gonorrhoea \le 12.2\%; Chlamydia 5.4\%; Trichomoniasis 0.2\%

Ethical & Practical Considerations

  • Asymptomatic carriage (e.g., Chlamydia, Gonorrhoea) drives silent transmission ⇒ routine screening in sexually active <25 y & pregnancy

  • Antimicrobial resistance (gonococci) demands stewardship, surveillance & novel therapeutics/vaccines

  • Congenital infections (syphilis, chlamydia, gonorrhoea) highlight importance of antenatal screening & treatment

  • Social stigma may delay care; patient education & confidentiality critical

Study Connections & Exam Tips

  • Compare pathogenesis strategies: stealth (T. pallidum) vs inflammatory overload (N. gonorrhoeae)

  • Recognise laboratory gold standards & rapid tests; match each pathogen with specimen type & media

  • Memorise treatment first-line drugs + resistance caveats

  • Link PID complications to infertility & ectopic risk – high-yield clinical scenario

  • Remember dermatological mimicry: painless (syphilis) vs painful (chancroid); indurated vs soft ulcer

  • Be able to sketch Chlamydia life-cycle & identify diagnostic window for serology vs NAAT

Selected References (See full list in transcript)

  • Rowley J. et al. Global & regional STI estimates 2016 — WHO

  • LaFond & Lukehart 2006. Biological basis for syphilis — CMR

  • CDC, WHO surveillance reports (Gonococcal AMR, syphilis maps)