Microbiology Cheat Sheet
Sti Patterns & Specimens
STI patterns and specimens for NEET-PG: gonorrhoea NAAT vs culture, chlamydia EB and RB, syphilis VDRL prozone, chancroid, donovanosis, and LGV groove sign.
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Sti Patterns & Specimens
STI patterns and specimens for NEET-PG: gonorrhoea NAAT vs culture, chlamydia EB and RB, syphilis VDRL prozone, chancroid, donovanosis, and LGV groove sign.
This topic covers the etiopathogenesis, clinical patterns, specimen collection, and laboratory diagnosis of sexually transmitted infections including genital ulcer and discharge syndromes.
High-yield lines
- Neisseria gonorrhoeae is a Gram-negative kidney-bean diplococcus that ferments only glucose, distinguishing it from N. meningitidis which also ferments maltose.
- Intracellular Gram-negative diplococci in urethral polymorphs in a symptomatic male are over 95 percent sensitive for gonorrhoea.
- NAAT is the diagnostic gold standard for gonorrhoea and chlamydia at all sites, while culture remains essential for antimicrobial susceptibility testing.
- Chlamydia trachomatis is an obligate intracellular energy parasite with an elementary body (infectious) and a reticulate body (replicating), not diagnosed by Gram stain.
- Chlamydia is the most common bacterial STI globally and the leading cause of non-gonococcal urethritis.
- Treponema pallidum cannot be cultured in vitro and is visualised by darkfield microscopy of chancre exudate in primary syphilis.
- Non-treponemal tests (VDRL, RPR) are used for screening, disease activity, and treatment monitoring, while treponemal tests (TPHA, FTA-ABS) confirm and remain positive for life.
- The prozone phenomenon causes a false-negative VDRL or RPR in high-titre secondary syphilis, corrected by testing serial dilutions.
- A painless indurated genital ulcer suggests primary syphilis, while a painful ulcer with a tender suppurative bubo suggests chancroid or herpes.
- Chancroid is caused by Haemophilus ducreyi, which requires X factor but not V factor and shows a school-of-fish pattern on Gram stain.
- Donovanosis (Klebsiella granulomatis) shows painless beefy-red ulcers with Donovan bodies as bipolar safety-pin inclusions in macrophages.
- The LGV groove sign is inguinal lymphadenopathy above and below the inguinal ligament, caused by C. trachomatis serovars L1 to L3.
- HPV E6 degrades p53 and E7 inactivates Rb; types 16 and 18 cause about 70 percent of cervical cancers and 6 and 11 cause most genital warts.
- Genital herpes is diagnosed by lesion PCR, while the Tzanck smear shows multinucleated giant cells but cannot distinguish HSV from VZV.
Mapped competency codes
- MI7.2
- MI7.3
Continue into the full chapter
This summary maps to MI7-genitourinary-and-sexually-transmitted-infections.
Frequently Asked Questions
How is gonorrhoea diagnosed and why is culture still needed?
NAAT is the sensitive diagnostic anchor at all sites, but culture is essential because it alone provides antimicrobial susceptibility information.
Why can a VDRL be falsely negative in secondary syphilis?
The prozone phenomenon from antibody excess in high-titre secondary syphilis; testing serial dilutions corrects it.
Which genital ulcers are painless versus painful?
Syphilis chancre, donovanosis, and early LGV are painless, while chancroid and herpes are painful.
How is chlamydia diagnosed?
By NAAT from urine or site-specific swabs, since Chlamydia is an obligate intracellular organism not visible on Gram stain.
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