Spirochetes, Mycoplasma and Chlamydia

5 DOH Abu Dhabi (HAAD) lab licensing practice questions on Spirochetes, Mycoplasma and Chlamydia, part of Microbiology. The first 5 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   Why is Mycoplasma pneumoniae not seen on a Gram stain of sputum?

   1. AIt is too thin to see by light microscopy
   2. BIt lacks a peptidoglycan cell wall
   3. CIts waxy wall resists the crystal violet
   4. DIt is an obligate intracellular parasite

   Show answerHide answer

   Answer: B · It lacks a peptidoglycan cell wall

   Mycoplasmas are the smallest free-living bacteria and have no peptidoglycan cell wall, only a sterol-containing membrane. They therefore do not hold the Gram stain, and they are naturally resistant to beta-lactam antibiotics. Diagnosis rests mainly on NAAT of a throat or nasopharyngeal swab, since culture on special media such as SP4 is slow. Unlike chlamydiae, they are not obligate intracellular parasites.

   • A: Being too thin for light microscopy describes Treponema pallidum.
   • C: A waxy, lipid-rich wall is a feature of mycobacteria.
   • D: Obligate intracellular growth describes Chlamydia, not Mycoplasma.

2. Question 2 · Medium

   A man has persistent urethritis after a course of doxycycline. A NAAT detects Mycoplasma genitalium. The most useful further test before choosing treatment is:

   1. ACulture on SP4 medium for susceptibility
   2. BMacrolide resistance mutation testing
   3. CRepeat Gram stain for intracellular diplococci
   4. DSerology for M. genitalium IgG

   Show answerHide answer

   Answer: B · Macrolide resistance mutation testing

   M. genitalium is diagnosed only by NAAT, because culture takes weeks to months and is not available clinically. Macrolide resistance from 23S rRNA gene mutations is common, so CDC recommends resistance-guided therapy where testing is available. Treatment usually starts with doxycycline, followed by azithromycin if the strain is macrolide susceptible or moxifloxacin if it is resistant. Serology has no role in diagnosis.

   • A: Culture of M. genitalium takes weeks to months and is not used clinically.
   • C: A Gram stain looks for gonorrhoea and does not guide this choice.
   • D: Serology does not diagnose M. genitalium infection.

3. Question 3 · Hard

   A man who has sex with men has rectal pain, bloody discharge and tender inguinal nodes. A rectal swab is positive for Chlamydia trachomatis by NAAT. What should the laboratory do to confirm suspected lymphogranuloma venereum?

   1. ASend the sample for LGV-specific (L1-L3) PCR
   2. BCulture the swab on McCoy cells only
   3. CRepeat the same NAAT on a urine specimen
   4. DMeasure chlamydial IgM in serum

   Show answerHide answer

   Answer: A · Send the sample for LGV-specific (L1-L3) PCR

   Routine C. trachomatis NAATs detect all serovars but do not separate LGV strains (serovars L1 to L3) from the common genital serovars D to K. LGV proctitis needs a longer course of doxycycline, so a positive rectal sample from a patient with this picture is sent for an LGV-specific PCR, such as one targeting the pmpH gene. Chlamydial serology is not specific enough to confirm LGV, and culture is slow and insensitive. Presumptive treatment should not wait for the genotyping result.

   • B: Culture is slow and insensitive and does not identify the serovar.
   • C: A urine NAAT would not show the serovar and can miss rectal infection.
   • D: Chlamydial serology cannot reliably confirm LGV.

4. Question 4 · Easy

   For Chlamydia trachomatis NAAT in a man, the preferred urine specimen is:

   1. AMidstream clean-catch urine
   2. BFirst-catch urine, the first part of the stream
   3. CUrine collected after prostate massage
   4. DA 24-hour urine collection

   Show answerHide answer

   Answer: B · First-catch urine, the first part of the stream

   Chlamydia infects urethral epithelial cells, and the first part of the urine stream washes these cells out. NAATs on first-catch urine are about as sensitive as urethral swabs in men and are non-invasive. Midstream urine comes after the urethra has been flushed and so contains fewer organisms. Many kits ask that the patient has not passed urine for at least an hour before collection.

   • A: Midstream urine contains fewer urethral cells and organisms.
   • C: Prostate massage is not used for chlamydia NAAT.
   • D: A 24-hour collection dilutes the organisms and is not validated.

5. Question 5 · Medium

   A farm worker has had high fever, myalgia and conjunctival suffusion for 3 days. Leptospirosis is suspected. Which test is most likely to confirm it at this stage?

   1. APCR on whole blood or plasma
   2. BIgM ELISA on serum
   3. CDarkfield microscopy of urine
   4. DMicroscopic agglutination test on serum

   Show answerHide answer

   Answer: A · PCR on whole blood or plasma

   In the first week of illness leptospires circulate in the blood, while antibodies are usually not yet detectable. PCR on blood, and on CSF if indicated, is therefore the most useful test early; from the second week organisms appear in urine. IgM ELISA usually turns positive after about 5 to 7 days, and the microscopic agglutination test, the reference serological method, needs paired sera showing a rise in titre. Darkfield microscopy is insensitive and prone to artefacts that look like leptospires.

   • B: IgM usually becomes detectable only after about 5 to 7 days.
   • C: Darkfield microscopy is insensitive, and artefacts mimic leptospires.
   • D: MAT antibodies are usually not detectable this early, and paired sera are needed.