CSF from a 20-year-old man with fever and a petechial rash shows gram-negative diplococci inside neutrophils. When the culture grows, a technologist makes a dense saline suspension of the colonies for latex agglutination and does oxidase and catalase tests on the open bench, without a biosafety cabinet. The isolate is oxidase positive, produces acid from glucose and maltose but not from lactose or sucrose, and is confirmed as Neisseria meningitidis serogroup W. The technologist is well and received a MenACWY vaccine 6 years ago. What is the most appropriate action for the technologist?

Offer antibiotic chemoprophylaxis now

No action, as the vaccine protects fully

Give a MenACWY booster as the only step

Take a throat swab; treat only if positive

A blood culture isolate is confirmed as Neisseria meningitidis. Which further test on the isolate is most useful for public health purposes?

Serogrouping by agglutination or PCR

Repeating the carbohydrate utilization tests

A vancomycin MIC by broth microdilution

Beta-lactamase testing with a nitrocefin disk

Genital specimens for Neisseria gonorrheae culture are plated on a non-selective chocolate agar as well as on modified Thayer-Martin agar. The reason is that:

Some strains are inhibited by the vancomycin in the selective medium

Colonies grow larger and faster on the selective medium

Chocolate agar makes the sugar reactions easier to read

The selective medium cannot support growth in carbon dioxide

A 15-year-old has pharyngitis, and an oxidase-positive gram-negative diplococcus grows from the throat swab on selective agar. Why must a throat isolate be identified by two different confirmatory methods before Neisseria gonorrheae is reported?

Commensal Neisseria and Moraxella grow on selective media too

Pharyngeal isolates grow too slowly for a single test

Selective media change the sugar reactions of the pathogen

A single enzyme test cannot be performed on throat specimens

Neisseria and Moraxella

10 DOH Abu Dhabi (HAAD) lab licensing practice questions on Neisseria and Moraxella, part of Microbiology. The first 10 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
CSF from a 20-year-old man with fever and a petechial rash shows gram-negative diplococci inside neutrophils. When the culture grows, a technologist makes a dense saline suspension of the colonies for latex agglutination and does oxidase and catalase tests on the open bench, without a biosafety cabinet. The isolate is oxidase positive, produces acid from glucose and maltose but not from lactose or sucrose, and is confirmed as Neisseria meningitidis serogroup W. The technologist is well and received a MenACWY vaccine 6 years ago. What is the most appropriate action for the technologist?
1. ANo action, as the vaccine protects fully
2. BOffer antibiotic chemoprophylaxis now
3. CGive a MenACWY booster as the only step
4. DTake a throat swab; treat only if positive
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Answer: B · Offer antibiotic chemoprophylaxis now
The Gram stain and the glucose and maltose reactions identify N. meningitidis, and making dense suspensions and doing tests outside a cabinet can expose the worker to droplets and aerosols. Laboratory-acquired meningococcal disease is rare but often fatal, so an exposed laboratorian should be offered chemoprophylaxis (for example ciprofloxacin, rifampin or ceftriaxone) without delay. Vaccination does not remove this need, because protection wanes over time and vaccines do not cover every serogroup. A throat culture is insensitive and waiting for it delays protection, and a booster alone acts too slowly. Work with suspected meningococcal isolates belongs in a class II biosafety cabinet.
- A: Protection from a vaccine given 6 years ago wanes, and vaccination does not replace prophylaxis.
- C: A booster takes days to act and does not replace prophylaxis after an exposure.
- D: A throat culture is insensitive, and waiting for it delays protection.
Question 2 · Medium
A blood culture isolate is confirmed as Neisseria meningitidis. Which further test on the isolate is most useful for public health purposes?
1. ASerogrouping by agglutination or PCR
2. BRepeating the carbohydrate utilization tests
3. CA vancomycin MIC by broth microdilution
4. DBeta-lactamase testing with a nitrocefin disk
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Answer: A · Serogrouping by agglutination or PCR
The serogroup is set by the capsular polysaccharide and decides which vaccine protects contacts and travellers, so it is the information public health teams need. Serogrouping is done by slide agglutination with grouping sera or, more reliably, by PCR of the capsule genes. Beta-lactamase production is rare in meningococci and penicillin remains active for most isolates. Repeating the sugars only re-confirms what is already known.
- B: The sugar reactions have already identified the isolate.
- C: Vancomycin is not used to treat meningococcal disease.
- D: Beta-lactamase is rare in meningococci and adds little.
Question 3 · Hard
Genital specimens for Neisseria gonorrheae culture are plated on a non-selective chocolate agar as well as on modified Thayer-Martin agar. The reason is that:
1. AColonies grow larger and faster on the selective medium
2. BChocolate agar makes the sugar reactions easier to read
3. CSome strains are inhibited by the vancomycin in the selective medium
4. DThe selective medium cannot support growth in carbon dioxide
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Answer: C · Some strains are inhibited by the vancomycin in the selective medium
A minority of gonococcal strains are susceptible to the vancomycin used to suppress gram-positive flora, so they fail to grow on modified Thayer-Martin or Martin-Lewis agar. A parallel chocolate plate recovers these strains, at the cost of also growing commensal flora. Both plates are incubated at 35 to 37 °C in 5% carbon dioxide and read at 24 and 48 hours. Genital specimens from sites with heavy flora are still plated on the selective medium, because growth there would otherwise be overgrown.
- A: Colony size is not the issue; the selective agents may prevent growth altogether.
- B: Sugar reactions are performed on the isolate, not read from the primary plate.
- D: Selective media support growth in carbon dioxide perfectly well.
Question 4 · Medium
A gram-negative diplococcus from a blood culture is oxidase positive and hydrolyzes gamma-glutamyl-beta-naphthylamide. This result supports:
1. ANeisseria lactamica
2. BMoraxella catarrhalis
3. CNeisseria meningitidis
4. DNeisseria gonorrheae
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Answer: C · Neisseria meningitidis
Gamma-glutamyl aminopeptidase activity is characteristic of N. meningitidis and is the basis of the enzyme-substrate identification kits. N. gonorrheae instead has hydroxyprolyl aminopeptidase, and N. lactamica has beta-galactosidase, detected with ONPG. These three enzyme reactions identify the pathogenic species within minutes of growth. The result should agree with the sugar reactions before the identification is reported.
- A: N. lactamica is identified by beta-galactosidase (ONPG) activity.
- B: M. catarrhalis lacks this enzyme and is butyrate esterase positive.
- D: N. gonorrheae has hydroxyprolyl aminopeptidase instead.
Question 5 · Hard
A 15-year-old has pharyngitis, and an oxidase-positive gram-negative diplococcus grows from the throat swab on selective agar. Why must a throat isolate be identified by two different confirmatory methods before Neisseria gonorrheae is reported?
1. APharyngeal isolates grow too slowly for a single test
2. BCommensal Neisseria and Moraxella grow on selective media too
3. CSelective media change the sugar reactions of the pathogen
4. DA single enzyme test cannot be performed on throat specimens
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Answer: B · Commensal Neisseria and Moraxella grow on selective media too
Vancomycin, colistin, nystatin and trimethoprim do not suppress every commensal, so N. cinerea, N. lactamica, N. subflava and M. catarrhalis can all appear on a selective plate. Misreporting pharyngeal gonorrhea has serious personal and public health consequences, so two methods based on different principles, such as sugar utilization plus an enzyme or antigen test, are used. Some strains of the pathogen are also inhibited by the vancomycin in the medium, which is why a chocolate agar plate is set up as well. Molecular confirmation may be needed for extragenital specimens.
- A: Growth rate is not the reason; the problem is specificity.
- C: Selective agents inhibit growth but do not alter the sugar reactions.
- D: Enzyme tests work on any pure isolate, whatever the source.
Question 6 · Medium
Which rapid test best separates Moraxella catarrhalis from commensal Neisseria species in a respiratory isolate?
1. AOxidase test on a fresh colony
2. BSuperoxol test with 30% hydrogen peroxide
3. CCatalase test with 3% hydrogen peroxide
4. DButyrate esterase (tributyrin) hydrolysis
Answer and explanation in LearnFree account
Question 7 · Easy
In the carbohydrate utilization test, Neisseria meningitidis produces acid from:
1. AMaltose and sucrose
2. BGlucose, maltose and lactose
3. CGlucose only
4. DGlucose and maltose
Answer and explanation in LearnFree account
Question 8 · Hard
A 29-year-old man treated for gonorrhea 10 days ago still has urethral discharge, and a repeat NAAT remains positive. What should the laboratory advise?
1. AReport the NAAT as proof of resistance
2. BCollect a swab for culture so susceptibility testing can be done
3. CRepeat the NAAT in one week
4. DTest the patient's serum for gonococcal antibodies
Answer and explanation in LearnFree account
Question 9 · Hard
A gram-negative diplococcus grown from a throat swab on modified Thayer-Martin agar is oxidase positive and produces acid from glucose and maltose, and the ONPG test is positive. The best identification is:
1. ANeisseria gonorrheae
2. BNeisseria meningitidis
3. CNeisseria lactamica
4. DMoraxella catarrhalis
Answer and explanation in LearnFree account
Question 10 · Medium
A urethral exudate from a 24-year-old man grows oxidase-positive gram-negative diplococci on modified Thayer-Martin agar. When 30% hydrogen peroxide is placed on a colony, immediate vigorous bubbling appears. This supports:
1. ANeisseria meningitidis
2. BNeisseria gonorrheae
3. CNeisseria cinerea
4. DNeisseria lactamica
Answer and explanation in LearnFree account

Neisseria and Moraxella

A blood culture isolate is confirmed as Neisseria meningitidis. Which further test on the isolate is most useful for public health purposes?

Genital specimens for Neisseria gonorrheae culture are plated on a non-selective chocolate agar as well as on modified Thayer-Martin agar. The reason is that:

A gram-negative diplococcus from a blood culture is oxidase positive and hydrolyzes gamma-glutamyl-beta-naphthylamide. This result supports:

A 15-year-old has pharyngitis, and an oxidase-positive gram-negative diplococcus grows from the throat swab on selective agar. Why must a throat isolate be identified by two different confirmatory methods before Neisseria gonorrheae is reported?

Which rapid test best separates Moraxella catarrhalis from commensal Neisseria species in a respiratory isolate?

In the carbohydrate utilization test, Neisseria meningitidis produces acid from:

A 29-year-old man treated for gonorrhea 10 days ago still has urethral discharge, and a repeat NAAT remains positive. What should the laboratory advise?

A gram-negative diplococcus grown from a throat swab on modified Thayer-Martin agar is oxidase positive and produces acid from glucose and maltose, and the ONPG test is positive. The best identification is:

A urethral exudate from a 24-year-old man grows oxidase-positive gram-negative diplococci on modified Thayer-Martin agar. When 30% hydrogen peroxide is placed on a colony, immediate vigorous bubbling appears. This supports: