Two days after a cesarean section, a 29-year-old woman has fever of 39.4 °C, hypotension and a red, tender wound. Two blood culture sets grow catalase-negative cocci in chains that form small colonies with wide zones of beta-hemolysis. Results: PYR positive; bacitracin 0.04 U, susceptible; latex grouping, Lancefield group A. The result is phoned to the ward as a critical value. No other group A streptococcal infection has been recorded on the maternity unit this year. Following CDC guidance on postpartum and postsurgical group A streptococcal infection, what else should the laboratory do?

Store the isolate and inform infection control

Ask for throat swabs from all maternity staff

Test penicillin susceptibility before reporting

Report it and discard the isolate as usual

Pharyngitis in a 19-year-old: the culture yields catalase-negative streptococci with wide beta-hemolysis and colonies over 0.5 mm across. Which species fits the disk pattern shown?

Streptococcus dysgalactiae (group C or G)

Streptococcus pyogenes (group A)

Streptococcus agalactiae (group B)

On sheep blood agar in 5% CO2, Streptococcus pneumoniae and most viridans streptococci show which type of hemolysis?

Alpha (partial lysis, green zone)

Gamma (no lysis, no color change)

Beta (complete lysis, clear zone)

Double zone of lysis around each colony

Streptococci and enterococci

21 DOH Abu Dhabi (HAAD) lab licensing practice questions on Streptococci and enterococci, part of Microbiology. The first 21 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
Urine from a 66-year-old woman grows more than 10^8 CFU/L (10^5 CFU/mL) of small grey, non-hemolytic colonies on blood agar and small yellow colonies on CLED agar. A colony picked from the blood agar gives slow, weak bubbling in 3% hydrogen peroxide, so a staphylococcal identification card is set up; it returns 'unidentified organism'. Further results: Gram stain, gram-positive cocci in pairs and short chains; catalase on a colony from the CLED agar, negative; PYR positive; LAP positive; bile esculin positive; growth in 6.5% NaCl broth; vancomycin susceptible. The most likely organism is:
1. AEnterococcus species
2. BAerococcus viridans
3. CStreptococcus agalactiae
4. DStaphylococcus saprophyticus
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Answer: A · Enterococcus species
Weak, slow bubbling from a blood agar colony is a classic false-positive catalase: red cells picked up with the colony contain catalase, and some enterococci make a weak pseudocatalase on media that contain blood. The negative catalase on growth from CLED agar is the valid result, so the organism is a catalase-negative coccus in pairs and chains. PYR and LAP positivity, bile esculin hydrolysis and growth in 6.5% NaCl identify Enterococcus. Aerococcus viridans is LAP negative and grows in tetrads, and group B streptococci are PYR and bile esculin negative. Catalase should be tested on growth from a medium without blood.
- B: Aerococcus viridans is LAP negative and forms tetrads, not chains.
- C: Group B streptococci are PYR negative and bile esculin negative.
- D: Staphylococci are strongly catalase positive on any medium and grow in clusters.
Question 2 · Hard
Two days after a cesarean section, a 29-year-old woman has fever of 39.4 °C, hypotension and a red, tender wound. Two blood culture sets grow catalase-negative cocci in chains that form small colonies with wide zones of beta-hemolysis. Results: PYR positive; bacitracin 0.04 U, susceptible; latex grouping, Lancefield group A. The result is phoned to the ward as a critical value. No other group A streptococcal infection has been recorded on the maternity unit this year. Following CDC guidance on postpartum and postsurgical group A streptococcal infection, what else should the laboratory do?
1. AAsk for throat swabs from all maternity staff
2. BTest penicillin susceptibility before reporting
3. CStore the isolate and inform infection control
4. DReport it and discard the isolate as usual
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Answer: C · Store the isolate and inform infection control
PYR positivity, bacitracin susceptibility and the group A antigen identify Streptococcus pyogenes. A single postpartum or postsurgical case of invasive group A streptococcal infection can be the first sign of spread from a carrier among staff, so infection control must be told and the isolate stored frozen so that it can be typed against any later cases. Screening staff is usually reserved for when a second case occurs or the investigation links staff to the case. S. pyogenes remains uniformly susceptible to penicillin, so routine penicillin testing is not needed and must not delay the report. Discarding the isolate would make later typing impossible.
- A: Screening every staff member is not advised after a single case; it is used if further cases occur.
- B: S. pyogenes is uniformly penicillin susceptible; testing is not needed and must not delay the report.
- D: Discarding the isolate prevents comparison with any later cases.
Question 3 · Medium
Pharyngitis in a 19-year-old: the culture yields catalase-negative streptococci with wide beta-hemolysis and colonies over 0.5 mm across. Which species fits the disk pattern shown?
Figure: MedExams
1. AStreptococcus anginosus group
2. BStreptococcus pyogenes (group A)
3. CStreptococcus dysgalactiae (group C or G)
4. DStreptococcus agalactiae (group B)
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Answer: C · Streptococcus dysgalactiae (group C or G)
The isolate grows up to the bacitracin (A) disk, so it is resistant, but has a clear zone around the SXT disk, so it is susceptible. Group A streptococci are bacitracin susceptible and SXT resistant, and group B streptococci are resistant to both. Large-colony beta-hemolytic streptococci that are bacitracin resistant and SXT susceptible are presumptively group C or G, S. dysgalactiae subsp. equisimilis, a recognised cause of pharyngitis. Minute colonies with the same disk pattern would suggest the S. anginosus group, so colony size and Lancefield grouping are both used.
- A: The S. anginosus group forms minute colonies under 0.5 mm.
- B: Group A streptococci are bacitracin susceptible and SXT resistant.
- D: Group B streptococci are resistant to both bacitracin and SXT.
Question 4 · Easy
An alpha-hemolytic, catalase-negative coccus was suspended in saline and in sodium deoxycholate (DOC), with the results shown. The isolate is presumptively:
Figure: MedExams
1. AStreptococcus mitis group
2. BStreptococcus pneumoniae
3. CEnterococcus faecalis
4. DStreptococcus gallolyticus
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Answer: B · Streptococcus pneumoniae
The deoxycholate tube has cleared while the saline control stays turbid, so the isolate is bile soluble. Bile salts activate the pneumococcal autolysin (LytA), which breaks down the cell wall and lyses the cells, so a positive test identifies S. pneumoniae. Viridans streptococci, group D streptococci and enterococci are bile insoluble, and both of their tubes stay turbid. The saline control shows that the clearing came from the bile salt and not from spontaneous autolysis.
- A: Viridans streptococci such as the S. mitis group are bile insoluble.
- C: Enterococci are bile insoluble, although they grow on bile esculin agar.
- D: Group D streptococci such as S. gallolyticus are bile insoluble.
Question 5 · Easy
On sheep blood agar in 5% CO2, Streptococcus pneumoniae and most viridans streptococci show which type of hemolysis?
1. AAlpha (partial lysis, green zone)
2. BGamma (no lysis, no color change)
3. CBeta (complete lysis, clear zone)
4. DDouble zone of lysis around each colony
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Answer: A · Alpha (partial lysis, green zone)
Alpha-hemolysis is partial lysis of the red cells with a green or brownish discoloration around the colony, often due to hydrogen peroxide oxidizing hemoglobin. Pneumococci and most viridans streptococci show this pattern, so tests such as optochin and bile solubility are needed to separate them. Beta-hemolysis is a clear zone of complete lysis, typical of groups A and B. Plates are best read with light shining through the agar.
- B: Gamma means no hemolysis, as in many enterococci.
- C: Complete clear lysis is beta-hemolysis, typical of groups A and B.
- D: A double zone is typical of Clostridium perfringens, not streptococci.
Question 6 · Medium
Which colony appearance on sheep blood agar at 24 hours best fits Streptococcus agalactiae?
1. ASmall grey colonies with a wide, sharp zone of beta-hemolysis
2. BFlat alpha-hemolytic colonies with a sunken center
3. CPinpoint colonies with a wide beta-hemolytic zone
4. DGrey, fairly large colonies with a narrow zone of beta-hemolysis
Answer and explanation in LearnFree account
Question 7 · Hard
A beta-hemolytic, catalase-negative coccus in pairs and short chains from a diabetic foot wound is PYR positive. It is bacitracin resistant, bile esculin positive and grows in 6.5% NaCl. The most likely organism is:
1. AEnterococcus faecalis
2. BStreptococcus dysgalactiae
3. CStreptococcus agalactiae
4. DStreptococcus pyogenes
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Question 8 · Medium
On bile esculin agar, the black color produced by enterococci comes from:
1. ABile salts precipitating in acid conditions
2. BHydrogen sulfide reacting with iron salts
3. CTellurite reduced to metallic tellurium
4. DEsculetin reacting with ferric citrate
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Question 9 · Easy
What is the Lancefield grouping of streptococci based on?
1. ACarbohydrate antigens in the cell wall
2. BM protein types on the cell surface
3. CThe type of hemolysis on blood agar
4. DCapsular polysaccharide serotypes
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Question 10 · Hard
Three beta-hemolytic, catalase-negative streptococci give these disk results (S = susceptible, R = resistant, SXT = trimethoprim-sulfamethoxazole): isolate 1, bacitracin S and SXT R; isolate 2, bacitracin R and SXT R; isolate 3, bacitracin R and SXT S. The presumptive groups, in order, are:
1. AGroup C, F or G; group B; group A
2. BGroup A; group B; group C, F or G
3. CGroup B; group A; group C, F or G
4. DGroup A; group C, F or G; group B
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Question 11 · Medium
A rectal screening swab from an ICU patient grows an Enterococcus that is motile and forms yellow-pigmented colonies. The most likely species is:
1. AEnterococcus gallinarum
2. BEnterococcus faecalis
3. CEnterococcus faecium
4. DEnterococcus casseliflavus
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Question 12 · Hard
An enterococcus from the blood culture of a 70-year-old on a hematology ward is non-motile and non-pigmented. It ferments arabinose and does not use pyruvate. The most likely species is:
1. AEnterococcus faecium
2. BEnterococcus gallinarum
3. CEnterococcus casseliflavus
4. DEnterococcus faecalis
Answer and explanation in LearnFree account
Question 13 · Medium
In the bile solubility test, sodium deoxycholate clears a suspension of Streptococcus pneumoniae by:
1. ABinding calcium needed by the cell wall
2. BDissolving the polysaccharide capsule
3. CBlocking cell wall synthesis, like penicillin
4. DActivating the organism's own autolysin
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Question 14 · Easy
Which pair of results gives a presumptive identification of Enterococcus?
1. AOptochin susceptible and bile soluble
2. BBile esculin positive and growth in 6.5% NaCl
3. CBacitracin susceptible and PYR positive
4. DCAMP positive and hippurate positive
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Question 15 · Hard
A throat culture from a 20-year-old grows pinpoint (under 0.5 mm) beta-hemolytic colonies that react with group A antiserum. The catalase-negative cocci in chains are PYR negative. Why should this not be reported as Streptococcus pyogenes?
1. APinpoint colonies are always a sign of a mixed culture
2. BGroup A latex cross-reacts with all viridans streptococci
3. CThe S. anginosus group can carry group A antigen
4. DS. pyogenes is PYR negative in most throat isolates
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Question 16 · Medium
Among catalase-negative gram-positive cocci, a positive PYR test is typical of:
1. AStreptococcus pneumoniae and viridans group
2. BStreptococcus agalactiae and enterococci
3. CStreptococcus pyogenes and enterococci
4. DStreptococcus pyogenes and S. agalactiae
Answer and explanation in LearnFree account
Question 17 · Medium
Why does an arrowhead of enhanced hemolysis form in a positive CAMP test?
1. AHippurate breakdown releases a hemolytic acid
2. BGroup B streptococci make a stronger streptolysin O
3. CS. aureus coagulase clots the sheep red cells
4. DCAMP factor acts with the S. aureus beta-lysin
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Question 18 · Hard
A laboratory switches its throat-culture plates from sheep blood agar to horse blood agar. Soon many beta-hemolytic colonies turn out to be small gram-negative coccobacilli. What is the most likely explanation?
1. AHorse blood supports beta-hemolytic Haemophilus haemolyticus
2. BHorse blood inactivates streptolysin O in group A streptococci
3. CHorse blood makes viridans streptococci appear beta-hemolytic
4. DHorse blood makes any gram-negative organism look hemolytic
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Question 19 · Medium
When a throat swab is plated on sheep blood agar, the agar is often stabbed with the loop. The purpose is to:
1. AGive anaerobic throat bacteria a chance to grow
2. BShow hemolysis by oxygen-labile streptolysin O
3. CHelp separate colonies of mixed throat flora
4. DPush the bacteria deeper to protect them from drying
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Question 20 · Hard
Blood cultures from a 2-day-old with sepsis grow catalase-negative cocci in chains with a narrow zone of beta-hemolysis. Results: bacitracin 0.04 U, susceptible; PYR, negative; CAMP, positive; hippurate hydrolysis, positive. What is the most likely organism?
1. AStreptococcus agalactiae
2. BEnterococcus faecalis
3. CStreptococcus pyogenes
4. DListeria monocytogenes
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Question 21 · Hard
Sputum from a 72-year-old with lobar pneumonia grows alpha-hemolytic, lancet-shaped diplococci with sunken centers. The optochin zone is 8 mm (6-mm disk; 14 mm or more = susceptible), but a deoxycholate suspension clears within 15 minutes while the saline control stays turbid. The best identification is:
1. AOptochin-resistant Streptococcus anginosus
2. BOptochin-resistant Enterococcus faecium
3. COptochin-resistant Streptococcus pneumoniae
4. DOptochin-resistant Streptococcus mitis group
Answer and explanation in LearnFree account

Streptococci and enterococci

Pharyngitis in a 19-year-old: the culture yields catalase-negative streptococci with wide beta-hemolysis and colonies over 0.5 mm across. Which species fits the disk pattern shown?

An alpha-hemolytic, catalase-negative coccus was suspended in saline and in sodium deoxycholate (DOC), with the results shown. The isolate is presumptively:

On sheep blood agar in 5% CO2, Streptococcus pneumoniae and most viridans streptococci show which type of hemolysis?

Which colony appearance on sheep blood agar at 24 hours best fits Streptococcus agalactiae?

A beta-hemolytic, catalase-negative coccus in pairs and short chains from a diabetic foot wound is PYR positive. It is bacitracin resistant, bile esculin positive and grows in 6.5% NaCl. The most likely organism is:

On bile esculin agar, the black color produced by enterococci comes from:

What is the Lancefield grouping of streptococci based on?

Three beta-hemolytic, catalase-negative streptococci give these disk results (S = susceptible, R = resistant, SXT = trimethoprim-sulfamethoxazole): isolate 1, bacitracin S and SXT R; isolate 2, bacitracin R and SXT R; isolate 3, bacitracin R and SXT S. The presumptive groups, in order, are:

A rectal screening swab from an ICU patient grows an Enterococcus that is motile and forms yellow-pigmented colonies. The most likely species is:

An enterococcus from the blood culture of a 70-year-old on a hematology ward is non-motile and non-pigmented. It ferments arabinose and does not use pyruvate. The most likely species is:

In the bile solubility test, sodium deoxycholate clears a suspension of Streptococcus pneumoniae by:

Which pair of results gives a presumptive identification of Enterococcus?

A throat culture from a 20-year-old grows pinpoint (under 0.5 mm) beta-hemolytic colonies that react with group A antiserum. The catalase-negative cocci in chains are PYR negative. Why should this not be reported as Streptococcus pyogenes?

Among catalase-negative gram-positive cocci, a positive PYR test is typical of:

Why does an arrowhead of enhanced hemolysis form in a positive CAMP test?

A laboratory switches its throat-culture plates from sheep blood agar to horse blood agar. Soon many beta-hemolytic colonies turn out to be small gram-negative coccobacilli. What is the most likely explanation?

When a throat swab is plated on sheep blood agar, the agar is often stabbed with the loop. The purpose is to:

Blood cultures from a 2-day-old with sepsis grow catalase-negative cocci in chains with a narrow zone of beta-hemolysis. Results: bacitracin 0.04 U, susceptible; PYR, negative; CAMP, positive; hippurate hydrolysis, positive. What is the most likely organism?