A 48-year-old man has had three Escherichia coli urinary infections in a year, with perineal discomfort between episodes. A short course of nitrofurantoin is given first to clear his bladder urine, and a four-glass localization test is then done. Colony counts of E. coli (CFU/mL): VB1, first-voided 10 mL, 1 × 10^2; VB2, midstream urine, no growth; EPS, expressed prostatic secretion, 2 × 10^4; VB3, first 10 mL voided after prostatic massage, 3 × 10^3. No other organism grows, and all four specimens were plated within 30 minutes. The most likely interpretation is:

Recurrent cystitis (bladder source)

A suprapubic aspirate from a febrile 4-month-old was plated with a 0.01 mL (10 µL) calibrated loop, and the plate is shown. How should the culture be reported?

1.6 × 10^6 CFU/L (1,600 CFU/mL); significant

1.6 × 10^5 CFU/L (160 CFU/mL); contaminant

1.6 × 10^6 CFU/L (1,600 CFU/mL); not significant

1.6 × 10^7 CFU/L (16,000 CFU/mL); significant

Urine cultures

16 DOH Abu Dhabi (HAAD) lab licensing practice questions on Urine cultures, part of Microbiology. The first 16 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
A 48-year-old man has had three Escherichia coli urinary infections in a year, with perineal discomfort between episodes. A short course of nitrofurantoin is given first to clear his bladder urine, and a four-glass localization test is then done. Colony counts of E. coli (CFU/mL): VB1, first-voided 10 mL, 1 × 10^2; VB2, midstream urine, no growth; EPS, expressed prostatic secretion, 2 × 10^4; VB3, first 10 mL voided after prostatic massage, 3 × 10^3. No other organism grows, and all four specimens were plated within 30 minutes. The most likely interpretation is:
1. AUrethral colonization only
2. BContamination of the specimens
3. CRecurrent cystitis (bladder source)
4. DChronic bacterial prostatitis
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Answer: D · Chronic bacterial prostatitis
The four-glass test localizes infection by comparing counts in sequential samples: VB1 reflects the urethra, VB2 the bladder, and EPS and VB3 the prostate. Bacterial prostatitis is shown when the EPS and VB3 counts are at least 10 times higher than those of VB1 and VB2. Here the EPS count is 200 times and the VB3 count 30 times the VB1 count, and VB2 is sterile, so the E. coli comes from the prostate. Nitrofurantoin was given first because it sterilizes bladder urine but does not reach the prostate, which makes localization possible. Chronic bacterial prostatitis is a common cause of relapsing urinary infection with the same organism in men.
- A: Urethral colonization gives the highest count in VB1, not in EPS and VB3.
- B: One organism with a clear rise after massage, plated promptly, is not contamination.
- C: VB2 shows no growth, so the bladder urine is not infected.
Question 2 · Hard
A suprapubic aspirate from a febrile 4-month-old was plated with a 0.01 mL (10 µL) calibrated loop, and the plate is shown. How should the culture be reported?
Figure: MedExams
1. A1.6 × 10^5 CFU/L (160 CFU/mL); contaminant
2. B1.6 × 10^6 CFU/L (1,600 CFU/mL); not significant
3. C1.6 × 10^7 CFU/L (16,000 CFU/mL); significant
4. D1.6 × 10^6 CFU/L (1,600 CFU/mL); significant
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Answer: D · 1.6 × 10^6 CFU/L (1,600 CFU/mL); significant
The plate holds 16 colonies of a single lactose-fermenting type. With a 0.01 mL loop each colony represents 100 CFU/mL, so the count is 1,600 CFU/mL, or 1.6 × 10^6 CFU/L. A suprapubic aspirate bypasses the urethra, so any growth of a uropathogen is significant, unlike the higher cut-offs used for midstream urine. The 0.01 mL loop is used for these specimens so that low counts are detected.
- A: The factor for a 0.01 mL loop is 100, and an aspirate avoids urethral flora.
- B: Midstream cut-offs do not apply; any growth from a suprapubic aspirate counts.
- C: That uses the 0.001 mL loop factor; this plate was made with a 0.01 mL loop.
Question 3 · Medium
Expressed in SI units, the colony count of the midstream urine plate shown is:
Figure: MedExams
1. A3.4 × 10^4 CFU/L
2. B3.4 × 10^7 CFU/L
3. C3.4 × 10^8 CFU/L
4. D3.4 × 10^6 CFU/L
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Answer: B · 3.4 × 10^7 CFU/L
The plate has 34 colonies of one type. A 1 µL loop delivers 0.001 mL, so each colony represents 1,000 CFU per mL: 34 × 1,000 = 34,000 CFU/mL. Multiplying by 1,000 mL per litre gives 3.4 × 10^7 CFU/L. Stopping at 34,000 and writing it as CFU/L leaves out the mL-to-litre step, a common slip when changing to SI units.
- A: 34,000 is the count per mL; per litre it is 1,000 times higher.
- C: That is ten times too high; the count is 34,000 CFU/mL, or 3.4 × 10^7 CFU/L.
- D: That uses the factor of 100 for a 0.01 mL loop, not 1,000 for a 1 µL loop.
Question 4 · Hard
Urine from an 80-year-old man with dysuria shows gram-positive cocci in clusters. At 24 hours, blood agar has tiny alpha-hemolytic colonies at more than 100,000 CFU/mL. The isolate is catalase negative, PYR negative and LAP positive. The most likely organism is:
1. AEnterococcus faecalis
2. BAerococcus viridans
3. CAerococcus urinae
4. DStaphylococcus saprophyticus
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Answer: C · Aerococcus urinae
Aerococcus urinae is a catalase-negative coccus that grows in clusters and tetrads, so it is easily mistaken for staphylococci on Gram stain or for viridans streptococci on the plate. It causes urinary infection mainly in older men with urinary tract problems, and sometimes bacteremia and endocarditis. A PYR-negative, LAP-positive pattern separates it from A. viridans (PYR positive, LAP negative) and from enterococci (both positive). MALDI-TOF identifies it reliably.
- A: Enterococci are PYR positive and grow in pairs and short chains.
- B: A. viridans is PYR positive and LAP negative.
- D: Staphylococci are catalase positive.
Question 5 · Easy
In a midstream urine from a woman, which isolate is most often regarded as a contaminant from the urethra or vagina?
1. AEscherichia coli
2. BLactobacillus species
3. CStaphylococcus saprophyticus
4. DKlebsiella pneumoniae
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Answer: B · Lactobacillus species
Lactobacilli are the main bacteria of the healthy vagina and are common in midstream urine from women. They rarely cause urinary infection, so most laboratories report them as genital or mixed flora without identification or susceptibility testing. Diphtheroids, viridans streptococci and coagulase-negative staphylococci other than S. saprophyticus are handled the same way. Heavy lactobacilli with many squamous cells in the sediment point to a poorly collected specimen.
- A: E. coli is the commonest cause of urinary infection.
- C: S. saprophyticus is a true uropathogen in young women.
- D: K. pneumoniae is a common uropathogen, especially in hospital patients.
Question 6 · Medium
Catheter urine from an ICU patient with no fever or urinary symptoms grows Candida albicans at 10,000 CFU/mL. What does this most likely represent?
1. AContamination of the sample by skin flora
2. BInvasive candidiasis needing urgent treatment
3. CA laboratory error in the colony count
4. DColonization of the catheter and bladder
Answer and explanation in LearnFree account
Question 7 · Medium
A midstream urine from a patient without a urinary catheter grows Staphylococcus aureus at more than 100,000 CFU/mL. Why should the report suggest blood cultures?
1. AIt is usually a skin contaminant from poor collection
2. BIt often reflects kidney seeding from the bloodstream
3. CIt reflects harmless colonization of the bladder
4. DThe urine was probably left too long at room temperature
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Question 8 · Easy
CLED agar is often used in place of MacConkey agar for urine culture. What does its low electrolyte content achieve?
1. AInhibits the growth of gram-positive cocci
2. BShows which colonies produce urease
3. CStops Proteus from swarming across the plate
4. DSupplies the X and V factors for Haemophilus
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Question 9 · Medium
Suprapubic aspirates are plated with a 0.01 mL loop instead of the 0.001 mL loop used for midstream urine. What is the reason?
1. ABladder urine is too viscous for the smaller loop
2. BThe larger loop removes the need for a Gram stain
3. CLow counts matter, and one colony equals 100 CFU/mL
4. DThe larger loop stops Proteus species from swarming
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Question 10 · Easy
Voided urine is cultured with a calibrated loop and reported as a colony count, rather than simply as growth or no growth. Why?
1. ACalibrated loops recover fastidious organisms better
2. BA colony count identifies the organism to species level
3. CUrethral flora contaminates even well-collected urine
4. DCounting colonies replaces susceptibility testing
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Question 11 · Hard
A febrile 10-month-old's urine was collected in an adhesive perineal bag. It grows Escherichia coli at 50,000 CFU/mL with Enterococcus faecalis and a coagulase-negative staphylococcus. What is the best next step?
1. AReport E. coli as the cause of infection
2. BRe-plate the same urine with a 0.01 mL loop
3. CTest all three isolates for susceptibility
4. DRequest a catheter or suprapubic specimen
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Question 12 · Hard
A midstream urine was plated with a 0.01 mL loop by mistake, but the count was worked out as if the 0.001 mL loop had been used. The plate has 120 colonies of Escherichia coli, and the report says 120,000 CFU/mL. What is the correct count?
1. A12,000 CFU/mL
2. B1,200,000 CFU/mL
3. C120,000 CFU/mL
4. D1,200 CFU/mL
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Question 13 · Hard
A woman with cystitis took a first dose of nitrofurantoin 3 hours before giving a urine sample. Microscopy shows many white cells and many rod-shaped bacteria, but the culture has no growth at 48 hours. The most likely explanation is:
1. AA 0.001 mL loop is too small to detect bacteriuria
2. BAntibiotic in the urine inhibited growth on the plate
3. CThe bacteria are anaerobes that need special culture
4. DRefrigerating the urine before plating killed them
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Question 14 · Hard
At 20 weeks of pregnancy, a woman's routine urine culture grows group B streptococcus (Streptococcus agalactiae) at 20,000 CFU/mL with a few lactobacilli. Following the CDC 2010 group B streptococcal guidance, what should the laboratory do?
1. AAsk for a repeat specimen before reporting any GBS
2. BReport mixed genital flora and do no further work
3. CReport GBS, which signals a need for intrapartum prophylaxis
4. DReport no significant growth below 100,000 CFU/mL
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Question 15 · Medium
A 24-year-old woman with dysuria, frequency and pyuria has a pure growth of Escherichia coli at 1,000 CFU/mL (10^6 CFU/L) in a clean-catch urine. The best interpretation is:
1. AContamination, because the count is under 100,000 CFU/mL
2. BColonization that needs no report or treatment
3. CProbable cystitis; low counts can matter with symptoms
4. DPyelonephritis, because pyuria is present
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Question 16 · Medium
A 0.001 mL calibrated loop is used, and 150 colonies grow. What is the colony count?
1. A15,000 CFU/mL
2. B1,500 CFU/mL
3. C1,500,000 CFU/mL
4. D150,000 CFU/mL
Answer and explanation in LearnFree account

Urine cultures

A suprapubic aspirate from a febrile 4-month-old was plated with a 0.01 mL (10 µL) calibrated loop, and the plate is shown. How should the culture be reported?

Expressed in SI units, the colony count of the midstream urine plate shown is:

Urine from an 80-year-old man with dysuria shows gram-positive cocci in clusters. At 24 hours, blood agar has tiny alpha-hemolytic colonies at more than 100,000 CFU/mL. The isolate is catalase negative, PYR negative and LAP positive. The most likely organism is:

In a midstream urine from a woman, which isolate is most often regarded as a contaminant from the urethra or vagina?

Catheter urine from an ICU patient with no fever or urinary symptoms grows Candida albicans at 10,000 CFU/mL. What does this most likely represent?

A midstream urine from a patient without a urinary catheter grows Staphylococcus aureus at more than 100,000 CFU/mL. Why should the report suggest blood cultures?

CLED agar is often used in place of MacConkey agar for urine culture. What does its low electrolyte content achieve?

Suprapubic aspirates are plated with a 0.01 mL loop instead of the 0.001 mL loop used for midstream urine. What is the reason?

Voided urine is cultured with a calibrated loop and reported as a colony count, rather than simply as growth or no growth. Why?

A febrile 10-month-old's urine was collected in an adhesive perineal bag. It grows Escherichia coli at 50,000 CFU/mL with Enterococcus faecalis and a coagulase-negative staphylococcus. What is the best next step?

A midstream urine was plated with a 0.01 mL loop by mistake, but the count was worked out as if the 0.001 mL loop had been used. The plate has 120 colonies of Escherichia coli, and the report says 120,000 CFU/mL. What is the correct count?

A woman with cystitis took a first dose of nitrofurantoin 3 hours before giving a urine sample. Microscopy shows many white cells and many rod-shaped bacteria, but the culture has no growth at 48 hours. The most likely explanation is:

At 20 weeks of pregnancy, a woman's routine urine culture grows group B streptococcus (Streptococcus agalactiae) at 20,000 CFU/mL with a few lactobacilli. Following the CDC 2010 group B streptococcal guidance, what should the laboratory do?

A 24-year-old woman with dysuria, frequency and pyuria has a pure growth of Escherichia coli at 1,000 CFU/mL (10^6 CFU/L) in a clean-catch urine. The best interpretation is:

A 0.001 mL calibrated loop is used, and 150 colonies grow. What is the colony count?