A patient on peritoneal dialysis has abdominal pain and cloudy dialysate with 600 white cells/µL (0.6 x 10^9/L), 85% neutrophils. Which organism group causes most of these infections?

Coagulase-negative staphylococci

Foul-smelling pleural fluid from a patient with poor dentition who aspirated shows many neutrophils and mixed gram-positive cocci, gram-positive rods and thin gram-negative rods. No antibiotics were given. Aerobic plates grow only a few streptococci. The best explanation is:

Most organisms are anaerobes not recovered aerobically

The fluid was contaminated by skin flora

The patient has tuberculous pleurisy

The smear was overdecolorized during staining

Body fluids from normally sterile sites

6 ASCP MLS practice questions on Body fluids from normally sterile sites, part of Microbiology. The first 6 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 · Medium
A patient on peritoneal dialysis has abdominal pain and cloudy dialysate with 600 white cells/µL (0.6 x 10^9/L), 85% neutrophils. Which organism group causes most of these infections?
1. ACoagulase-negative staphylococci
2. BEnterococcus species
3. CYeasts such as Candida
4. DAnaerobic gram-negative rods
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Answer: A · Coagulase-negative staphylococci
Peritonitis is the main infectious complication of peritoneal dialysis, and cloudy effluent with more than 100 white cells/µL, mostly neutrophils, supports the diagnosis. Most episodes are caused by skin bacteria, especially coagulase-negative staphylococci, carried in by touch contamination of the catheter or tubing. Staphylococcus aureus, gram-negative rods and, less often, fungi cause the rest. Inoculating the effluent into blood culture bottles, or culturing the sediment of a large centrifuged volume, improves recovery.
- B: Enterococci cause some episodes but far fewer than staphylococci.
- C: Fungal peritonitis is uncommon, though serious, and often follows antibiotic courses.
- D: Anaerobes are uncommon and suggest a bowel leak or perforation.
Question 2 · Easy
The most common cause of bacterial septic arthritis in the native knee of an older adult is:
1. AEscherichia coli
2. BStreptococcus pyogenes
3. CStaphylococcus aureus
4. DKingella kingae
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Answer: C · Staphylococcus aureus
Staphylococcus aureus causes most cases of nongonococcal septic arthritis in adults, usually by spread from the blood. Beta-hemolytic streptococci are the next most common group, and gram-negative rods cause a smaller share, mainly in older or immunocompromised patients. Kingella kingae is a leading cause in children under about 4 years. Joint fluid should be Gram stained and cultured before antibiotics are started.
- A: Gram-negative rods such as E. coli cause only a minority of adult cases.
- B: Streptococci are the second most common group, well behind S. aureus.
- D: Kingella kingae affects mainly young children, not older adults.
Question 3 · Medium
Foul-smelling pleural fluid from a patient with poor dentition who aspirated shows many neutrophils and mixed gram-positive cocci, gram-positive rods and thin gram-negative rods. No antibiotics were given. Aerobic plates grow only a few streptococci. The best explanation is:
1. AMost organisms are anaerobes not recovered aerobically
2. BThe fluid was contaminated by skin flora
3. CThe patient has tuberculous pleurisy
4. DThe smear was overdecolorized during staining
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Answer: A · Most organisms are anaerobes not recovered aerobically
Empyema after aspiration is usually a mixed infection with oral anaerobes such as Prevotella, Fusobacterium and anaerobic gram-positive cocci, often with the Streptococcus anginosus group. A putrid smell and several morphotypes on the smear that do not grow on aerobic plates strongly suggest anaerobes. Pleural fluid should be sent in an anaerobic transport container and plated on anaerobic media. The Gram stain is a useful check on whether culture has recovered all the organisms seen.
- B: Skin contaminants are few and do not give a putrid, mixed smear like this.
- C: Tuberculous pleural fluid is usually lymphocytic, and mycobacteria stain poorly with Gram stain.
- D: Overdecolorization changes how cells stain but does not stop them growing.
Question 4 · Hard
Synovial fluid from a 24-year-old man with a hot knee has 55,000 white cells/µL (55 x 10^9/L), mostly neutrophils, and no crystals. At 48 hours the sheep blood agar has no growth, but chocolate agar in CO2 has small gray, oxidase-positive colonies of gram-negative diplococci. What is the most likely organism?
1. AHaemophilus influenzae
2. BMoraxella catarrhalis
3. CNeisseria meningitidis
4. DNeisseria gonorrhoeae
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Answer: D · Neisseria gonorrhoeae
Neisseria gonorrhoeae is fastidious and grows on chocolate agar in CO2 but poorly or not at all on sheep blood agar. Neisseria meningitidis and Moraxella catarrhalis are also oxidase-positive gram-negative diplococci, but both grow well on blood agar. Haemophilus influenzae also needs chocolate agar, but it is a small coccobacillus rather than a diplococcus. Gonococcal arthritis is a leading cause of septic arthritis in young sexually active adults, so genital, rectal and throat samples should also be tested.
- A: Haemophilus influenzae is a coccobacillus, not a diplococcus.
- B: Moraxella catarrhalis grows well on sheep blood agar.
- C: Neisseria meningitidis grows well on sheep blood agar.
Question 5 · Easy
In a patient with cirrhosis and ascites, spontaneous bacterial peritonitis is most often caused by:
1. AStaphylococcus epidermidis
2. BCandida albicans
3. CBacteroides fragilis
4. DEscherichia coli
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Answer: D · Escherichia coli
Spontaneous bacterial peritonitis arises without a bowel perforation, when gut bacteria reach the ascitic fluid through the blood or lymph. It is usually caused by a single organism, most often Escherichia coli, followed by Klebsiella pneumoniae and streptococci. Anaerobes and mixed growth suggest secondary peritonitis from a perforated bowel instead. An ascitic neutrophil count of 250/µL (0.25 x 10^9/L) or more supports the diagnosis even when the culture is negative.
- A: Coagulase-negative staphylococci are typical of peritoneal dialysis peritonitis.
- B: Candida is an uncommon cause, seen mainly after bowel surgery or perforation.
- C: Bacteroides fragilis and other anaerobes point to secondary peritonitis from bowel perforation.
Question 6 · Medium
Ascitic fluid from a patient with suspected spontaneous bacterial peritonitis is put into blood culture bottles at the bedside rather than sent in a sterile tube. The main reason is:
1. ABacteria are few, so a larger inoculum improves recovery
2. BThe Gram stain is usually positive and needs confirmation
3. CThe bottles stop skin bacteria from contaminating it
4. DThe infection is usually polymicrobial with anaerobes
Answer and explanation in LearnFree account

Body fluids from normally sterile sites

A patient on peritoneal dialysis has abdominal pain and cloudy dialysate with 600 white cells/µL (0.6 x 10^9/L), 85% neutrophils. Which organism group causes most of these infections?

The most common cause of bacterial septic arthritis in the native knee of an older adult is:

Foul-smelling pleural fluid from a patient with poor dentition who aspirated shows many neutrophils and mixed gram-positive cocci, gram-positive rods and thin gram-negative rods. No antibiotics were given. Aerobic plates grow only a few streptococci. The best explanation is:

In a patient with cirrhosis and ascites, spontaneous bacterial peritonitis is most often caused by:

Ascitic fluid from a patient with suspected spontaneous bacterial peritonitis is put into blood culture bottles at the bedside rather than sent in a sterile tube. The main reason is: