Free AP-MICRO Bacteriology: Gram-Positive Identification Questions and Answers 1 — Questions and Answers
Question 1: A Gram-positive coccus isolated from a urine culture is catalase-positive and coagulase-negative. Which of the following tests would be most appropriate to differentiate Staphylococcus saprophyticus from other coagulase-negative staphylococci?
- Bacitracin susceptibility
- Bile esculin hydrolysis
- Novobiocin susceptibility (Correct answer)
- CAMP test
Correct answer: Novobiocin susceptibility
Staphylococcus saprophyticus, a common cause of urinary tract infections in young, sexually active females, is differentiated from other coagulase-negative staphylococci (like S. epidermidis) by its resistance to the antibiotic novobiocin. Most other coagulase-negative staphylococci are susceptible to novobiocin.
Question 2: A laboratory technologist is working on the identification of an alpha-hemolytic, Gram-positive coccus isolated from a sputum culture. Which of the following test results would presumptively identify the organism as Streptococcus pneumoniae?
- Susceptibility to bacitracin (A disk)
- Positive bile esculin test
- Positive CAMP test
- Susceptibility to optochin (P disk) (Correct answer)
Correct answer: Susceptibility to optochin (P disk)
Streptococcus pneumoniae is characteristically susceptible to optochin, which is the chemical ethylhydrocupreine hydrochloride. A zone of inhibition of 14 mm or greater around a 6-mm optochin disk is a key feature for its presumptive identification and differentiation from other alpha-hemolytic streptococci (viridans group), which are typically resistant.
Question 3: A beta-hemolytic, Gram-positive coccus is isolated from a throat swab. The organism is catalase-negative. Which single rapid test provides the most definitive presumptive identification of Streptococcus pyogenes (Group A Streptococcus)?
- Coagulase test
- PYR (pyrrolidonyl arylamidase) test (Correct answer)
- Bile solubility test
- Growth in 6.5% NaCl
Correct answer: PYR (pyrrolidonyl arylamidase) test
The PYR (pyrrolidonyl arylamidase) test is a rapid and highly specific test for the presumptive identification of Streptococcus pyogenes (Group A Strep) and Enterococcus species. S. pyogenes produces the enzyme L-pyrrolidonyl arylamidase, which hydrolyzes the PYR substrate to produce a beta-naphthylamine, resulting in a red color after the addition of a cinnamaldehyde reagent. This test is more specific than bacitracin susceptibility.
Question 4: A technologist observes an arrowhead-shaped zone of enhanced hemolysis at the junction where a beta-hemolytic Streptococcus is streaked perpendicular to a Staphylococcus aureus streak on a sheep blood agar plate. This phenomenon is characteristic of which organism?
- Streptococcus pneumoniae
- Enterococcus faecalis
- Streptococcus pyogenes
- Streptococcus agalactiae (Correct answer)
Correct answer: Streptococcus agalactiae
The arrowhead-shaped zone of enhanced hemolysis describes a positive CAMP test. This test is used to identify Streptococcus agalactiae (Group B Streptococcus), which produces an extracellular protein (CAMP factor) that acts synergistically with the beta-hemolysin produced by Staphylococcus aureus to increase the lysis of red blood cells.
Question 5: A Gram-positive coccus growing in chains is isolated from a blood culture. The organism is catalase-negative, does not produce hemolysis on sheep blood agar (gamma-hemolytic), and grows in 6.5% NaCl broth. It also hydrolyzes esculin in the presence of bile. Which of the following is the most likely identification?
- Staphylococcus aureus
- Streptococcus bovis
- Enterococcus faecalis (Correct answer)
- Streptococcus pneumoniae
Correct answer: Enterococcus faecalis
The combination of key characteristics—catalase-negative, growth in 6.5% NaCl, and a positive bile esculin test—is definitive for the identification of Enterococcus species. While Streptococcus bovis (Group D) is bile esculin positive, it does not typically grow in 6.5% NaCl.
Question 6: Which of the following biochemical tests is based on the principle that an organism can hydrolyze esculin to esculetin in the presence of bile, with the resulting esculetin reacting with ferric citrate to form a dark brown or black precipitate?
- Catalase test
- Bile esculin test (Correct answer)
- Coagulase test
- Optochin susceptibility test
Correct answer: Bile esculin test
The bile esculin test is designed to identify organisms, primarily Enterococcus and Group D Streptococcus, that can hydrolyze the glycoside esculin in the presence of bile. The hydrolysis product, esculetin, reacts with ferric citrate in the medium to produce a dark brown to black phenolic iron complex, indicating a positive result.
Free AP-MICRO Bacteriology: Gram-Positive Identification Questions and Answers 1
A Gram-positive coccus isolated from a urine culture is catalase-positive and coagulase-negative.
Which of the following tests would be most appropriate to differentiate Staphylococcus saprophyticus from other coagulase-negative staphylococci?