FREE AP-MICRO Bacteriology: Gram-Positive Identification 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?

Accepted Answer

Novobiocin susceptibility

Answer

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?

Accepted Answer

Susceptibility to optochin (P disk)

Answer

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)?

Accepted Answer

PYR (pyrrolidonyl arylamidase) test

Answer

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?

Accepted Answer

Streptococcus agalactiae

Answer

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?

Accepted Answer

Enterococcus faecalis

Answer

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.

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FREE AP-MICRO Bacteriology: Gram-Positive Identification Questions and Answers