Microbiology: Gram-Positive Cocci Identification Flashcards

A medical laboratory technologist is analyzing a culture from a urine sample of a 22-year-old female patient with symptoms of a urinary tract infection. The Gram stain reveals Gram-positive cocci in clusters. The organism is catalase-positive and coagulase-negative. Which of the following tests is most crucial for the presumptive identification of *Staphylococcus saprophyticus*?

Answer: Novobiocin susceptibility

*Staphylococcus saprophyticus* is a common cause of urinary tract infections in young, sexually active females. A key characteristic that differentiates it from other coagulase-negative staphylococci, such as *Staphylococcus epidermidis*, is its resistance to the antibiotic novobiocin. Bacitracin is used for streptococci, bile esculin for enterococci and Group D streptococci, and PYR is primarily for *Enterococcus* and *Streptococcus pyogenes*.

An alpha-hemolytic, catalase-negative, Gram-positive coccus is isolated from a sputum culture. The colonies are mucoid and demonstrate a zone of inhibition of 16 mm around a 5 µg optochin disk. What is the most likely identification of this organism?

Answer: Streptococcus pneumoniae

*Streptococcus pneumoniae* is characteristically alpha-hemolytic and is susceptible to optochin, typically showing a zone of inhibition of 14 mm or greater around a 6mm, 5 µg disk. Its presence in a sputum culture, along with the mucoid colony appearance (suggesting a capsule), strongly indicates this identification. *S. agalactiae* and *S. pyogenes* are beta-hemolytic, and *Enterococcus faecalis* is typically non-hemolytic or alpha-hemolytic but is optochin-resistant.

A technologist performs a CAMP test to identify a beta-hemolytic Streptococcus isolated from a neonatal blood culture. An arrowhead-shaped zone of enhanced hemolysis is observed where the isolate meets the streak of *Staphylococcus aureus*. This result is indicative of which organism?

Answer: Streptococcus agalactiae (Group B)

The CAMP test is a classic method used for the presumptive identification of *Streptococcus agalactiae* (Group B Streptococcus). This organism produces a protein called CAMP factor that acts synergistically with the beta-hemolysin produced by *Staphylococcus aureus*, resulting in an arrowhead-shaped zone of enhanced hemolysis. *S. agalactiae* is a significant cause of neonatal sepsis and meningitis.

Which of the following Gram-positive cocci is a key pathogen known for being catalase-positive, coagulase-positive, and often beta-hemolytic on blood agar?

Answer: Staphylococcus aureus

*Staphylococcus aureus* is distinguished from other staphylococci by its production of coagulase, an enzyme that clots plasma. It is also catalase-positive, which differentiates it from streptococci and enterococci. Many strains of *S. aureus* produce hemolysins that cause beta-hemolysis on blood agar.

A Gram-positive coccus, growing in chains, is isolated from a throat swab. It is beta-hemolytic and catalase-negative. A rapid PYR test is performed and yields a bright cherry-red color. Which organism is most likely present?

Answer: Streptococcus pyogenes

*Streptococcus pyogenes* (Group A Streptococcus) is a primary cause of bacterial pharyngitis. It is beta-hemolytic, catalase-negative, and gives a positive result in the PYR (pyrrolidonyl arylamidase) test, which produces a red color. *Streptococcus agalactiae* is PYR-negative. *Streptococcus pneumoniae* is alpha-hemolytic and PYR-negative. *Staphylococcus saprophyticus* is catalase-positive.

An isolate from a wound infection shows Gram-positive cocci in pairs and chains. The organism is catalase-negative and grows on bile esculin agar, turning the slant black. It also demonstrates growth in 6.5% NaCl broth. What is the most probable identification?

Answer: Enterococcus faecalis

The ability to hydrolyze esculin in the presence of bile (turning the agar black) and to tolerate a high salt concentration (growth in 6.5% NaCl) are hallmark characteristics of the genus *Enterococcus*. While some Group D streptococci (like the *Streptococcus bovis* group) are bile esculin positive, they do not grow in 6.5% NaCl. *S. pyogenes* and *S. agalactiae* are negative for both tests.