Microbiology Flashcards

A catalase-positive, coagulase-negative staphylococcus from a young woman's urine is resistant to novobiocin. What is the most likely identification?

Answer: Staphylococcus saprophyticus

S. saprophyticus is the novobiocin-resistant CoNS associated with UTIs in young women.

A beta-hemolytic, catalase-negative coccus from a vaginal/rectal swab is CAMP positive and hippurate hydrolysis positive. Which organism is it?

Answer: Streptococcus agalactiae

Group B Streptococcus (S. agalactiae) is CAMP positive and hydrolyzes hippurate.

Which combination of results presumptively identifies Streptococcus pyogenes?

Answer: PYR positive, bacitracin susceptible

Group A strep is PYR positive and susceptible to bacitracin (A disk).

An alpha-hemolytic, catalase-negative, lancet-shaped diplococcus is optochin susceptible. Which additional test confirms the identification?

Answer: Bile solubility

S. pneumoniae is lysed by bile (sodium deoxycholate), distinguishing it from viridans streptococci.

A catalase-negative gram-positive coccus hydrolyzes esculin in the presence of bile and grows in 6.5% NaCl. What is the presumptive genus?

Answer: Enterococcus

Enterococcus is bile esculin positive and salt tolerant, while S. bovis group fails to grow in 6.5% NaCl.

A catalase-positive, small gram-positive rod from a neonate's CSF shows tumbling motility at room temperature. What is the likely organism?

Answer: Listeria monocytogenes

Listeria monocytogenes shows characteristic tumbling motility at 20-25°C and causes neonatal meningitis.

On Tinsdale medium, which feature indicates Corynebacterium diphtheriae?

Answer: Black colonies with a brown halo

C. diphtheriae reduces tellurite (black colonies) and has cysteinase producing a brown halo.