A colony that is catalase-positive and nonhemolytic is most likely which organism?

The organism that fits the description of being catalase-positive and nonhemolytic is Staphylococcus epidermidis. This species is a type of coagulase-negative staphylococcus that is commonly found on human skin and mucous membranes. It exhibits catalase activity, meaning it can break down hydrogen peroxide into water and oxygen, which is an important characteristic that differentiates it from the catalase-negative genera such as Streptococcus and Enterococcus.

Staphylococcus epidermidis is also known for being nonhemolytic, meaning it does not lyse red blood cells or change the appearance of blood agar, making it distinct from other hemolytic species such as Staphylococcus aureus, which is hemolytic and produces the enzyme coagulase. Streptococcus pneumoniae and Enterococcus faecalis are also catalase-negative organisms and would not fit the criteria of being catalase-positive.

The ability of Staphylococcus epidermidis to resist hemolysis and exhibit catalase positivity is significant in clinical microbiology, particularly regarding skin flora and opportunistic infections in immunocompromised patients. Thus, Staphylococcus epidermidis is the best representative organism that aligns with the characteristic features described in the question.