What is the most probable identification of an organism showing the following results: Coagulase = + at 4 hours, coagulase = Neg at 18 hours, and Novobiocin = Sensitive?

The organism showing these specific results — coagulase positive at 4 hours, coagulase negative at 18 hours, and Novobiocin sensitive — is most likely Staphylococcus aureus.

Staphylococcus aureus is known for its ability to produce coagulase, an enzyme that causes blood plasma to clot. A positive result within a short timeframe, such as 4 hours, is indicative of this species. However, as the culture grows longer, some strains may show coagulase negativity after longer incubation, which aligns with the results observed (positive initially, negative later).

Furthermore, Staphylococcus aureus is generally sensitive to Novobiocin, while Staphylococcus saprophyticus is typically resistant to this antibiotic. This additional information reinforces the identification of the organism as Staphylococcus aureus, as it fits the profile of coagulase activity and antibiotic sensitivity.

Thus, considering the combination of these biochemical tests, Staphylococcus aureus emerges as the most probable identification for the organism in question.