Final answer:
Staphylococcus aureus's ability to cause infections would be reduced in the presence of an anti-clotting drug due to its reliance on coagulase to form blood clots as part of its infection strategy.
Step-by-step explanation:
The bacterium that would have a reduced ability to infect cells in the presence of an anti-clotting drug is Staphylococcus aureus (B). S. aureus differs from most other staphylococci due to its production of the enzyme coagulase, which induces blood clot formation. This is a significant factor in its ability to cause infections. Anti-clotting drugs would potentially impede S. aureus's ability to use this mechanism, thereby reducing its ability to infect cells.