How might a cell take in a bacterium?

What causes ingestion of bacteria: Special bacterial recognition surface receptors on the cell's plasma membrane recognize the bacteria as bacteria and thus food.

How these cause it to ingest the bacteria: These receptors cause signalling within the cell to occur. The signal is passed down through ATP-activated signalling proteins specialized for ingestion until it reaches the nucleus. In the nucleus, the signal is given to code for the "ingestion" action. It is coded onto mRNA which codes for cell-wide signalling. These signals reach all the organelles, which then work as one unit to engulf the bacteria with the plasma membrane.