Bacteria can run but they can't hide

I just stumbled across a video (https://www.youtube.com/watch?v=I_xh-bkiv_c&feature=kp) of a neutrophil chasing down and engulfing a bacterium. Although it’s one of those old, slightly grainy science films from the 1950s, there is something really compelling about the frantic vibrations of the bacterium, the remorseless pursuit by the neutrophil, and their eerily silent battle among the red blood cells. It’s a little Hitchcockian, don't you think? I leaned forward while I watched it, and now I can’t help anthropomorphizing the heck out of it while I discuss it. Although that isn’t completely out of line, since this very scene is being repeated a million times over right now inside me and you and every human. At any rate, it was interesting enough to get me to Google around a bit to refresh my memory on just what a neutrophil is and does. So you go, 1950s filmstrip.

As is so often the case in biology, what looks like a simple process – see bacteria, catch bacteria, kill bacteria – is actually quite complex. Neutrophils are one variety of a kind of white blood cells called granular leucocytes, which form a major part of the immune system. The cell membrane of each neutrophil is studded with chemical receptors. These receptors are specialized to detect the proteins that immune cells release when they encounter an infection or inflammation. When neutrophils receive such calls for help, they follow their chemical gradient back to the source with great speed. Upon arrival, foreign bacteria are destroyed through phagocytosis – the neutrophils engulf and then digest the invaders. The bacteria do get revenge of a sort. Eventually the neutrophils die, and are themselves phagocytosed and subsequently turned into pus, which isn’t a particularly glorious end for such staunch defenders.

Neutrophil (yellow) engulfing anthrax bacteria (orange). Scale bar is 5um. From PLoS Pathogens Vol. 1(3) November 2005.