Which enzyme in tears and saliva attacks bacterial walls?

Lysozyme, which cuts the sugar backbone of bacterial cell walls so the cell bursts under its own internal pressure. It is in tears, saliva, mucus and egg white - a chemical defence at every surface where the outside world gets in.

Fleming found it in 1922, six years before penicillin, and by the same method: he had a cold, a drop from his nose landed on a culture plate, and the bacteria around it died. He hoped he had found a universal antibiotic. He had not - lysozyme works well on relatively harmless bacteria and poorly on the dangerous ones, whose walls are protected by an outer membrane - and the disappointment is part of why he took the penicillin mould seriously when the same thing happened again.

It has a second life as a laboratory workhorse. Lysozyme was the first enzyme to have its three-dimensional structure solved, in 1965, and the first for which a detailed mechanism was proposed from that structure - the moment when it became possible to see how an enzyme does what it does rather than infer it. It is still the standard test protein for new crystallography and microscopy methods, because everyone already knows what the answer should look like.