Pathogens not only attempt to shield themselves from the weaponry of the host, they also have destructive weaponry of their own. The bacterium that causes most simple skin infections, Staphylococcus aureus, secretes a neuropeptide
that blocks the action of Hageman’s factor, a crucial first step in use- ful inflammation. Bacteria that cannot secrete this peptide do not cause infection. Even the common streptococcal bacteria that cause so many sore throats make streptolysin-O, which kills white blood cells. Vaccinia, the virus that causes cowpox, makes a protein that inhibits the complement system, an important host defense, as noted previously. Why doesn’t the complement system attack our own cells? In part because our cells have a layer of sialic acid, a chemical that protects them from attack by the complement system. Sure enough, certain bacteria, in this case the KI strain of the common E. coli that live in our guts, are able to cover themselves in sialic acid and thus gain protection from the complement system.
One of the great dangers of serious infection with certain kinds of bacteria is shock, a decrease in blood pressure that can be rapidly fatal. Shock is caused by chemical lipopolysaccharide (LPS) formed by the bacteria. Superficially, it would seem that LPS is a toxin made by bacteria to harm us, but, as researcher Edmund LeGrand has noted, this is unlikely, because LPS is a necessary component of the cell wall of this whole group of bacteria. Hosts recognize this reliable cue to the presence of dangerous infection and react strongly-some- times too strongly. Here is an example of a defensive weapon that can turn on its bearer.