a | Some bacteriocins, and in particular many of those that inhibit Gram-positive bacteria, function by targeting the cell envelope. Some class I bacteriocins inhibit lipid II on the cell membrane, thereby abrogating peptidoglycan synthesis. Other bacteriocins form pores to inhibit or kill their target bacterium. For example, class II bacteriocins such as lactococcin A bind to the pore-forming receptor mannose phosphotransferase system (Man-PTS). Nisin and some other class I bacteriocins both inhibit peptidoglycan synthesis and form pores. Other class I peptides, such as the thiopeptides and bottromycins, control Gram-positive bacteria by targeting translation (not shown). b | Many bacteriocins that inhibit Gram-negative bacteria (and thus need to be transported through the outer and, in many cases, inner membranes before functioning) control their target bacteria by interfering with DNA, RNA and protein metabolism. For example, microcin B17 (MccB17) inhibits DNA gyrase, MccJ25 inhibits RNA polymerase, and MccC7-C51 inhibits aspartyl-tRNA synthatase. There are also exceptions, such as MccE492, that function through pore formation.