on the fact that each plaque on a lawn of bacteria,although it contains 106 to 107 virions along with bacterial debris, represents a single infecting phage that entered one cell at the start of the culture. The infection then “spread” as the viruses reproduced and cells lysed, eventually forming a visible plaque (figure 37.3). The titer of a phage suspension, therefore, is determined by counting the number of plaques that form from a given volume of suspension. Phage titer is expressed as laqueforming units (PFU) per milliliter (ml).