ASZ1 and infection: Figure 6C shows limited calpain activity in uninfected cells with the espA mutant causing a near-significant increase by 4 h post infection (P = 0.07; Fig. 1). Interestingly, while WT EPEC infection significantly increased calpain activity relative to espA-infected cells (P < 0.001), there was no significant difference between activity in espG/orf3- and espA-infected cells (P = 0.3). By contrast, cells infected with the tir mutant exhibited a significant (∼twofold) increase in calpain activity compared with WT EPEC (P < 0.001; Fig. 6C).