While appendicitis is clearly associated with bacterial pro- liferation within the appendix, no single organism is respon- sible. A mixed growth of aerobic and anaerobic organisms is usual. The initiating event causing bacterial proliferation is controversial. Obstruction of the appendix lumen has been widely held to be important, and some form of luminal obstruction, either by a faecolith (Figure 72.5) or a stricture, is found in the majority of cases.