7.4 Clostridium perfringens
C. perfringens (Figure 10) is an anaerobic organism present in the intestines of humans and animals at much lower numbers than E. coli. It is a common cause of food poisoning. The cells are rod-shaped (about 5 µm by 1 µm) and can form endospores. They can cause severe abdominal cramp and diarrhoea if present in water that is ingested.
Endospores – or spores, as they are commonly referred to – are hardy structures that certain bacteria can form when their environment becomes unfavourable for growth. The purpose of the endospore is survival; it is very resistant to heat and desiccation, and may survive for many years at normal temperatures. When environmental conditions are favourable for growth, the endospore reactivates to form a normal, single cell. This ability leads to C. perfringens being present in unchanged numbers long after other faecal indicators have died out. Thus its presence in the absence of E. coli indicates intermittent faecal contamination.