Primary Isolation of Shiga Toxigenic Escherichia coli from Environmental Sources

Since the time of the first microbe hunters, primary culture and isolation of bacteria has been a foundation of microbiology. Like other microbial methods, bacterial culture and isolation methodologies continue to develop. Although fundamental concepts like selection and enrichment are as relevant today as they were over 100 yr ago, advances in chemistry, molecular biology and bacterial ecology mean that today’s culture and isolation techniques serve additional supporting roles. The primary isolation of Shiga toxigenic Escherichia coli (STEC) from environmental sources relies on enriching the target while excluding extensive background flora. Due to the complexity of environmental substrates, no single method can be recommended; however, common themes are discussed. Brilliant Green Bile Broth, with or without antibiotics, is one of many broths used successfully for selective STEC enrichment. Stressed cells may require a pre-enrichment recovery step in a nonselective broth such as buffered peptone water. After enrichment, immunomagnetic separation with serotype specific beads drastically increases the chances for recovery of STEC from environmental or insect sources. Some evidence suggests that acid treating the recovered beads can further enhance isolation. Although it is common in human clinical, food safety, and water quality applications to plate the recovered beads on Sorbitol MacConkey Agar, other chromogenic media, such as modified CHROMagar, have proven helpful in field and outbreak applications, allowing the target to be distinguished from the numerous background flora. Optimum conditions for each sample and target must be determined empirically, highlighting the need for a better understanding of STEC ecology. Primary Isolation of Shiga Toxigenic Escherichia coli from Environmental Sources