Can EPEC (Enteropathogenic Escherichia coli) overgrow in stored samples?

EPEC E. coli Can Overgrow in Stored Samples

Yes, EPEC (Enteropathogenic Escherichia coli) can overgrow in stored samples, which is why specimens should be processed as soon as possible after collection and refrigerated if immediate processing is not possible.

Storage Conditions and Their Impact on E. coli

Recommended Storage Guidelines

• Specimens should be sent to the laboratory as soon as possible after collection 1
• Samples should ideally be processed immediately upon receipt by the laboratory 1
• If immediate processing is not possible:
  • Refrigerate specimens at 2-8°C until testing 1
  • Unpreserved specimens should not be held for >24 hours 1
  • Specimens in transport medium should not be held for >48 hours 1

Effects of Improper Storage

Improper storage conditions can significantly impact the viability and composition of E. coli in samples:

• Storage at room temperature allows for bacterial overgrowth, potentially altering the original microbial composition 2
• Overnight storage at 0-5°C can significantly reduce E. coli counts in water samples (mean reduction of 25%) 3
• Samples stored at -20°C show considerably lower bacterial survival rates compared to those stored at -70°C 2

Factors Affecting E. coli Growth in Stored Samples

Temperature

• Room temperature storage promotes bacterial overgrowth 2
• Refrigeration (2-8°C) slows growth but should be limited to short periods 1
• Freezing at -70°C with glycerol broth provides the best preservation of the original E. coli population 2

Time

• The longer the storage time, the greater the potential for changes in the microbial composition 2
• Repeated freeze-thaw cycles further alter the bacterial population 1
• Even refrigerated samples show changes in E. coli counts after 24 hours 3

Storage Medium

• Addition of glycerol broth helps maintain the original E. coli flora composition during frozen storage 2
• Transport media (e.g., Cary-Blair) can extend acceptable storage time to 48 hours 1
• Direct toxin testing requires refrigeration without transport medium 1

Best Practices for Sample Handling

Immediate Processing

• Process samples as soon as they are received by the laboratory 1
• For optimal results, samples should be processed within hours of collection 1

If Immediate Processing Is Not Possible

• Refrigerate at 2-8°C for short-term storage (up to 24 hours unpreserved) 1
• Use appropriate transport medium if longer storage is necessary (up to 48 hours) 1
• For long-term storage, freeze at -70°C to -80°C with 20% glycerol or another suitable storage medium 1

Special Considerations

• Minimize freeze-thaw cycles as they significantly impact microbial cell integrity 1
• Aliquot fresh samples before initial freezing to avoid multiple freeze-thaw cycles 1
• For certain resistance phenotypes, selective pressure may be needed to maintain the desired phenotype during storage 1

Clinical Implications

The potential for EPEC overgrowth in stored samples has important clinical implications:

• False results may occur if samples are not properly handled
• Delayed processing without proper refrigeration may lead to overestimation of EPEC prevalence
• Improper storage can alter the original composition of the E. coli population, potentially masking the true clinical picture 2
• For accurate diagnosis and appropriate treatment decisions, proper sample handling is essential

By following proper sample collection, transport, and storage protocols, the risk of EPEC overgrowth can be minimized, leading to more accurate diagnostic results and better patient outcomes.