E. coli: Characteristics and Classification

Laboratory Identification of E. coli

• Escherichia coli (E. coli) is a bacterium commonly found in the intestines of humans and animals but certain pathogenic strains can cause severe illnesses.

Learning Outcomes

• Understand the Enterobacteriaceae family.
• Understand lactose fermenter organisms.
• Recognize the lab identification of E. coli & Klebsiella.

Enterobacteriaceae Genera

• The Enterobacteriaceae family includes Escherichia coli, Klebsiella, Citrobacter, Salmonella, Shigella, Proteus, Serratia, Enterobacter, Morganella, Providencia, and Yersinia.

Classification of Enterobacteriaceae

• Enterobacteriaceae is divided based on lactose fermentation.
• Lactose fermenters produce rose pink colonies on MacConkey's medium, including E. coli, Klebsiella, Enterobacter, and Citrobacter.
• Lactose non-fermenters produce pale yellow colonies on MacConkey's medium, including Salmonella, Shigella, and Proteus.

Uses of MacConkey Agar

• MacConkey agar is selective and differential.
• It isolates Gram-negative enteric bacteria.
• It differentiates between lactose-fermenting and non-lactose-fermenting Gram-negative rods by colony color.

What is E. coli?

• E. coli is a Gram-Negative, Rod-Shaped bacterium found in the intestines of humans and animals.
• E. coli is a facultative anaerobic bacterium that can thrive in both oxygen-rich and oxygen-deprived environments.
• Most E. coli strains are commensal, aiding in digestion and nutrient absorption.
• Certain E. coli strains have virulence factors causing illness, like food poisoning and urinary tract infections.

Importance of E. coli Identification

• E. coli identification is crucial for protecting public health, especially with foodborne outbreaks or water contamination.
• Accurate identification is essential for diagnosing E. coli infections, allowing for appropriate treatment and preventing complications.
• Identifying E. coli strains helps researchers track outbreaks, identify contamination sources, and develop effective prevention strategies.

E. coli Morphology and Characteristics

• E. coli bacteria are Gram-negative, rod-shaped, 2-3 micrometers long and 0.5-1 micrometer wide, commonly found in animal intestines, including humans.
• E. coli possesses flagella, hair-like appendages for movement in liquid environments.
• Some E. coli strains have a capsule for protection against host defenses and environmental stresses.
• Pili are thin, hair-like structures that facilitate adherence to host cells, promoting colonization.

Additional Information

• E. coli's natural host is the intestinal tract of humans and animals and can grow on ordinary medium.
• E. coli can cause fecal pollution of water.
• E. coli are Gram-negative bacilli, motile, and non-spore-forming; some strains are capsulated.
• E. coli is aerobic and facultative anaerobic and is isolated from extraintestinal sources.

Diseases Caused by E. coli (Intestinal)

• There are 5 types of diarrheagenic E. coli.
• Enterotoxigenic E. coli
• Enteropathogenic E. coli
• Enteroinvasive E. coli
• Enteroaggregative E. coli
• Enterohaemorrhagic E. coli causes bloody diarrhea or hemorrhagic colitis, produces verocytotoxigenic toxins, and is antigenically O157:H7.

Specimen Collection and Handling

• Choose the appropriate specimen, like stool, urine, or blood based on the suspected infection.
• Follow aseptic techniques.
• Use transport media to preserve the viability and integrity of E. coli bacteria.
• Label the specimen container with patient information and collection date/time.
• Transport the specimen to the laboratory promptly.

Primary Isolation and Culture Media

• Selective media, like MacConkey agar, inhibit the growth of other bacteria, allowing E. coli to flourish.
• Differential media differentiate E. coli from others based on metabolic characteristics.
• Eosin methylene blue (EMB) agar differentiates lactose fermenters from non-fermenters.

Biochemical Identification Tests

• Indole Production identifies bacteria that produce indole from tryptophan.
• Methyl Red Test determines the ability of bacteria to produce acid from glucose fermentation.
• Voges-Proskauer Test identifies bacteria that produce acetoin, a neutral product of glucose fermentation, indicated by a pink color change.
• Citrate Utilization determines the ability of bacteria to use citrate as a sole carbon source, leading to a change in the medium's color.

Interpretation of E. coli Test Results

• A positive test result indicates E. coli presence, requiring investigation to determine the specific strain and associated risks.
• A negative result indicates E. coli absence but does not rule out other bacterial or viral infections.

Laboratory Diagnosis

• Specimen: diarrheal stool sample, urine pus, CSF, and blood.
• Samples are examined by direct smear stained with Gram stain, culture on MacConkey's medium, and identification of lactose-fermenting colonies by morphology and biochemical reactions.
• For diarrhea, isolated E. coli is tested by serotyping using specific O antigen and for toxin production by latex agglutination or ELISA.

E. coli Pathogenic Strains

• Enterohemorrhagic E. coli (EHEC) produces Shiga toxins, causing bloody diarrhea and potentially life-threatening complications, such as hemolytic uremic syndrome (HUS).
• Enterotoxigenic E. coli (ETEC) causes traveler's diarrhea, characterized by watery diarrhea, abdominal cramps, and vomiting.
• Enteroinvasive E. coli (EIEC) invades the intestinal lining, leading to bloody diarrhea and dysentery-like symptoms.
• Enteropathogenic E. coli (EPEC) attaches to the intestinal lining, causing diarrhea, vomiting, and dehydration, particularly in infants.

Sorbitol Fermentation

• In cases caused by enterohaemorrhagic E. coli, culture of the isolated bacteria is done on MacConkey’s sorbitol agar to differentiate O157:H7 from normal commensal strains.
• Commensal E. coli produces rose pink colonies (sorbitol fermenter).
• E. coli 157: H7 produces yellow colonies (non-sorbitol fermenter).
• The type of fermentation: sorbitol fermentation.

Biochemical Reaction of E.coli O157:H7

• Ferments all sugar with production of acid and gas.
• Indole positive.
• Citrate negative.
• Positive methyl red.
• Urease negative.
• Lysine positive.
• TSI = A/A.
• H₂ S negative.
• Ornithine +/-.
• Motile.

IMViC Tests

• IMViC is a group of four tests: Indole, Methyl Red, Voges-Proskauer, and Citrate used to differentiate Enterobacteriaceae members, each with a unique result formula.

E. coli-Associated Diseases and Infections

• Gastroenteritis: common cause of diarrhea, abdominal cramps, and fever, often self-limiting but can be severe in infants, children, and the elderly.
• Urinary Tract Infections (UTIs): E. coli can cause UTIs; symptoms include pain during urination, frequent urination, and blood in the urine.
• Respiratory Infections cause pneumonia.
• Bacteremia can spread to the bloodstream, causing bacteremia, requiring immediate medical attention.
• Meningitis can be associated with E. coli infections.

Prevention and Control Measures

• Sanitation and Hygiene: Handwashing with soap and water and thorough disinfection of surfaces, especially in food preparation areas.
• Safe Food Handling: Cooking meat and poultry to the recommended internal temperature and prevent cross-contamination by separating raw meat.
• Water Treatment: Public water systems treat water to remove pathogens, including E. coli; home methods like boiling can also reduce contamination risk.
• Vaccination: No vaccine is available to protect against E. coli, but some vaccines for other diarrheal diseases can reduce the risk of complications.

Clinical Significance and Public Health Impact

• Human Health: E. coli infections can lead to severe complications, especially in vulnerable populations.
• Economic Burden: E. coli outbreaks can result in significant healthcare costs and economic losses.
• Food Safety: Contaminated food and water are major sources of E. coli outbreaks, impacting food security.
• Public Health Strategies: Prevention and control measures, including hygiene practices, are crucial for managing E. coli.

Explore E. coli, a bacterium in the Enterobacteriaceae family commonly found in intestines. Learn to identify lactose fermenters like E. coli and Klebsiella using MacConkey agar. Understand their classification and differentiation in the laboratory.