Enterobacteriaceae pt. 1

Introduction to Enteric Infections

• General Overview: Enterobacteriaceae family primarily includes intestinal bacteria.

  • Typically not harmful unless they become pathogenic (take over and cause disease).

Common Locations and Effects

• Intestinal Location: Most enteric bacteria are found in the intestines.

  • Result of Infections:

  • Diarrhea is almost always caused if an infection occurs in the intestines.

  • Presence of these bacteria can be found in stool during diarrheal disease.

• Urinary Tract Infections (UTIs):

  • These can occur, particularly in females (young girls, kids).

  • Important hygiene practice: Wipe front to back to prevent fecal matter from contaminating the urethra.

  • UTIs can ascend to the bladder, causing cystitis.

• Rare Cases:

  • Some enteric bacteria may cause pneumonia (e.g., Klebsiella pneumoniae).

  • Skin infections from enteric bacteria are rare except in cases of sacral wounds (bed sores).

Risk Factors for Severe Infections

• At-Risk Populations:

  • Older adults in nursing homes using diapers.

  • If left with fecal matter, risk of UTI increases, causing sepsis.

Bacterial Characteristics

• Morphology:

  • All enteric bacteria are gram-negative rods; some may be coccobacillary (bean-shaped).

  • They are leomorphic (variable shapes).

• Metabolism:

  • All ferment glucose.

  • Biochemical testing outcomes for Triple Sugar Iron (TSI):

  • TSI results will show either A/A or K/A (

    • A = acid from fermentation turning medium yellow

    • K = no acid, remains red).

• Nitrate Reduction:

  • All members reduce nitrate to nitrite.

• Motility:

  • Most are motile at 37 degrees Celsius, exceptions include Shigella and Klebsiella.

• Oxygen Requirements:

  • All are facultative anaerobes (can grow in the presence or absence of oxygen).

• Oxidase Test:

  • All are oxidase negative (important for identification).

Colony Morphology

• Appearance on media:

  • Large, moist, gray colonies common on standard media.

  • Odorous growth is typical due to intestinal origin.

  • Coccobacillary forms tend to look irregular (like kidney beans or footballs).

Identifying Lactose Fermentation

• MacConkey Agar:

  • Lactose fermenters turn the medium pink (e.g., non-fermenters remain colorless).

• If checking TSI results for lactose fermenters, usually A/A.

  • No need for a TSI if have confirmed lactose fermentation on MacConkey.

Pathogenicity and Antigenic Factors

• Primary vs. Opportunistic Pathogens:

  • Primary pathogens are not part of normal flora (e.g., Salmonella, Shigella).

  • Opportunistic pathogens are part of normal flora, causing disease under certain conditions (like in infections from E. coli).

• Antigenic Features:

  • O Antigen: Somatic antigen on the cell wall, heat-stable.

  • H Antigen: Flagellar antigen, heat-labile.

  • K Antigen: Capsular antigen, heat-labile and specific to certain species.

• E. Coli Pathogenicity:

  • O157:H7 serotype produces shiga toxin associated with hemolytic uremic syndrome.

Types of E. Coli

1. Enterotoxigenic E. coli (ETEC):

   • Transmission: Common in tropical and subtropical areas; cause of traveler's diarrhea (Montezuma's revenge).

   • Symptoms: Watery diarrhea, abdominal cramps, mild and self-limiting.

   • High Infectious Dose: Treatment involves fluid replacement.

2. Enteropathogenic E. coli (EPEC):

   • Symptoms: Low-grade fever, vomiting, diarrhea with mucus (mucusy diarrhea).

   • Primarily affects: Children under one year.

3. Enteroinvasive E. coli (EIEC):

   • Severe Symptoms: Intense cramps, watery diarrhea, penetrates mucosal lining.

   • Transmission: Fecal-oral route.

4. Enterohemorrhagic E. coli (EHEC):

   • Specifically O157:H7.

   • Symptoms: Causes hemolytic uremic syndrome, linked to undercooked meat.

   • Treatment: Limited options; requires thorough cooking of meat to prevent transmission.

5. Enteroadherent E. coli:

   • Associated with: Cystitis in children, pyelonephritis in pregnant women, diarrheal disease.

   • Mechanism: Adheres to intestinal microvilli, forms mucus biofilm.

Biochemical Tests for Identification

• Methyl Red (MR) test:

  • Positive for E. coli.

• Voges-Proskauer (VP) test:

  • Negative for E. coli.

• Indole Test:

  • Positive for E. coli.

• Citrate test:

  • Negative for E. coli.

• Summary of MVIC for E. coli:

  • Indole: +

  • Methyl Red: +

  • Voges-Proskauer: -

  • Citrate: -

Conclusion and Study Recommendations

• Study Tips:

  • Pay attention to MVIC results for understanding differentiation among organisms.

  • Know the characteristics of lactose fermenters vs non-lactose fermenters.

  • Familiarize with the testing steps for accurate microbial identification.

• Upcoming Tests: Expect to differentiate between organisms based on biochemical characteristics (TSI and MVIC).