Notes on Gram Negative Rods and Enterobacteriaceae

Gram Negative Rods

  • Family: Enterobacteriaceae

Microbes in Our Gut: Enemies or Allies?

  • Ilya Mechnikov (1845–1916), a Ukrainian microbiologist, researched the importance of normal gut flora.
    • Developed the theory that aging is caused by toxic bacteria in the gut.
    • Proposed that lactic acid could prolong life, introducing the concept of probiotics.
  • Some doctors believed in excessive colon removal to eliminate toxins.

Germ-Free Animals

  • Germ-free animals require 30% more calories as their gut bacteria help in food absorption.
  • They lack vitamin K, essential for blood clotting, which is normally produced by gut bacteria.
  • Observations on germ-free animals include:
    • Portions of the gut balloon out, interfering with other organs.
    • Improper development of intestinal walls and mucus production.
  • Benefits of Normal Gut Flora:
    • Helps digest food.
    • Excretes antibiotics.
    • Trains the immune system to distinguish between invaders and normal flora.
    • Produces vitamins, including K and B7.

Studies of Germ-Free Mice

  • Germ-free mice show improper intestine development compared to normal mice, impacting health.
  • Recent advancements allow scientists to analyze the genetic makeup of gut microbes, improving our understanding of their roles.

The Role of Gut Flora in Health

  • Digestion:
    • Bacteria digest healthy sugars in breast milk, important for infant health.
    • Certain bacteria metabolize fiber, producing short-chain fatty acids that promote gut health, may prevent diabetes, heart disease, and cancer.
  • Immune System:
    • The gut microbiome influences immune responses through communications with immune cells.
  • Brain Health:
    • Neuroactive substances from gut bacteria may affect emotions, cognition, and pain management.

Characteristics of the Enterobacteriaceae Family

  • Common members in the family include:
    • E. coli (except O157:H7)
    • Klebsiella
    • Enterobacter
    • Citrobacter
    • Proteus
  • Many are normal enteric flora but can also act as pathogens.

Pathogenic Species in Enterobacteriaceae

  • Pathogens (non-lactose fermenters): Shigella, Salmonella, Yersinia, and E. coli O157:H7.

Characteristics of Enterobacteriaceae Family

  • Gram-negative non-rods, facultative anaerobes.
  • Oxidase negative and ferment glucose.
  • Motility: may be motile by peritrichous flagella or non-motile.
  • They grow well on MacConkey agar.

Escherichia coli

  • Causes more urinary tract infections (UTIs) than any other organism.
    • Responsible for 80%-90% of UTIs, primarily in women.
  • Can lead to life-threatening conditions like septicemia and meningitis.
  • Known for causing "Traveler's Diarrhea" (Montezuma's Revenge), with some strains producing toxins similar to Shigella.
  • E. coli O157:H7 can cause severe gastroenteritis, associated with shiga toxins.

Symptoms and Complications of STEC Infections

  • Symptoms include severe stomach cramps, bloody diarrhea, and vomiting.
    • 5-10% of cases can lead to hemolytic uremic syndrome (HUS), a life-threatening condition.

Salmonella Species

  • Two key species: S. typhi and S. enteritidis.
  • Incubation time: 12 - 36 hours.
  • Symptoms: fever, cramps, diarrhea, potential bloodstream invasion, particularly in vulnerable populations.
  • Common sources include poultry and eggs, with a high risk of transmission.

Typhoid Fever

  • Caused by Salmonella typhi, with humans being the exclusive carriers.
  • Characterized by a long incubation period, fever, headache, and gastrointestinal symptoms.
  • Chronic carriers can shed the bacteria for extended periods.

Shigella

  • Causes severe diarrhea (bacillary dysentery) with an infective dose <10 organisms.
  • Antibiotics (e.g., fluoroquinolones) can shorten the period of infectivity.

Yersinia pestis: Plague

  • Historical pandemics, including the Black Death.
  • Symptoms onset after flea bite or pneumonia, with high mortality if untreated.
  • Rodents are natural reservoirs; transmission occurs via fleas.

Enterobacter and Other Species

  • Klebsiella pneumoniae, Serratia marcescens, and at times, Proteus mirabilis are opportunistic pathogens in humans.
  • Enterobacter spp. associated with wounds; resistance to numerous antibiotics is common due to acquired ESBLs.