GI Gram (-) Rods/ Bacilli

• Enterobacteriaceae


  • Escherichia coli (E. coli)
  • Klebsiella pneumoniae
  • Proteus mirabilis
  • Enterobacter
  • Serratia
  • Salmonella
  • Shigella
  • Vibrio cholera
  • Campylobacter jejuni
  • Helicobacter pylori
  • Pseudomonas aeruginosa
• Enterobacteriaceae


  Gram (-) rods that can infect the GI system in humans and animals

  A variety of species can infect other organs and cause significant pathology

  Produce a wide variety of powerful exotoxins
• Enterobacteriaceae / GI Gram (-) Rods a

  ll usually grow well on MacConkey Agar which inhibits growth of gram (+) bacteria
• Enterobacteriaceae

  Bile salts & crystal violet dye inhibit G(+) bacteria

  Neutral red 🡪 pink if LACTOSE fermenter
• Escherichia coli (E. coli)


  G(-) rod

  Facultative anaerobic

  Lactose fermenter

  Indole (+)
• Indole Test


  Ability of bacteria to convert tryptophan into indole (by tryptophanase)

  (+) 🡪 red/red-violet layer

  (-) 🡪 yellow

  Variable 🡪 orange
• Escherichia coli (E. coli)

  Transmission: - Non-pathogenic strains (0.1%) are part of the normal GI flora (coliforms) - Pathogenic strains may be acquired from contaminated food and water or transmitted from person-to-person
• Escherichia coli (E. coli)

  Virulence factors: - Majority lack virulence factors and comprise the bulk of the normal GI flora
• Enteropathogenic E. coli (EPEC)


  Causes watery, non-bloody infectious diarrhea especially in infants

  Possesses factors which allow adherence and effacement of GI mucosa
• Enterotoxigenic E. coli (ETEC)


  Causes a watery, non-bloody infectious diarrhea ("Traveler's Diarrhea")

  • GI mucosal adherence factors
  • Heat Labile exotoxin (LT) - promotes fluid secretion by enterocytes
  • Heat-stable exotoxin (ST) worsens diarrhea by further enhancing fluid secretion by enterocytes
• Shiga-toxin E. coli (STEC)


  Causes severe, bloody infectious diarrhea ("Hemorrhagic Colitis")

  • GI mucosal adherence factors
  • Exotoxin Shiga Toxin
  • Some strains (especially O157:H7) can result in a Microangiopathic Hemolytic Anemia, classically Hemolytic Uremic Syndrome (HUS)
• Enteroinvasive E. coli (EIEC)


  • The only strain which can invade the enterocytes of the GI mucosa and thus induces a systemic immune response
  • Causes bloody infectious diarrhea, fever and white cells (pus) in the stool
• Uropathogenic E. coli (UPEC)


  • Possess a specialized pili, termed P. Fimbriae, which allows adhesion to the urothelium
  • Most common cause of UTI especially in young adults - dysuria, frequency, and urgency
  • If left untreated, complications include cystitis and acute pyelonephritis
• Klebsiella pneumoniae


  G(-), non-motile, encapsulated rod

  Facultative anaerobic

  Lactose fermenter

  Indole negative

  Transmission: part of the normal skin, oropharyngeal and intestine flora in a small proportion of individuals
• Pneumonia


  Very infrequent cause of community-acquired pneumonia except in alcoholics (due to aspiration) - Can be a cause of hospital-acquired pneumonia - hemorrhagic with a red, jelly-like, purulent sputum and it is caused by klebsiella pneumoniae
• Proteus mirabilis

  • Lactose Non- Fermenter
  • Indole (-)
  • found usually in soil and water
  • Virulence factors: - Urease (not exotoxin) splits urea, liberating ammonium that increases (alkalinizes) the urine pH - Abnormally increased urine pH can lead to precipitation of compounds that increases the risk of renal and urinary tract stones, especially of the "Struvite" subtype
  • Culture & detection: - highly motile - rather than forming distinct colonies on a plate, characteristically "swarms" across the entire surface of selective media
• Serratia

  • Lactose non-fermenter
  • indole negative
  • Some strains are characterized by a bright red pigment
• Serratia marcescens


  Infection is usually acquired in hospitals and only causes pathology in those who are ill with reduced immunity, especially hospitalized patients
• Salmonella

  • Lactose non-fermenter
  • Indole (-), motile
  • facultative intracellular anaerobe 1. Non-Typhoidal (NTS) 2. Typhoidal
• Non-Typhoidal Salmonella bongori


  Zoonotic bacteria that can live and grow in a variety of animals, especially birds such as poultry and exotic reptiles
• Transmission

  • Primarily through ingestion of contaminated food/water
  • Occurs in major outbreaks due to the modern centralization of the food supply
• The vast majority of cases of Salmonellosis are caused by Non-Typhoidal Salmonella bongori
• Ingestion of contaminated food

  Followed by a usually watery, non-bloody infectious diarrhea with an incubation time of more than 8 hours
• Self-limited diarrhea accompanied by nausea, vomiting, and abdominal pain
• Asplenic patients are especially prone to NTS sepsis
• Those with Sickle Cell Disease are prone to NTS osteomyelitis
• Non-Typhoidal Salmonella bongori


  Zoonotic bacteria that can live and grow in a variety of animals, especially birds such as poultry and exotic reptiles
• Transmissionof Salmonella Bongori (NTS)

  • Primarily through ingestion of contaminated food/water
  • Occurs in major outbreaks due to the modern centralization of the food supply
• Characteristics of Salmonellosis

  • Food poisoning
  • Watery, non-bloody infectious diarrhea
  • Incubation time of more than 8h
  • Self-limited diarrhea
  • Nausea
  • Vomiting
  • Abdominal pain
• Asplenic patients


  • Prone to NTS sepsis
• Those with Sickle Cell Disease

  • Prone to NTS osteomyelitis
• Typhoidal Salmonella Enterica


  Generally only infect humans (i.e. are not zoonotic) and are transmitted via a fecal-oral transmission
• Due to modern advances in hygiene Typhoid Fever is now rare in developed countries
• Invasion and dissemination of Typhoidal Salmonella Enterica

  1. Invade through enterocytes

  2. Phagocytosed by macrophages in Peyer's Patches

  3. Evade killing within phagosomes

  4. Disseminate throughout the body within macrophages

  5. Colonize immune organs such as the spleen, lymph nodes, liver, and intestinal lymphoid tissue (i.e. Peyer's Patches)
• Typhoid Fever

  • Manifests with fevers and constitutional symptoms along with abdominal pain and in some cases hepatosplenomegaly
  • Patients display a characteristic skin rash composed of small "Rose Spots" on their belly
• If untreated

  The intestinal Peyer's Patches may undergo necrosis and life-threatening lower GI bleeding ensues
• Some untreated patients return to health but can become "Chronic Carriers", shedding Salmonella from their feces for months or years and thus contaminating the food/water supply, thus contributing to fecal-oral transmission
• Shigella

  Lactose non-fermenter

  Indole negative

  Facultative anaerobic

  Similar To E.coli

  Purely human pathogen and does not possess an animal reservoir - Fecal-oral transmission and infection can occur with a very small infective dose (as low as 100 organisms) - Consequently, Shigellosis is usually a disease of cramped quarters with poor sanitation such as campaigning armies and child-care facilities
• Shiga Toxin


  Exotoxin of the A-B toxin family and displays enterotoxin behavior
• Shiga Toxin

  • B subunit directs absorption of the toxin complex
  • A subunit inhibits the mammalian ribosome leading to cell death