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Introduction
1. Anatomy
2. Microbiology
2.1 General bacteriology
2.2 Introduction to systemic bacteriology
2.3 Gram positive cocci
2.4 Gram negative cocci
2.5 Gram positive bacilli
2.6 Gram negative bacilli
2.6.1 Introduction
2.6.2 E.coli
2.6.3 Klebsiella pneumoniae
2.6.4 Proteus
2.6.5 Vibrio cholerae
2.6.6 Other Vibrios
2.6.7 Salmonella
2.6.8 Pseudomonas
2.6.9 Minor pathogens
2.7 Other important bacteria
2.8 Virology
2.9 Parasitology
2.10 Mycology
3. Physiology
4. Pathology
5. Pharmacology
6. Immunology
7. Biochemistry
8. Cell and molecular biology
9. Biostatistics and epidemiology
10. Genetics
11. Behavioral science
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2.6.9 Minor pathogens
Achievable USMLE/1
2. Microbiology
2.6. Gram negative bacilli

Minor pathogens

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Enterobacter

Colonies are mucoid and lactose-fermenting, similar to Klebsiella colonies. It is mainly a nosocomial pathogen associated with UTIs, pneumonia, and sepsis following instrumentation.

Serratia

Produces a non-diffusible red pigment in culture. It is a slow lactose fermenter. It is associated with hospital-acquired UTIs, pneumonia, sepsis, wound infections, and endocarditis in intravenous drug abusers.

Citrobacter

A minor pathogen that can cause UTIs, meningitis, and otitis media. It may show false-positive tests for Salmonella due to antigen sharing.

Shigella

It is the causative organism of dysentery (passage of blood and mucus in loose stools).

Human pathology

Some strains produce exotoxins. The most dangerous is verocytotoxin, which is the same as the EHEC verotoxin (as discussed above under EHEC). Even as few as 10 bacilli can cause infection.

Patients present with:

  • Abdominal cramps
  • Passage of blood and mucus in stools
  • Tenesmus

Fever may be present. Intestinal ulcers may develop. Secondary complications can occur, including arthritis, conjunctivitis, intussusception, parotitis, and HUS.

Laboratory diagnosis of shigellosis

Stool samples show pus cells and RBCs. Culture on Mac Conkey’s agar shows non-lactose-fermenting colonies.

No H2S is produced, so there is no blackening of:

  • TSI
  • SS agar (Salmonella Shigella agar)
  • Wilson and Blair medium

Biochemical tests help differentiate the strains. Mannitol is fermented by all strains except Shigella dysenteriae. Shigella sonnei ferments sucrose and lactose late.

Enterobacter

  • Mucoid, lactose-fermenting colonies (resembles Klebsiella)
  • Nosocomial pathogen: UTIs, pneumonia, sepsis (post-instrumentation)

Serratia

  • Produces non-diffusible red pigment in culture
  • Slow lactose fermenter
  • Causes hospital-acquired UTIs, pneumonia, sepsis, wound infections, endocarditis (IV drug abusers)

Citrobacter

  • Minor pathogen: UTIs, meningitis, otitis media
  • May give false-positive Salmonella tests (antigen sharing)

Shigella

  • Causative agent of dysentery (blood and mucus in stools)
  • Some strains produce exotoxins, notably verocytotoxin (same as EHEC verotoxin)
  • Extremely low infectious dose (as few as 10 bacilli)

Shigella: Human pathology

  • Symptoms: abdominal cramps, blood/mucus in stools, tenesmus, fever
  • Complications: intestinal ulcers, arthritis, conjunctivitis, intussusception, parotitis, HUS

Laboratory diagnosis of shigellosis

  • Stool: pus cells and RBCs
  • Culture: non-lactose-fermenting colonies on Mac Conkey’s agar
  • No H2S production (no blackening on TSI, SS agar, Wilson and Blair medium)
  • Biochemical tests:
    • Mannitol fermented by all except S. dysenteriae
    • S. sonnei ferments sucrose and lactose late

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Minor pathogens

Enterobacter

Colonies are mucoid and lactose-fermenting, similar to Klebsiella colonies. It is mainly a nosocomial pathogen associated with UTIs, pneumonia, and sepsis following instrumentation.

Serratia

Produces a non-diffusible red pigment in culture. It is a slow lactose fermenter. It is associated with hospital-acquired UTIs, pneumonia, sepsis, wound infections, and endocarditis in intravenous drug abusers.

Citrobacter

A minor pathogen that can cause UTIs, meningitis, and otitis media. It may show false-positive tests for Salmonella due to antigen sharing.

Shigella

It is the causative organism of dysentery (passage of blood and mucus in loose stools).

Human pathology

Some strains produce exotoxins. The most dangerous is verocytotoxin, which is the same as the EHEC verotoxin (as discussed above under EHEC). Even as few as 10 bacilli can cause infection.

Patients present with:

  • Abdominal cramps
  • Passage of blood and mucus in stools
  • Tenesmus

Fever may be present. Intestinal ulcers may develop. Secondary complications can occur, including arthritis, conjunctivitis, intussusception, parotitis, and HUS.

Laboratory diagnosis of shigellosis

Stool samples show pus cells and RBCs. Culture on Mac Conkey’s agar shows non-lactose-fermenting colonies.

No H2S is produced, so there is no blackening of:

  • TSI
  • SS agar (Salmonella Shigella agar)
  • Wilson and Blair medium

Biochemical tests help differentiate the strains. Mannitol is fermented by all strains except Shigella dysenteriae. Shigella sonnei ferments sucrose and lactose late.

Key points

Enterobacter

  • Mucoid, lactose-fermenting colonies (resembles Klebsiella)
  • Nosocomial pathogen: UTIs, pneumonia, sepsis (post-instrumentation)

Serratia

  • Produces non-diffusible red pigment in culture
  • Slow lactose fermenter
  • Causes hospital-acquired UTIs, pneumonia, sepsis, wound infections, endocarditis (IV drug abusers)

Citrobacter

  • Minor pathogen: UTIs, meningitis, otitis media
  • May give false-positive Salmonella tests (antigen sharing)

Shigella

  • Causative agent of dysentery (blood and mucus in stools)
  • Some strains produce exotoxins, notably verocytotoxin (same as EHEC verotoxin)
  • Extremely low infectious dose (as few as 10 bacilli)

Shigella: Human pathology

  • Symptoms: abdominal cramps, blood/mucus in stools, tenesmus, fever
  • Complications: intestinal ulcers, arthritis, conjunctivitis, intussusception, parotitis, HUS

Laboratory diagnosis of shigellosis

  • Stool: pus cells and RBCs
  • Culture: non-lactose-fermenting colonies on Mac Conkey’s agar
  • No H2S production (no blackening on TSI, SS agar, Wilson and Blair medium)
  • Biochemical tests:
    • Mannitol fermented by all except S. dysenteriae
    • S. sonnei ferments sucrose and lactose late