Enterobacteriaceae: Characteristics and Properties

• Enterobacteriaceae is the largest and most heterogeneous group of bacilli

Key traits

• Gram-negative
• Ubiquitous microorganisms found in soil, water, and vegetation
• Part of the normal intestinal flora of many animals, including humans
• Some are normal commensal microflora that can produce opportunistic infections (E. Coli, Klebsella pneumonae, proteus mirabilis)
• Can become pathogenic when they acquire virulence genes

General properties

• Non-spore forming
• Share a common antigen
• Can grow in selective media (blood) and selective media (MacConkey)
• Facultative anaerobes
• Ferment glucose
• Reduce nitrates
• Catalase-positive and oxidase-negative
• Ferment lactose (changes the color of culture media)
• Used to distinguish lactose-fermenting strains (Escherichia, Klebsiella, Enterobacter, Citrobacter, and Serratia): colonies of pink-purple color on MacConkey agar
• Resistance to bile salts

Klebsiella

• Presents a characteristic mucoid appearance (humid, elevated, viscous)
• Notable capsules surrounded by a viscous, diffusible, and loose biofilm

Lipopolysaccharide (LPS)

• Main antigen of the cell wall
• Composed of 3 components:
  • O-polysaccharide (outer)
  • Central polysaccharide
  • Lipid A
• Polysaccharide core is important for microorganism classification
• O polysaccharide classifies strains within a species
• Lipid A component of LPS is responsible for endotoxin activity (virulence factor)

Epidemiological classification

• 3 major groups of antigens:
  • O somatic polysaccharides
  • K capsule antigens
  • H proteins
• Most are mobile, except for Klebsiella, Shigella, and Yersinia
• Mobile strains surrounded by flagella
• Have fimbriae (pili), which are subdivided into 2 general classes:
  • Common fimbriae encoded by the chromosome and sex pili
  • Capacity to adhere to specific receptors on the host cell

Important Enterobacteria

• Escherichia coli (Urinary infection)
• Salmonella enterica, typhi, paratyphi, choleraesuis, typhimurium, enteritidis (Typhoid fever)
• Shigella dysenteriae, flexneri (Bacillary dysentery, presence of mucus and blood in feces)
• Salmonella serotype typhi, shigella, yersinia pestis are always associated with disease in humans

Pathogenicity and immunity

• Enterobacteria can be pathogenic and cause diseases in humans
• Salmonella serotype Typhi, Shigella, and Yersinia pestis are always associated with human illness

Virulence factors

• Endotoxin is a shared virulence factor among Gram-negative aerobic and anaerobic bacteria
• Depends on the lipid A component of lipopolysaccharide
• Released during cell lysis
• Systemic manifestations are initiated by endotoxin, release of cytokines, leukocytosis, thrombocytopenia
• Encapsulated enterobacteria are protected from phagocytosis by hydrophilic capsular antigens
• Repel the hydrophobic surface of phagocytic cells
• Variation of antigenic phase
  • Expression of somatic O antigens, capsular K antigens, and flagellar H antigens
  • Under genetic control of the microorganism

Secretion systems

• Type III secretion systems are present in Yersinia, Salmonella, Shigella, Pseudomonas, and Chlamydia
• Common effector system for transferring virulence factors
• Type III secretion system facilitates the transfer of virulence factors into target host cells
• Sequestration of growth factors:
  • Iron, a growth factor for bacteria, is bound to heme proteins (Hb and myoglobin) or iron-chelating proteins (transferrin or lactoferrin)
  • Bacteria counteract this binding by producing siderophores
• Resistance to the bactericidal effect of serum + the bacterial capsule can protect microorganisms from this bactericidal effect
• Antimicrobial resistance

Pathogens

• Possible causative agents of diarrheal diseases include:
  • ETEC
  • EHEC
  • SUH
  • EIEC
• Salmonella (Typhi)
• Shigella
• Yersinia
• Yersinia enterocolitica