Enterobacteriaceae Flashcards

Classification of Enterobacteriaceae

• Classified based on biochemical properties, antigenic structure, DNA-DNA hybridization, and 16S rRNA sequencing.
• Distinction made due to diverse pathogenic capabilities.

Human Diseases Caused

• Responsible for approximately one-third of all bacteremias.
• Account for over 70% of urinary tract infections (UTIs).
• Include numerous intestinal infections.

Pathogenic vs. Commensal Organisms

• Salmonella serotype Typhi, Shigella species, and Yersinia pestis are always linked to human disease.
• Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis are normally commensal but can lead to opportunistic infections.

Pathogenicity Acquisition

• Commensal organisms may become pathogenic after acquiring virulence genes via plasmids, bacteriophages, or pathogenicity islands.

Antigens

• Enterobacterial common antigen serves as a common antigen among this family.
• Key to serological classification includes O polysaccharides, K antigens, and H proteins.

Growth and Characteristics

• Enterobacteriaceae can be motile with peritrichous flagella or nonmotile; spores are not formed.
• Aerobically and anaerobically grown on nonselective (e.g., blood agar) and selective media (e.g., MacConkey agar).
• Nutritional requirements are simple; they ferment glucose, reduce nitrate, are catalase-positive, and oxidase-negative.

Lactose Fermentation

• Lactose-fermenting strains include Escherichia, Klebsiella, Enterobacter, Citrobacter, and Serratia, which produce pink-purple colonies on MacConkey agar.
• Non-lactose-fermenting strains like Proteus, Salmonella, Shigella, and Yersinia spp. yield colorless colonies on MacConkey agar.

Resistance and Virulence

• Shigella and Salmonella exhibit resistance to bile salts.
• Klebsiella and some Enterobacter and Escherichia strains possess capsules that protect against phagocytosis.
• Lipid A in LPS is crucial for endotoxic activity and as a virulence factor.

Serologic Classification

• The epidemiologic classification is performed based on antigen types: somatic O polysaccharides, K antigens, and H flagellar proteins.
• Antigen detection is achieved through agglutination with specific antibodies.

Adherence and Genetic Transfer

• Common fimbriae enhance adherence to host cell receptors.
• Sex pili facilitate genetic transfer between bacterial cells.

Common Virulence Factors

• Include endotoxin, capsule, antigenic phase variation, type III secretion systems, growth factor sequestration, serum resistance, and antimicrobial resistance.

Endotoxin Effects

• Systemic manifestations of gram-negative infections feature activation of complement, cytokine release, leukocytosis, thrombocytopenia, and potentially lead to shock and death.

Encapsulation Impact

• Encapsulated strains are shielded from phagocytosis due to hydrophilic capsular antigens that repel phagocytic cells.

Phase Variation

• Offers protection from antibody-mediated cell death, as K, H, and O antigens may vary in expression.

Type III Secretion System

• Serves as a common mechanism for delivering virulence factors into eukaryotic cells, crucial for pathogenicity.

Test your knowledge on Enterobacteriaceae with these flashcards. Explore their biochemical properties, diseases they cause, and key genera. This quiz is perfect for students studying microbiology or related fields.