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Enterobacteriaceae Overview

Jul 6, 2025

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Overview

This lecture discusses the opportunistic members of the Enterobacteriaceae family, focusing on their identification, key characteristics, pathogenic mechanisms, and clinical significance.

Structure of Enterobacteriaceae Lecture

  • The lecture is divided into three main parts:
    • Biochemical tests for Enterobacteriaceae (see recorded lecture for details).
    • Opportunistic members and their associated infections.
    • Pathologic members of Enterobacteriaceae.
  • Emphasis on watching the recorded lecture for biochemical identification methods.

Escherichia coli (E. coli)

  • E. coli is part of the normal flora of the bowel and genital tract but is also a significant human pathogen.
  • Associated with a wide range of clinical syndromes: urinary tract infections (UTI), diarrheal diseases, and central nervous system infections.
  • Leading cause of nosocomial infections and the primary marker for fecal contamination in water quality testing.
  • Antigenic determinants:
    • O antigen: cell wall, heat-stable.
    • H antigen: flagella, heat-labile.
    • K antigen: capsule, heat-labile polysaccharide covering the O antigen.
    • Pili: responsible for attachment to epithelial surfaces.
  • O and H antigens are important for identifying E. coli strains, especially those causing serious gastrointestinal diseases.
  • On selective media:
    • EMB agar: green metallic sheen colonies.
    • MacConkey agar: pink, mucoid colonies (lactose fermenter), surrounded by precipitated bile salts.
  • Biochemical characteristics:
    • Ferments glucose, lactose, trehalose, and xylose.
    • Indole and methyl red positive; Voges-Proskauer (VP) and citrate negative.
    • Does not produce H2S; negative for DNase, urease, and phenylalanine deaminase.
  • Uropathogenic E. coli:
    • Most common cause of UTIs.
    • Virulence factors: pili (adherence to epithelial cells), cytolysins (hemolysins), and aerobactin.
    • Cytolysins can kill immune cells and inhibit phagocytosis and chemotaxis.
  • Pathogenic E. coli strains:
    • ETEC (Enterotoxigenic): Traveler’s diarrhea; watery, non-bloody diarrhea; transmitted via contaminated food/water; produces heat-labile and heat-stable toxins that disrupt electrolyte balance, causing watery stool.
    • EPEC (Enteropathogenic): Second most common cause of infantile diarrhea in developing countries; affects small intestine; causes watery, non-bloody diarrhea; plasmid-mediated attachment leads to effacement of microvilli and malabsorption.
    • EIEC (Enteroinvasive): Causes dysentery-like illness; invades large intestine; fever, watery diarrhea progressing to bloody, mucus-containing stool; plasmid-mediated invasion leads to epithelial cell destruction and ulcer formation.
    • EHEC (Enterohemorrhagic): Causes bloody diarrhea with little or no fever; associated with hemolytic-uremic syndrome (HUS), a serious kidney complication; transmitted via undercooked meat (e.g., contaminated hamburgers), unpasteurized milk, or contaminated produce.
    • EAEC (Enteroaggregative): Causes infant diarrhea; affects small intestine; watery stool and vomiting; plasmid-mediated aggregative adherence shortens microvilli and disrupts absorption.
  • Other notable Escherichia species:
    • E. hermannii: yellow-pigmented colonies, isolated from CSF, wounds, and blood.
    • E. vulneris: associated with infected wounds.
    • E. albertii: associated with diarrhea in children.

Klebsiella

  • Major species: Klebsiella pneumoniae (Friedlander’s bacillus).
    • Non-motile, capsule present (main virulence factor), lactose fermenter, produces mucoid and glistening colonies.
    • Identified by positive string test (mucoid string formation) and capsular swelling test.
  • Main clinical significance:
    • Causes lobar pneumonia (currant jelly sputum), especially in alcoholics, immunocompromised, and those with chronic lung disease.
    • Can also cause UTIs, bacteremia, and meningitis.
  • Other Klebsiella species:
    • K. oxytoca: indole positive, similar infections as K. pneumoniae.
    • K. ozaenae: causes atrophic rhinitis (fetid nasal and sinus infection).
    • K. rhinoscleromatis: causes chronic nasal and pharyngeal infections.
    • K. granulomatis: causes granuloma inguinale (Donovanosis), a chronic genital ulcerative disease; does not grow in artificial media, diagnosed by identifying Donovan bodies in tissue samples.

Enterobacter

  • Common species: E. cloacae and E. aerogenes.
    • Motile, colonies may resemble Klebsiella on MacConkey agar (pink colonies).
    • Biochemical features: methyl red negative, VP positive.
  • Clinical significance:
    • Isolated from wounds, urine, blood, and CSF.
    • E. agglomerans: associated with outbreaks of septicemia from contaminated IV fluids.
    • E. gergoviae: found in respiratory samples, rarely in blood.

Serratia

  • Most important species: Serratia marcescens.
    • Produces red pigment (prodigiosin), delayed lactose fermenter.
    • Causes hospital-acquired infections: UTIs, respiratory tract infections, bacteremia, outbreaks in nurseries and burn units.
    • Can contaminate antiseptic solutions.
  • S. odorifera: emits a musty odor; two biogroups (one from respiratory tract, one from blood/CSF); biogroups differ in sucrose and ornithine reactions.

Hafnia

  • Single species: Hafnia alvei.
    • Two biotypes; biotype 1 found in beer, not clinically significant.
    • Associated with gastroenteritis; delayed positive citrate reaction.

Proteus

  • Main pathogenic species: P. mirabilis and P. vulgaris.
    • P. mirabilis: swarming motility, burnt chocolate odor, causes UTIs and struvite kidney stones, can ascend urinary tract and cause glomerulonephritis.
    • P. vulgaris: similar but differentiated by indole and ornithine tests (P. mirabilis: indole negative, ornithine positive; P. vulgaris: indole positive, ornithine negative).
  • Swarming colonies: coordinated surface movement on non-selective media.

Morganella and Providencia

  • Morganella morganii:
    • Motile, non-swarming, causes UTIs and neonatal sepsis.
  • Providencia:
    • Includes P. stuartii (burn unit outbreaks, UTIs), P. rettgeri, and P. alcalifaciens (diarrhea in children and travelers).
    • Isolated from urine and immunocompromised patients.

Other Notable Members

  • Edwardsiella tarda: H2S and indole positive, lysine decarboxylase positive, causes wound infection and bacteremia.
  • Erwinia: plant pathogen, mainly of academic interest.
  • Citrobacter freundii: H2S and urease positive, can be mistaken for Salmonella; causes nosocomial infections and endocarditis in drug users; differentiated from Salmonella by positive urease test.

Key Terms & Definitions

  • Nosocomial infection: hospital-acquired infection.
  • Lactose fermenter: bacteria that ferment lactose, producing acid and pink colonies on MacConkey agar.
  • Indole test: detects bacteria that convert tryptophan to indole.
  • Swarming motility: coordinated movement of bacteria across a solid surface (seen in Proteus).
  • Hemolytic-uremic syndrome (HUS): kidney disorder caused by EHEC.

Action Items / Next Steps

  • Watch the recorded lecture on biochemical tests for Enterobacteriaceae.
  • Review and memorize key biochemical and antigenic characteristics of opportunistic Enterobacteriaceae.
  • Study the clinical syndromes and transmission routes for each genus and species discussed.

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