BSCI 424 — PATHOGENIC MICROBIOLOGY — Fall 2000

Enterobacteriaceae Summary

Morphology & Physiology:

  Small Gram-negative non-sporeforming enteric bacilli

  All Enterobacteriaciae:

1. ferment glucose with acid production

2. reduce nitrates (NO₃ to NO₂ or all the way to N₂)
   

3. are oxidase negative

  All are aerobic but can be facultatively anaerobic

  Motile via peritrichous flagella except Shigella and Klebsiella which are non-motile

  Capsule, slime layer, or neither

  Possess fimbriae (pili)

  Complex cell wall

  Antigenic Structure: plays an important role for some species in epidemiology and classification

 K (capsular) antigens: capsular polysaccharide, particularly heavy in Klebsiella

 H (flagellar) antigens: flagellar proteins of motile genera and species; used for typing; absent in nonmotile genera (Shigella and Klebsiella)

 O (somatic) antigens: O-specific polysaccharide side chain of lipopolysaccharide; used for typing

  

Taxonomy of Enterobacteriaceae:

  Over 30 genera and 120 species

  More than 95% of clinically significant strains fall into 10 genera and less than 25 species

  Family Enterobacteriaceae (clinically important enterics)

 Citrobacter

 Citrobacter freundii  

  Citrobacter diversus

  Enterobacter spp.

 Enterobacter aerogenes  

  Enterobacter agglomerans

  Enterobacter cloacae

  Escherichia coli

 Opportunistic Escherichia coli 

  ETEC = enterotoxigenic E. coli

  EIEC = enteroinvasive E. coli  

  EPEC = enteropathogenic E. coli 

  EHEC = enterohemorrhagic E. coli

  EaggEC = enteroaggregative E. coli

  UPEC = uropathogenic E. coli

   Klebsiella spp.

  Klebsiella pneumoniae  

  Klebsiella oxytoca

  Morganella morganii

  Proteus spp.

  Proteus mirabilis

  Proteus vulgaris

  Providencia spp.

 Providencia alcalifaciens  

  Providencia rettgeri

  Providencia stuartii

 Salmonella spp.

 Salmonella enterica (proper nomenclature; encompasses all Salmonella; taxonomically only one species of Salmonella) 

  Common nomenclature still in use:

Salmonella typhi

Salmonella paratyphi

Salmonella enteritidis

Salmonella cholerasuis

Salmonella typhimurium

 Serratia spp.

  Serratia marcesans

  Serratia liquifaciens

 Shigella spp.

 Shigella dysenteriae  

  Shigella flexneri

  Shigella boydii

  Shigella sonnei

 Yersinia spp.

  Yersinia enterocolitica

  Yersinia pestis

  Yersinia pseudotuberculosis

Clinical Syndromes:

  Some members of the Enterobacteriaceae are true pathogens

 Salmonella spp.

 Shigella spp.

 Yersinia spp.

  Certain strains of Escherichia coli

 

  ETEC = enterotoxigenic E. coli

  EIEC = enteroinvasive E. coli  

  EPEC = enteropathogenic E. coli 

  EHEC = enterohemorrhagic E. coli

  EaggEC = enteroaggregative E. coli

  UPEC = uropathogenic E. coli

  Most members of the Enterobacteriaceae are opportunistic or cause secondary infections of wounds, the urinary and respiratory tracts, and the circulatory system

 Gram-negative sepsis

 Life-threatening

  Usually nosocomial

  Commonly caused by E. coli

  Clinical Progression:

  Early Phase (REVERSIBLE)

Decrease in arterial resistance; Increased cardiac output
Kinins (protein) in plasma due to tissue damage, endotoxin, or antigen-antibody complexes

  Second Phase (REVERSIBLE)

Increase in arterial resistance; Decreased cardiac output

  Third Phase (IRREVERSIBLE) Vascular collapse with organ failure

Endotoxin induces DIC which leads to hemorrhage and death

 Urinary tract infections

 Greatest incidence in young individuals and middle-aged females

  Incidence increases with age in males

  Most commonly caused by E. coli

  Diagnosis by microscopic and cultural examination of urine

   Obtain urine by catheter through urethra into bladder, suprapubic tap

 Pneumonia

 Nosocomial; Spread by personnel and equipment

  Frequently caused by K. pneumoniae

  Often in middle-aged males who abuse alcohol

  Difficult to diagnose because of commensals in sputum

 Abdominal sepsis

 Caused by flora of the gastrointestinal tract

  Infections usually polymicrobi

 Meningitis

 Usually nosocomial

  Frequently caused by E. coli

  Diagnosis by microscopic and cultural examination of cerebrospinal fluid

 Spontaneous bacterial peritonitis

 Usually in patients with liver ailments

  Commonly caused by E. coli, but also anaerobes and Gram-positive cocci (S. pneumoniae)

 Endocarditis

 Vascular endocardial surface inflammation

  Mostly caused by Gram-positive cocci, but 1-3% caused by aerobic Gram-negative rods

  Diagnosis by blood culture

  Difficult to treat; treatment is of long duration

Epidemiology:

  Found in soil, water and decaying matter

  Some of the normal inhabitants of the small and large intestine included in the family: therefore, are sometimes referred to as enteric bacilli or simply enterics

  Enterics are responsible for a majority of nosocomial infections

 About two million patients per year in the United States are estimated to acquire nosocomial infections

 Approximately 5% to 10% of the total hospital population acquire such infections

Pathogenesis & Immunity:

  Endotoxin: important virulence factor with wide-ranging effects on host

  Capsule: antiphagocytic

  Antigenic phase variation: capability to alternately express or not express either capsule or flagella and thus avoid host immunity

  Sequestration of nutritional factors, in particular, iron by production of siderophores which are extracellular iron-chelating compounds (e.g., enterobactin, aerobactin)

 Iron is important compound for both host and pathogen and is limited in supply and thus must be competed for

 Much of iron in host body is sequestered in heme proteins (e.g., hemoglobin, myoglobin) and in iron-chelating proteins (e.g., transferrin, lactoferrin)

  Resistance to serum killing: many bacteria are inherently sensitive to nonspecific bloodborne components and to circulating complement and the resultant complement-mediated clearance, but Enterobacteriaceae and other bloodborne pathogens can resist such killing

  Antimicrobial resistance

  True pathogenic members of the Enterobacteriaceae may possess specific virulence features, which are unique to individual genera or species

 Adhesins

 Exotoxins, (e.g., enterotoxins which act in the small intestine)

  Antigens which stimulate antibody production by the host include:

1. K - polysaccharide capsular antigens
2. H - flagellar antigen
3. O - somatic polysaccharide antigen (associated with LPS)

Laboratory Identification:

  Specimens whether pus, tissue, sputum, fluids, rectal swabs, or feces should be cultured immediately or placed on special media to prevent overgrowth

  Special isolation media: contain various substances including indicators, inhibitors, etc.

  Media and tests to differentiate the genera of the family

  Tests that divide species of the genera, e.g., patterns of acid production from various carbohydrates

  Various species differ in the carbohydrates from which acid may be produced and end products that may be formed from various substrates

  Culture:

 Colony morphology: moist, gray (except Serratia marcescans which appears red) smooth colonies on non-selective media

 Special differential and selective media used for separation of genera and species

 Some strains are beta hemolytic on blood agar

Treatment, Prevention & Control:

  Major problem

  Various options; must consider resistance

Miscellaneous Opportunists of the Enterobacteriaceae

Citrobacter: environmental sources and in feces of man and animals

• Can infect any body site opportunistically, but most isolates from the urinary tract; neonatal meningitis and brain abscesses by Citrobacter diversus

• Citrobacter freundii can be enterotoxigenic

• Antigenic structure: O, H, K antigens

Enterobacter: inhabit soil and water and to a lesser extent, the human intestine

• Similar to and must be distinguished from Klebsiella in specimens

• Cultural characteristics: motile, four species biochemically

• Antigenic structure: O, H, K antigens

• Disease in body tissues, most frequently urinary tract; Enterobacter cloacae found most often

• Strains of E. cloacae are enteroxigenic and may be antibiotic resitant

• Treat with any antimicrobials used for enterobacterial diseases

Serratia: inhabitants of water and soil

• Major agent in nosocomial infections; three species represent major human pathogens

• Cultural characteristics: produces DNase, red pigment production (in nature, but rarely in strains isolated from infection) enhanced at 25° C

• Antigenic structure: O, H, K antigens

• Serratia marcescens: involved in nosocomial pneumonia, septicemia, urinary tract and wound infections; endocarditis in drug abusers

• Treat with either aminoglycosides, chloramphenicol, or trimethoprimsulfa-methoxazole; resistant to most of the "common" antibiotics

Morganella / Providencia: airborne organisms that frequently colonize burn wounds and are often antibiotic resistant in vivo

• Antigenic structure: O, H, K antigens

• Morganella morganii: formerly Proteus morganii, causes infections similar to Proteus

• Providencia: causes nosocomial infections of urinary tract, blood, respiratory tract and wounds

Edwardsiella: rarely causes wound infections; also rarely seen in sepsis, meningitis, and gastroenteritis in humans

• Produces hydrogen sulfide, but doesn't ferment lactose
   
  

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