trances

• Family Enterobacteriaceae includes many genera and species

  
• The latest edition of Bergey's Manual of Systematic Bacteriology describes 176 named species among 44 different genera
• Clinical isolates in general acute care facilities consist primarily of

  • Escherichia coli
  • Klebsiella pneumoniae
  • Proteus mirabilis
• It is nonetheless important to be aware of the other species because they also cause significant infectious diseases
• This chapter is divided into three major areas

  • Clinically significant enteric species that cause opportunistic infections
  • Primary intestinal pathogens and their related human infections
  • Methods of identification of these organisms

  
• Members of Enterobacteriaceae

  • Gram-negative, non-spore-forming, facultatively anaerobic bacilli
  • On Gram-stained smears, they can appear as coccobacilli or as straight rods

  
• Colony morphology on nonselective media, such as SBA or chocolate (CHOC) agar, is of little value in initial identification
• Exceptions to colony morphology on nonselective media

  • Klebsiella and sometimes Enterobacter produce characteristically large and very mucoid colonies
  • Many isolates of E. coli are β-hemolytic

  
• Differential and selective media available for presumptive identification of enteric pathogens

  • Eosin-methylene blue (EMB) agar
  • MacConkey (MAC) agar
  • Hektoen enteric (HE) agar
  • Xylose-lysine desoxycholate (XLD) agar

  
• Fermentation

  Indicated by a color change on the medium, which results from a decrease in pH detected by a pH indicator incorporated into the medium
• Non-fermenting species are differentiated by lack of color change, and colonies retain the original color of the medium
• Species that produce hydrogen sulfide (H2S) may be readily distinguished when placed on HE or XLD agar
• HE and XLD agars contain sodium thiosulfate and ferric ammonium citrate, which produce blackening of H2S-producing colonies
• Definitive identification depends on the biochemical reactions and serologic antigenic structures demonstrated by the particular species

  
• Tribes

  Bacterial species with similar biochemical characteristics are grouped together
• Within the tribes, organisms are classified further into genera and species
• Differentiation of each genus and definitive identification of species are based on biochemical characteristics and DNA homology

  
• Virulence of Enterobacteriaceae

  • Affected by ability to adhere, colonize, produce toxins, and invade tissue
  • Some species harbor plasmids that can provide antimicrobial resistance genes

  
• Antigens used in identification of different serologic groups

  • Antigen, or somatic antigen
  • H antigen, or flagellar antigen
  • K antigen, or capsular antigen
• K antigen


  Heat-labile polysaccharide found only in certain encapsulated species, e.g. K1 antigen of E. coli and the Vi antigen of Salmonella enterica subsp. enterica serotype Typhi

  
• Members of the family Enterobacteriaceae are ubiquitous in nature

  
• With few exceptions, they share a common niche - reside in the GIT
• Except for Salmonella, Shigella, and Yersinia, they can be resident microbiota if confined to their natural environment
• Often commensals, causing no harm, but can be responsible for a large number of opportunistic infections when introduced into inappropriate body sites
• Members of the family Enterobacteriaceae may be divided into two broad categories

  • Opportunistic pathogens
  • Primary pathogens