Entero

• The Enterobacteriaceae is the most common group of gram-negative rods cultured in clinical laboratories both as normal flora and as agents of disease
• Along with staphylococci and streptococci, Enterobacteriaceae are the most common bacteria that cause disease
• Enterobacteriaceae


  A large diverse family of bacteria that exhibit a considerable degree of relatedness
• The taxonomy of the Enterobacteriaceae is complex and rapidly changing since the introduction of techniques that measure evolutionary distance, such as nucleic acid hybridization and nucleic acid sequencing

  
• Revisions in bacterial taxonomy and nomenclature, and recognition and acknowledgement of novel bacteria are published in the International Journal of Systematic and Evolutionary Microbiology

  
• Changes published in the years 2016 and 2017 include the major reorganization of the family Enterobacteriaceae and the revisions within the genus Enterobacter

  
• In this unit module, taxonomic refinements will be minimized, and the names commonly used in the medical literature are generally used
• Enterobacteriaceae


  • Gram-negative bacilli
  • Non-spore-forming
  • Most are motile (peritrichous); Plesiomonas has lophotrichous flagellation; some are nonmotile (Klebsiella; Shigella; Yersinia at 35-37 oC, motile at room temperature)
  • Some are encapsulated (Klebsiella), others are non-encapsulated
  • Facultative anaerobes
  • Are oxidase negative, EXCEPT Plesiomonas
  • Ferment glucose but otherwise vary in the utilization of other carbohydrates
  • Most reduce nitrates to nitrites
• O antigen (somatic antigen)

  • Associated with the outer membrane (OM) of the cell wall found in all species of enteric bacilli
  • Repeating units of polysaccharides that consist the outer region of the lipopolysaccharides (LPS)
  • Heat-stable, i.e., the bacteria keep their immunogenicity, agglutinating, and antibody-binding capacity after boiling
  • Alcohol-stable
• Although each genus of enteric bacillus is associated with specific O groups, a single organism may carry several O antigens. Thus, cross-reactivity between species exists. For example, most shigellae share one or more O antigens with Escherichia coli. E. coli may cross-react with some Providencia, Klebsiella, and Salmonella species
• K antigen (capsular antigen or fimbrial antigen)


  • Mostly associated with the capsule (others are associated with the fimbriae) found in some, not all, enteric bacilli
  • Polysaccharides (capsular) or proteins (fimbrial) in nature
  • Heat-labile, that is inactivated by boiling
• K antigens are expressed by the following enteric bacilli: Klebsiella spp., Salmonella enterica subsp. enterica serotype Typhi (S. Typhi) - called the Vi antigen, Some strains of Escherichia coli

  
• Since the capsule covers the cell wall in some bacteria, the K antigen Inhibits agglutination with type-specific O antigen. To detect the O antigen, after determination of the K antigen, the bacterial suspension must be boiled for 30 minutes, and then retested
• Neufeld's Quellung Reaction


  The K antigens of Klebsiella species can be identified when bacterial cells are mixed with specific anticapsular antibodies resulting in bacterial cell agglutination. Microscopic examination reveals bacteria whose capsules appear to be swollen. However, preparation of Salmonella Typhi and Escherichia coli, unlike in Klebsiella species, does not reveal the capsule. Their capsule usually collapse during sample preparation. It is only with the use of certain technique, i.e. stabilization of the capsule with antibody before sample preparation, that the capsule can be demonstrated by electron microscopy

  
• H antigen (flagellar antigen)


  • Found in the flagella of motile enteric bacilli
  • Consists of proteins
  • Heat- and alcohol labile
• Various types of bacteria employ phase variation with their H antigen. Phase variation is a process of switching between different types of the protein flagellin within a single serotype. As a result, flagella with different antigenic structures are assembled. Once an immune response has been mounted against one type of flagellin, switching types renders previous immune responses against the flagella ineffective
• Coliforms


  Defined as all the aerobic and facultative anaerobic, nonspore-forming, gram-negative rods that are cytochrome-oxidase negative, capable of growth in the presence of bile salts, and ferment lactose at either 35 or 37 oC with the production of acid, gas and aldehyde within 24-48 hours (lactose fermenters)
• Coliforms


  • Escherichia
  • Klebsiella
  • Enterobacter
  • Citrobacter
• Fecal coliforms


  • Ferment lactose at 44.5 oC, thus are also termed the thermotolerant coliforms
  • Refer to Escherichia coli
• E. coli is the most common facultative bacterium in the intestine of man and warm-blooded animals, but is outnumbered by strict anaerobic bacteria. It is the ideal indicator organism that provides evidence for fecal contamination of water supply, food, or other materials because it is always present in feces, not normally found in the environment unless there is fecal contamination, and is readily detectable by simple methods, and easy to isolate, identify and enumerate
• Nonfecal coliforms


  • Do NOT ferment lactose at 44.5 oC
  • Include species of Klebsiella, Enterobacter, and Citrobacter
• Nonfecal coliforms can be a part of natural aquatic flora. Thus, they are not a good index of fecal contamination. Their detection in water supply or other materials may mean false positive for fecal contamination
• Noncoliforms


  Do NOT ferment lactose at 35 or 37 oC within 24-48 hours (non-lactose fermenters)
• Noncoliforms


  • Proteus
  • Morganella
  • Providencia
  • Edwardsiella
  • Serratia
  • Hafnia
  • Salmonella
  • Shigella
  • Yersinia enterocolitica
  • Y. pseudotuberculosis
  • Yersinia pestis
• The Enterobacteriaceae and related bacteria is a large, heterogeneous group of bacteria whose natural habitat is the intestinal tract of humans and animals. They are, thus, often commonly called the enteric bacteria or the enterobacteria
• Many members of this group are ubiquitous in nature — in soil, water, and vegetation
• This Enterobacteriaceae and related enterobacteria is the most frequently encountered bacterial isolates recovered from clinical specimens - both as normal flora and as agents of disease
• The diseases caused by these agents usually involve systems other than the gastrointestinal tract, such as the lungs and urinary tract
• Enterics


  • Those associated with diseases within and outside the GIT
  • Those associated with diseases within the GIT
  • Those associated with diseased outside of the GIT
• Yersinia pestis is a non-enteric agent of disease
• Escherichia coli


  • Normal flora in the large intestine; may colonize the vagina and urethra
  • Acquired during birth and by fecal-oral route
• Escherichia coli causing urinary tract infection

  • Commonly causes UTIs in approximately 90% of cases
  • Results from endogenous spread of the bacteria from the large intestine as resident flora, and mechanically introduced into the urethra
  • Occurs primarily in women, attributed to short urethra, proximity of urethra to anus, and colonization of vagina by fecal flora
  • Virulence factors: P pili (P fimbriae), endotoxin, hemolysin

  
• Signs and symptoms of E. coli UTI include urinary frequency, dysuria, hematuria, pyuria, and flank pain. None of these symptoms or signs is specific for E coli infection

  
• P pili (P fimbriae)


  • Primary virulence factor of E. coli to cause UTIs
  • Allow the bacteria to attach to the urinary epithelial mucosa and not be washed out with urine flow
  • Named for their ability to bind to and agglutinate human RBCs carrying the P blood group antigen
• Endotoxin


  • Resides in the cell wall and consists of the lipid A of the LPS
  • Responsible for much of the morbidity and mortality resulting from infections associated with these bacteria
• Hemolysin


  • Cytolysin that creates pores in eukaryotic cell membranes
  • Inhibits phagocytosis and chemotaxis of certain white blood cells
• Aerobactin


  • A siderophore, i.e., iron chelating agent, enabling E. coli to bind and transport iron for use by bacteria in iron-poor environments such as the urinary tract
• Capsular polysaccharide


  • The determinant of the K antigen
  • Is antiphagocytic and inhibits the opsonizing and lytic activities of complement because of the identity of the capsular polysaccharides and the host carbohydrates
• E. coli is a major cause of meningitis and septicemia among newborns, along with Group B streptococci

  
• Newborns acquire E. coli by vertical transmission from the birth canal just before or during delivery, when the mother's vagina is heavily colonized or infection may also result if contamination of the amniotic fluid occurs