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Corynebacterium. Corynebacterium. Classification Corynebacterium  diphtheriae  and diphtheroids (look  like   C. diphtheriae )  are  Gram- positive, club  shaped  rods.   Some  are saprophytic Some   produce  disease   in   animals.  

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  • Classification
    • Corynebacterium  diphtheriae  and diphtheroids (look  like  C. diphtheriae)  are  Gram- positive, club  shaped  rods.  
      • Some  are saprophytic
      • Some   produce  disease   in   animals.  
      • C. diphtheriae is the most important pathogen in the group.
  • Morphology and cultural characteristics
    • Small G+B; arrangement=palisade or Chinese
    • letters
    • Growth on B.A – raised, translucent, gray colonies 
  • Loeffler‘s agar slant­ contains serum and egg that enhance the  formation  of metachromatic  granules (polymerizedpolyphosphoric acid) in C. diphtheriae.
    • Also called Babes-Ernst granules.
    • They  are visualized  by  staining  with methylene blue.
  • A medium containing tellurite should be used to select for Corynebacterium  and  other G+  organisms -it  inhibits  G­ organisms. Two kinds are used:
    • Cystine tellurite ­ has alonger  shelf  life
    • Tinsdale ­ helps  to  differentiate amongst the Corynebacterium.
      • Colonies on  either  appear black  or gray due to tellurite reduction.
      • S. aureus  and Listeria  also grow as black colonies.  
      • On  Tinsdale  C. diphtheriae,  ulcerans, and pseudotuberculosis  form  brown halos around the colonies due to formation of ferric sulfide.
    • 3 morphological  types of C. diphtheriae are  found on tellurite containing media:
      • Mitis – black colonies with a gray  periphery
      • Gravis – large,  gray  colonies
      • Intermedius – small, dull gray  to black.
      • All produce an immunologically identical toxin.
    • Incubation -35-370 C for 24 hours.
    • They prefer a pH of 7.8-8.0 for good growth.
    • They require access to oxygen (poor AnO2 growth).
  • Biochemistry
    • Catalase +
    • Nonmotile
    • C. ulcerans is urease +, C. diphtheriae is -, and C. pseudotuberculosis is usually +
  • Virulence factors­ C. diphtheriae
    • For C. diphtherias to cause diphtheria an exotoxin must be  produced.
      • Is a heat-labile polypeptide produced during lysogeny of  a  phage that carries the "tox” gene.
      • Alkaline pH of  7.8- 8.0, aerobic conditions, and a low environmental iron level are essential for toxin production (occurs late in the growth of the organism).
      • The  toxin  inhibits protein  synthesis  by  ADP-ribosylating elongation factor  2.
      • What other organism produces a similar toxin? 
    • Trypsin  cleaves  the toxin into 2 fragments, A and B, that are linked together by a disulfide bridge.
    • Fragment B is required for toxin  binding to tissue cells and fragment A contains  the toxic activity.
    • One molecule of toxin can inhibit  90%  of the  protein synthesis in a cell.
    • Systemic effects  include heart failure, paralysis and adrenal hypofunction leading to an Addison’s like disease.
  • C. ulcerans and C. pseudotuberculosis sometimes make a diphtheria-like  toxin.
c diphtheria toxin
C. diphtheria toxin
  • Toxin enters through receptor mediated endocytosis
  • Acidification of endocytic vesicle allows A to dissociate from B
  • A enters cycoplasm
  • To prove  that an isolate can cause diphtheria, one  must demonstrate  toxin  production.
    • This is most often done  on  an  Elek plate:
      • The  organism is streaked on a plate containing low iron.
      • A filter strip containing anti-toxin antibody is placed perpendicular to the streak of the organism.
      • Diffusion of the antibody into the medium and secretion of the toxin into the medium occur.
      • At the zone of equivalence, a precipitate will form.
    • Guinea pig or tissue culture toxicity assays may also be done.
  • Capsule – is protein in nature
  • Cord factor – is a complex glycolipid (trehalose 6,6’-dicorynemycolate) that has been shown to disrupt mouse mitochondria.
    • It has not been shown to play a role in the production of diphtheria.
  • Clinical Significance (C. diphtheria)
    • Is normally found in the throats  of  healthy carriers.
      • The organism infects only man and it has a limited capacity to invade.
    • Diphtheria -  Disease usually starts as a local infection of the  mucous membranes causing a membranous  pharyngitis
      • Local  toxin  effects result  in degeneration of epithelial cells.
      • Inflammation, edema, and production of a pseudomembrane composed of fibrin clots, leukocytes,  and dead epithelial cells and microorganisms occurs  in  the throat.
diphtheria pseudomembrane
Diphtheria - pseudomembrane
  • This may obstruct the  airway  and result  in suffocation.
    • The more dangerous  effects  occur when the toxin becomes systemic and  attacks the  heart(heart failure),  peripheral nerves (paralysis), and the adrenal glands (hypofunction).
    • Cutaneous  diphtheria­  More  common  in tropical and subtropical areas.
      • Necrotic lesions with occasional formation of a local pseudomembrane occur.
  • Antibiotic susceptibility and treatment
    • Antiserum ­ once the toxin has bound, however, the antiserum  against it is ineffective.
    • Penicillin­ to eliminate the organism.
    • Prevention- Active immunization with toxoid (alum precipitate).
      • Is part of the DPT vaccine.
    • Shick skin test­ like the Dick test in that it  tests  for circulating  antibody  to the toxin by injecting a small amount  of  toxin intradermally and observing for  a  local erythematous  and  necrotic reaction.
      • If this occurs it indicates  that  the person has no anti-toxin  antibodies  and  is, therefore, susceptible to diphtheria.
  •  Other Corynebacterium­ are part of the normal flora of  the skin and URT.
  • Are called diphtheroids and may occasionally cause disease, particularly in immunocompromised individuals.
    • C.  ulcerans­  toxigenic strains  may  produce  a  disease similar to, but less severe than diphtheria.
    • J-KGroup­  commonly  cause  infections  in  those   with underlying disease.  
      • Diseases include bacteremia, meningitis, peritonitis, wound infections, etc.  
      • It  is becoming more and more of a problem.
    • C.  pseudotuberculosis ­ found in those with exposure to animals.
      • Can cause pneumonia or lymphadenitis.
      • Produces a different exotoxin than C. diphtheriae.