MICROBIOLOGY

• Gram-negative, lactose-fermenting aerobic bacilli [Fig 1] of Enterobacterales order, Enterobacteriaceae family.
  • K. pneumoniae, K. oxytoca and K. granulomatis.
    • K. ozaenae and K. rhinoscleromatis are non-fermenting subspecies.[10]
  • Forms highly mucoid colonies with polysaccharide capsule [Fig 2], a virulence factor that inhibits phagocytosis.
  • Colonizes mammalian intestinal and respiratory tracts.[9]
  • Easily cultured on non-selective media for sterile specimens or MacConkey agar for contaminated specimens.
  • Contaminates sinks[7] and causes nosocomial outbreaks, esp. K. pneumoniae, K. oxytoca.[11]
• Resistance issues[14]
  • Beta-lactamases are constitutive, usually produced at low levels, and confer resistance against ampicillin, amoxicillin and ticarcillin.
    • Few klebsiellae lack these beta-lactamases.
  • Extended-spectrum beta-lactamases (ESBLs) may be plasmid-mediated or chromosomal, confer multidrug resistance (TEM or SHV types), and are detected by in vitro resistance to ceftazidime and aztreonam.
    
    • CTX-M type ESBLs are the most prevalent ESBL types worldwide and are commonly associated with ST131 E. coli.[16]
    • Non-susceptibility to ceftriaxone is often used as a proxy for ESBL production.[6]
  • Carbapenemases:
    
    • Klebsiella pneumoniae carbapenemase, KPC (Ambler Class A): confers broad resistance and is associated with mortality rate >50%.[13]
      • Many isolates in the U.S. are sequence type, ST258, a biologically fit lineage that is able to cause outbreaks.[8]
      • The phenotype includes strong potentiation of meropenem by vaborbactam.[8]
    • Metallo-beta-lactamases (Ambler Class B): types include IMP (imipenemase), VIM (Verona integron-encoded MBL), and NDM-1 (New Delhi metallo-beta-lactamase).
      • Require zinc ions in the active site for activity.
      • IMP and VIM are not resistant to aztreonam.
      • NDM-1 presence is strongly suggested by combined resistance to meropenem, ceftazidime, and cefepime together with clear susceptibility to aztreonam.[17]
    • OXA-type carbapenemases (Ambler Class D): include OXA-48, weakly hydrolyze carbapenems, broad-spectrum cephalosporins, and aztreonam; express resistance or decreased susceptibility to carbapenems.
      • Often associated with Acinetobacter baumannii.