- Gram-negative, aerobic bacilli of Enterobacteriaceae family.
- Human pathogens: K. pneumoniae, K. oxytoca, K. rhinoscleromatis, and K. granulomatis.
- Forms highly mucoid colonies w/ polysaccharide capsule, a virulence factor that inhibits phagocytosis. Easily cultured on non-selective media for sterile specimens or MacConkey agar for contaminated specimens.
- Resistance issues:
- Beta-lactamases are constitutive, usually produced at low levels, and provide resistance against ampicillin, amoxicillin and ticarcillin. Few klebsiellae lack these beta-lactamases.
- Extended-spectrum beta-lactamases (ESBLs): plasmid-mediated, confer multidrug resistance (TEM or SHV types), and are detected by in vitro resistance to ceftazidime and aztreonam.
- CTX-M type ESBLs, which hydrolyze ceftazidime much less than other 3rd and 4th generation cephalosporins, are more prevalent and have proliferated in the E. coli ST131 lineage.
- Klebsiella pneumoniae carbapenemases:
- KPC, Ambler Class A beta-lactamases: confer broad resistance and are associated with mortality rate >50%.
- Metallo-beta-lactamases (Ambler Class B): types include IMP (imipenemase), VIM (Verona integron-encoded MBL), and NDM-1 (New Delhi MBL). NDM-1 generally resistant to all antibiotics except tigecycline and colistin.
- OXA-type carbapenemases (Ambler Class D): include OXA-48, weakly hydrolyze carbapenems, broad-spectrum cephalosporins and aztreonam, but express resistance or decreased susceptibility to carbapenems.
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