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- Gram-negative, lactose fermenting aerobic bacilli of Enterobacteriaceae family.
- K. pneumoniae, K. oxytoca and K. granulomatis.
- K. ozaenae and K. rhinoscleromatis are non-fermenting subspecies.
- 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.
- Known to contaminate drains and serve as point source for nosocomial outbreaks, esp. K. pneumoniae, K. oxytoca (as well as E. coli., Pseudomonas and Acinetobacter species).
- 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): 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.
- Klebsiella pneumoniae carbapenemase, KPC (Ambler Class A): confers broad resistance and is associated with mortality rate >50%.
- Many isolates in US are sequence type, ST258.
- Susceptibility limited to gentamicin, tigecycline and colistin.
- Inhibited by clavulanic acid and tazobactam.
- Metallo-beta-lactamases (Ambler Class B): types include IMP (imipenemase), VIM (Verona integron-encoded MBL), and NDM-1 (New Delhi MBL).
- Require zinc ions in the active site for activity.
- IMP and VIM are not resistant to aztreonam.
- NDM-1 generally resistant to all antibiotics (including aztreonam and aminoglycosides) except tigecycline and colistin.
- 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.
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