Spheroplast-Mediated Carbapenem Tolerance in Gram-Negative Pathogens

Antimicrob Agents Chemother. 2019 Aug 23;63(9):e00756-19. doi: 10.1128/AAC.00756-19. Print 2019 Sep.


Antibiotic tolerance, the ability to temporarily sustain viability in the presence of bactericidal antibiotics, constitutes an understudied and yet potentially widespread cause of antibiotic treatment failure. We have previously shown that the Gram-negative pathogen Vibrio cholerae can tolerate exposure to the typically bactericidal β-lactam antibiotics by assuming a spherical morphotype devoid of detectable cell wall material. However, it is unclear how widespread β-lactam tolerance is. Here, we tested a panel of clinically significant Gram-negative pathogens for their response to the potent, broad-spectrum carbapenem antibiotic meropenem. We show that clinical isolates of Enterobacter cloacae, Klebsiella aerogenes, and Klebsiella pneumoniae, but not Escherichia coli, exhibited moderate to high levels of tolerance of meropenem, both in laboratory growth medium and in human serum. Importantly, tolerance was mediated by cell wall-deficient spheroplasts, which readily recovered wild-type morphology and growth upon removal of antibiotic. Our results suggest that carbapenem tolerance is prevalent in clinically significant bacterial species, and we suggest that this could contribute to treatment failure associated with these organisms.

Keywords: L-form; antibiotic; carbapenem; carbapenemase; meropenem; tolerance.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Amdinocillin / pharmacology
  • Anti-Bacterial Agents / pharmacology*
  • Drug Tolerance
  • Enterobacter aerogenes / drug effects*
  • Enterobacter aerogenes / growth & development
  • Enterobacter aerogenes / isolation & purification
  • Enterobacter cloacae / drug effects*
  • Enterobacter cloacae / growth & development
  • Enterobacter cloacae / isolation & purification
  • Escherichia coli / drug effects
  • Escherichia coli / growth & development
  • Escherichia coli / isolation & purification
  • Gram-Negative Bacterial Infections / drug therapy
  • Gram-Negative Bacterial Infections / microbiology
  • Klebsiella pneumoniae / drug effects*
  • Klebsiella pneumoniae / growth & development
  • Klebsiella pneumoniae / isolation & purification
  • Meropenem / pharmacology*
  • Microbial Sensitivity Tests
  • Spheroplasts / drug effects*
  • Spheroplasts / growth & development
  • Spheroplasts / isolation & purification


  • Anti-Bacterial Agents
  • Meropenem
  • Amdinocillin