Inoculum effect of methicillin-susceptible Staphylococcus aureus against broad-spectrum beta-lactam antibiotics

The Korea INfectious Diseases (KIND) study group

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16 Citations (Scopus)


Scarce information concerning the inoculum effect (InE) of methicillin-susceptible Staphylococcus aureus (MSSA) against broad-spectrum β-lactam antibiotics is available. We investigated the InE of MSSA against ceftriaxone, cefepime, meropenem, ampicillin/sulbactam and piperacillin/tazobactam. The bacteraemic MSSA isolates were collected at ten Korean general hospitals from Sep 2013 to Mar 2015. The InE was defined if MICs of antibiotics at high inoculum (HI, ~5 × 10 7  CFU/ml) increased beyond the susceptible range compared to those at standard inoculum (SI, ~5 × 10 5  CFU/ml). All isolates were sequenced for blaZ gene typing. Among 302 MSSA isolates, 254 (84.1%) were positive for blaZ; types A, B, C and D were 13.6%, 26.8%, 43.4% and 0.3%, respectively. Mean HI MICs of all tested antibiotics were significantly increased and increases in HI MIC of piperacillin/tazobactam (HI, 48.14 ± 4.08 vs. SI, 2.04 ± 0.08 mg/L, p < 0.001) and ampicillin/sulbactam (HI, 24.15 ± 1.27 vs. SI, 2.79 ± 0.11 mg/L, p < 0.001) were most prominent. No MSSA isolates exhibited meropenem InE, and few isolates exhibited cefepime (0.3%) and ceftriaxone (2.3%) InE, whereas 43.0% and 65.9% of MSSA isolates exhibited piperacillin/tazobactam and ampicillin/sulbactam InE, respectively. About 93% of type C blaZ versus 45% of non-type C exhibited ampicillin/sulbactam InE (p < 0.001) and 88% of type C blaZ versus 9% of non-type C exhibited piperacillin/tazobactam InE (p < 0.001). A large proportion of MSSA clinical isolates, especially those positive for type C blaZ, showed marked ampicillin/sulbactam InE and piperacillin/tazobactam.

Original languageEnglish
Pages (from-to)67-74
Number of pages8
JournalEuropean Journal of Clinical Microbiology and Infectious Diseases
Issue number1
Publication statusPublished - 2019 Jan 22

Bibliographical note

Funding Information:
Funding This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) (grant number NRF-2015R1C1A1A01054293).

Publisher Copyright:
© 2018, Springer-Verlag GmbH Germany, part of Springer Nature.

All Science Journal Classification (ASJC) codes

  • Microbiology (medical)
  • Infectious Diseases


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