Journal Article

Effect of inoculum density on susceptibility of <i>Plesiomonas shigelloides</i> to cephalosporins

Irith Wiegand and Sonja Burak

in Journal of Antimicrobial Chemotherapy

Published on behalf of British Society for Antimicrobial Chemotherapy

Volume 54, issue 2, pages 418-423
Published in print August 2004 | ISSN: 0305-7453
Published online August 2004 | e-ISSN: 1460-2091 | DOI:
Effect of inoculum density on susceptibility of Plesiomonas shigelloides to cephalosporins

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  • Medical Oncology
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Objectives: Resistance of Plesiomonas shigelloides to cephalosporins at higher cell densities has been reported. We investigated whether these inoculum effects are due to the production of β-lactamases.

Methods: β-Lactamase production of five P. shigelloides strains was characterized by activity tests, SDS–PAGE and isoelectric focusing. For all strains, MIC values of different cephalosporins were determined by microdilution methodology using inocula of 1 × 105 cfu/mL and 1 × 106 cfu/mL. Subsequently, the morphology of cells was determined by light microscopy. For one isolate, kill kinetics of cefpodoxime were determined using batch cultures with the lower and higher inocula.

Results: Four of five P. shigelloides strains were shown to be β-lactamase-positive, producing different amounts of constitutively expressed non-inducible enzymes. Inoculum effects for cephalosporin susceptibility were observed for all strains. Examination of cells revealed a very strong filamentation, with filament sizes ranging from 100 μm up to 2 mm. The kill kinetics with cefpodoxime showed similar killing capacities of the antibiotic at both inoculum sizes.

Conclusions: The reported resistance of P. shigelloides to cephalosporins at higher cell densities is not due to an inoculum-dependent regulation of β-lactamases, but can be explained by the formation of extensive filaments.

Keywords: filamentation; β-lactamases; inoculum effect

Journal Article.  4013 words.  Illustrated.

Subjects: Medical Oncology ; Critical Care

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