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JAC Advance Access originally published online on June 23, 2004
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Journal of Antimicrobial Chemotherapy 2004 54(2):418-423; doi:10.1093/jac/dkh322
JAC vol.54 no.2 © The British Society for Antimicrobial Chemotherapy 2004; all rights reserved.

Effect of inoculum density on susceptibility of Plesiomonas shigelloides to cephalosporins

Irith Wiegand* and Sonja Burak

Pharmaceutical Microbiology, University of Bonn, Meckenheimer Allee 168, 53115 Bonn, Germany

* Corresponding author. Tel: +49-22-873-5347; Fax: +49-22-873-5267; Email: wiegand{at}uni-bonn.de

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 ß-lactamases.

Methods: ß-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 x 105 cfu/mL and 1 x 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 ß-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 ß-lactamases, but can be explained by the formation of extensive filaments.

Keywords: filamentation , ß-lactamases , inoculum effect


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