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JAC Advance Access published online on June 23, 2004
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkh322
© 2004 by The British Society for Antimicrobial Chemotherapy
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Original article

Effect of inoculum density on susceptibility of Plesiomonas shigelloides to cephalosporins

Irith Wiegand 1* Sonja Burak 1

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

* To whom correspondence should be addressed. E-mail: Wiegand{at}uni-bonn.de.


  Abstract

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 {beta}-lactamases.

Methods: {beta}-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 {beta}-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 {beta}-lactamases, but can be explained by the formation of extensive filaments.

Keywords: filamentation; {beta}-lactamases; inoculum effect.
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