Effect of inoculum density on susceptibility of Plesiomonas shigelloides to cephalosporins.
OBJECTIVE
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 10(5) cfu/mL and 1 x 10(6) 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 microm 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.
Wiegand
Irith
I
Pharmaceutical Microbiology, University of Bonn, Meckenheimer Allee 168, 53115 Bonn, Germany. Wiegand@uni-bonn.de
Burak
Sonja
S
D002511:Cephalosporins / Q000494:pharmacology*; D015169:Colony Count, Microbial; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000187:drug effects; D007525:Isoelectric Focusing; D007700:Kinetics; D008826:Microbial Sensitivity Tests; D016981:Plesiomonas / Q000187:drug effects* / Q000201:enzymology / Q000648:ultrastructure; D001618:beta-Lactamases / Q000096:biosynthesis / Q000378:metabolism
D002511:Cephalosporins; D001618:beta-Lactamases