Effect of inoculum density on susceptibility of Plesiomonas shigelloides to cephalosporins.

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 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.