title: beta-Lactam-susceptibility patterns of Plesiomonas shigelloides strains: importance of inoculum and medium.
abstract: The susceptibility of 10 Plesiomonas shigelloides strains to 30 beta-lactam antibiotics was examined. Susceptibility testing was carried out with a microdilution procedure using 3 media (Isosensitest broth, Mueller-Hinton broth and cation-adjusted Mueller-Hinton broth) and 4 inocula [1 x 10(4), 1 x 10(5), 1 x 10(6) and 1 x 10(7) colony-forming units (CFU)/ml]. A high inoculum dependency of MIC values was found for numerous beta-lactams. Each strain showed 2 completely different susceptibility patterns to several cephalosporins and aztreonam, but the same patterns were found with little variation in each strain. Using an inoculum of 1 x 10(4) CFU/ml the strains showed a high susceptibility to aztreonam and to all cephalosporins in all media (pattern 1), whereas they showed a decreased susceptibility to aztreonam and numerous cephalosporins with an inoculum of 1 x 10(7) CFU/ml (pattern 2). Using an inoculum of 1 x 10(6) CFU/ml, 4/10 strains in lsosensitest broth and 2/10 strains in Mueller-Hinton media showed pattern 1. Susceptibility testing of numerous penicillins revealed a medium-independent inoculum dependency, characterized by a step-to-step correlation between MICs and inocula. The beta-lactam susceptibility patterns arising from different inocula point to new beta-lactamase expression and regulation mechanisms in Plesiomonas. The potential to be resistant to a variety of beta-lactams under conceivable testing conditions should question the use of numerous beta-lactams for the treatment of Plesiomonas infections.
authors.affiliation: Pharmazeutische Mikrobiologie, Universität Bonn, Germany.
authors.forename: I
authors.initials: I
authors.lastname: Stock
authors.forename: B
authors.initials: B
authors.lastname: Wiedemann
mesh_terms: D000818:Animals; D001398:Aztreonam / Q000494:pharmacology; D015780:Carbapenems / Q000494:pharmacology; D002511:Cephalosporins / Q000494:pharmacology; D003470:Culture Media; D004285:Dogs; D006801:Humans; D008826:Microbial Sensitivity Tests; D010406:Penicillins / Q000494:pharmacology; D016981:Plesiomonas / Q000145:classification* / Q000187:drug effects*; D047090:beta-Lactams / Q000494:pharmacology*
chemical_list: D015780:Carbapenems; D002511:Cephalosporins; D003470:Culture Media; D010406:Penicillins; D047090:beta-Lactams; D001398:Aztreonam