beta-lactam-susceptibility patterns of plesiomonas shigelloides strains: Importance of inoculum and medium

The susceptibility of 10 Plesiomonas shigelloides strains to 30 beta -lacta m antibiotics was examined. Susceptibility testing was carried out with a m icrodilution procedure using 3 media (Isosensitest broth, Mueller-Hinton br oth 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 inoc ulum dependency of MIC values was found for numerous beta -lactams. Each st rain showed 2 completely different susceptibility patterns to several cepha losporins and aztreonam, but the same patterns were found with little varia tion in each strain. Using an inoculum of 1 x 10(4) CFU/ml the strains show ed a high susceptibility to aztreonam and to all cephalosporins in all medi a (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 Isosensitest brot h and 2/10 strains in Mueller-Hinton media showed pattern 1. Susceptibility testing of numerous penicillins revealed a medium-independent inoculum dep endency, characterized by a step-to-step correlation between MICs and inocu la. The beta -lactam susceptibility patterns arising from different inocula point to new beta -lactamase expression and regulation mechanisms in Plesi omonas. The potential to be resistant to a variety of beta -lactams under c onceivable testing conditions should question the use of numerous beta -lac tams for the treatment of Plesiomonas infections.