Pharmacodynamic effects of antibiotics. Studies on bacterial morphology, initial killing, postantibiotic effect and effective regrowth time.

Pharmacodynamics of antibiotics deals with time course of drug activity and mechanisms of action of drugs on bacteria. In this thesis pharmacodynamic parameters have been studied after brief exposure of gram-positive bacteria to daptomycin, imipenem or vancomycin and after short exposure of gram-negative bacteria to amikacin, ampicillin, aztreonam, cefepime, cefotaxime, ceftazidime, ceftriaxone, cefuroxime, imipenem, mecillinam, or piperacillin. The studies have been focused on morphological alterations, initial killing, postantibiotic effect (PAE) and effective regrowth time (ERT) and a method, based on bioluminescence assay of intracellular ATP has been used. The basic principle behind this technique is that ATP in living cells is present in a relatively constant amount, and hence affords a measure of the number of microbial cells. The PAE describes the delayed regrowth of bacteria after brief exposure to antibiotics. The number of cells measured after this antibiotic exposure describes the initial killing and is also the start value for calculating the PAE. PAEs of 2-3 h were obtained by bioluminescence for gram-positive bacteria exposed to imipenem or vancomycin. This is in agreement with results obtained by viable count and is probably due to similar weak initial decrease in cell density when assayed by both methods. Long (greater than 3 h) concentration dependent PAEs and moderate (less than or equal to 1 log10) initial decrease in intracellular ATP were in general seen for gram-positive bacteria exposed to daptomycin and for gram-negative bacteria exposed to imipenem or amikacin when assayed by bioluminescence. These very long PAEs and rather weak initial killing have to be compared with the shorter PAEs and stronger initial killing reported by us and others using viable count. Furthermore, this study showed that there was a relatively good concordance between microscopy and bioluminescence, which are direct methods, in determining the initial killing and PAE of imipenem on Escherichia coli. The ERT, defined as the time for bacterial density to increase 1 log10 from the pre-exposure inoculum, was independent of the method used for measuring regrowth of E. coli after brief exposure to imipenem. The combination of mecillinam with ampicillin, aztreonam, ceftazidime or piperacillin and the combination of amikacin with ceftazidime, ceftriaxone or piperacillin induced longer PAEs on gram-negative bacteria than the sum of PAEs of the individual antibiotics. A strong initial killing in combination with a long PAE cause a long ERT and may allow the antibiotic concentration to stay below MIC during long periods of time without any regrowth. This may, in clinical practice, have implications for long dosing intervals.