Retention and biodistribution of microspheres injected into ischemic myocardium.

Intramyocardial injection of therapeutic agents may enhance heart repair after infarction. Incomplete retention of intramyocardial injections has been reported, but modes of loss are undefined. We determined the fate of neutron-activated microspheres injected into acutely ischemic rat myocardium using saline, Pluronic F127, or Matrigel as vehicle. Twenty minutes after injection in saline, 63% +/- 12% of 10-mum microspheres was retained in the heart. Similar retention was observed after 6 days. Injection site leakage accounted for 14% +/- 5% of the microspheres, whereas exit via coronary veins resulted in 11.2% +/- 9.5% collecting in the lungs. Microspheres distribution to other organs was minimal. Retention of 40-mum microspheres was similar to that observed with the 10-mum microspheres. Pluronic F127 and Matrigel reduced immediate leakage to 4% +/- 1% and 2% +/- 1%, respectively. Surprisingly, microsphere retention in the heart was not improved at 20 min using either gelling vehicle, suggesting that leakage occurs over a prolonged period. Thus, most injected particles are retained in the ischemic rat heart following direct injection, but significant fractions are lost from the injection site and through coronary veins. Gelling agents reduced short-term leakage, but failed to enhance longer-term retention. Hydrogels with stiffer mechanical properties might enhance retention and reduce variability.

Role of nutrient limitation and stationary-phase existence in Klebsiella pneumoniae biofilm resistance to ampicillin and ciprofloxacin.

Biofilms formed by Klebsiella pneumoniae resisted killing during prolonged exposure to ampicillin or ciprofloxacin even though these agents have been shown to penetrate bacterial aggregates. Bacteria dispersed from biofilms into medium quickly regained most of their susceptibility. Experiments with free-floating bacteria showed that stationary-phase bacteria were protected from killing by either antibiotic, especially when the test was performed in medium lacking carbon and nitrogen sources. These results suggested that the antibiotic tolerance of biofilm bacteria could be explained by nutrient limitation in the biofilm leading to stationary-phase existence of at least some of the cells in the biofilm. This mechanism was supported by experimental characterization of nutrient availability and growth status in biofilms. The average specific growth rate of bacteria in biofilms was only 0.032 h(-1) compared to the specific growth rate of planktonic bacteria of 0.59 h(-1) measured in the same medium. Glucose did not penetrate all the way through the biofilm, and oxygen was shown to penetrate only into the upper 100 micro m. The specific catalase activity was elevated in biofilm bacteria to a level similar to that of stationary-phase planktonic cells. Transmission electron microscopy revealed that bacteria were affected by ampicillin near the periphery of the biofilm but were not affected in the interior. Taken together, these results indicate that K. pneumoniae in this system experience nutrient limitation locally within the biofilm, leading to zones in which the bacteria enter stationary phase and are growing slowly or not at all. In these inactive regions, bacteria are less susceptible to killing by antibiotics.