Biology Reference
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which block their bioactivity [40, 46, 64, 65]. Our results demonstrate the capability
of liposomes to reduce the antibiotics' contact with polyanionic factors in the sputum
and enhance bacteria-antibiotic(s) interactions. The liposomal formulation protected
tobramycin from the inhibitory actions of DNA/F-actin at low concentrations while
neither polymyxin B formulations were inactivated. Our fi ndings are in agreement
with those reported by Hunt et al. [36] who found a reduction in tobramycin activity
in the presence of DNA (within a 2 hr exposure) even when it was pretreated with
recombinant human DNase (rhDNase). Weiner et al. [43] on the other hand, reported
DNA and F-actin aggregation (within a 5 hr exposure) with increasing concentrations
of tobramycin, yet bioactivity in a microbroth dilution assay (within an 18 hr expo-
sure) was not hindered by the presence of either DNA or F-actin. The inconsistencies
among the results of the different studies may be attributed to factors such as incuba-
tion time, co-incubation of DNA and F-actin, and that DNA/F-actin concentrations
were increased as tobramycin concentration was kept constant. The protective effect
of the liposomes at the lower DNA/F-actin concentrations may be attributed to the
neutral nature of the phospholipids comprising the liposomes which would not fa-
vor electrostatic interactions between phospholipids with DNA or F-actin. The lack
of effectiveness of tobramycin encapsulated within liposomes in the presence of the
higher concentrations of polyanionic factors cannot be explained from the results of
this study but it may be possible that a build up of F-actin/DNA aggregates leads to an
increase in viscoelasticity, which ultimately hinders liposome-bacteria interaction [64,
65]. Reports from other studies have shown that DNA greatly hampers nanosphere
diffusion through sputum and that the rhDNase improves its diffusion [53, 55, 56].
With regards to polymyxin B, reports from studies have shown G-actin polymer-
ization in the presence of polymyxin B [37] and DNA and polymyxin B precipitation
in vitro
[67]. In our studies, there was no loss of bioactivity when DNA, F-actin, or
both were incubated with polymyxin B (data not shown). The observation of consis-
tent bacterial killing by polymyxin B can be attributed to the ability of the antibiotic
to resist bundle formation, and having a higher affi nity for polyanionic LPS of the
bacterial outer wall than DNA or F-actin. Weiner et al. [43] reported no aggregation or
reduction of bioactivity between colymycin, an anionic colistin form, and DNA or F-
actin. However, the absence of aggregation may be due to the similar negative charges
of the antibiotic and DNA or F-actin.
The binding of free bacterial surface components (e.g., LPS and LTA) to polyca-
tionic antibiotics like polymyxin B may be benefi cial to the host in terms of suppress-
ing infl ammation however it will compromise the antibacterial effect of the antibiotic.
Tobramycin and polymyxin B tend to interact with the bacterial lipid membranes as
indicated by the results of this study where the bioactivity of both antibiotics was
reduced when co-incubated with LPS/LTA. However, the bioactivity of the antibiot-
ics within the liposomes fared better (Figure 3) although inhibited at the higher LPS/
LTA concentrations. The mechanism of inactivation of liposomal antibiotics by the
higher polyanionic LPS/LTA levels cannot be attributed to the release of antibiotics
from liposomes and subsequent inactivation because results from the liposomal stabil-
ity studies (Table 1) showed that the lipid bilayers were not lysed. This is consistent
with results from another study reported by Davies et al. [68] where divalent anions
