[3H]tobramycin bound to sodium alginate and to exopolysaccharide prepared from two mucoid strains of Pseudomonas aeruginosa. Binding to sodium alginate was similar to binding to exopolysaccharide, both in the dependence on tobramycin concentration and in the maximum binding observed at saturation. Incorporation of sodium alginate into agar plates reduced the zone sizes of growth inhibition caused by tobramycin. The reductions in zone sizes were quantitatively accounted for by the binding of tobramycin to sodium alginate during diffusion of the antibiotic away from the well in which it had been placed at the start of the experiment. However, the binding of tobramycin to the exopolysaccharide of P. aeruginosa, and the resulting inhibition of diffusion of the antibiotic, did not significantly increase the penetration time of a spherical microcolony with a radius of 125 ,um, such as might be found in the respiratory tract of a patient with cystic fibrosis (from a 90% penetration time of 12 s in the absence of exopolysaccharide to one of 35 s with an exopolysaccharide concentration of 1.0% [wt/voll).The question of whether bacterial exopolysaccharides reduce the penetration of antibiotics to their target sites (5, 22) is an important one in antibacterial chemotherapy. This is because exopolysaccharide-producing bacteria existing as biofilms are less susceptible to antibiotics than are freely suspended bacteria (6, 17), and mucoid Pseudomonas aeruginosa apparently forms microcolonies (12) when causing respiratory tract infections that are refractory to chemotherapy in patients with cystic fibrosis. Moreover, in a recent review (4), the exopolysaccharide material of the biofilm was specifically postulated to exclude antibacterial substances.Inhibition of the diffusion of aminoglycoside antibiotics occurs in the presence of alginate (21), a polyanionic polysaccharide similar in structure to the exopolysaccharide of mucoid P. aeruginosa (13), or in the presence of the exopolysaccharide from P. aeruginosa (21). A likely reason for the reduced rate of diffusion of aminoglycosides within the anionic polysaccharide matrix is that any antibiotic-binding sites act as sinks, thereby reducing the free concentration of antibiotic, which is effectively the driving force of diffusion. In apparent conflict with this suggestion, the binding of tobramycin and streptomycin to alginate has been reported (23) as not being detectable in a physiological buffer containing 0.10 M NaCl.We quantitatively investigated the inhibition of diffusion and assessed the binding of tobramycin to alginate and Pseudomonas exopolysaccharide using radiolabeled tobramycin. Here we report that in a physiological buffer solution, tobramycin binds to alginate and to exopolysaccharides isolated from two mucoid strains of P. aeruginosa and that the binding to alginate quantitatively accounts for the inhibition of diffusion reported previously (21). However, binding and consequent inhibition of diffusion cannot account for antibiotic resistance within microcolonie...

~~ ~ ~Cells of mucoid and non-mucoid Pseudomonas aeruginosa in colonies were at least onethousandfold less sensitive to the antibiotics tobramycin or cefsulodin than were cells of the same bacteria in dispersed suspension. We did not detect any difference between the mucoid form and the non-mucoid form in the antibiotic sensitivity of colonies, from which we infer that the exopolysaccharide of the mucoid form does not contribute to colony-resistance by forming a barrier to antibiotic diffusion. Mathematical models were constructed in order to estimate timecourses of penetration of tobramycin and cefsulodin into biofilms and microcolonies of mucoid and non-mucoid P. aeruginosa. For tobramycin penetration, adsorption of antibiotic to the exopolysaccharide of the glycocalyx and antibiotic uptake by cells were taken into account in the calculations. The longest time-period for the concentration of tobramycin at the base of a biofilm 100 prn deep to rise to 90% of the concentration outside the biofilm was predicted to be 2.4 h. For cefsulodin penetration, irreversible hydrolysis catalysed by p-lactamase was taken into account, using P-lactamase levels taken from the literature. The calculations predicted that the cefsulodin concentration at the base of a biofilm 100pm deep would rise to 90% of the external concentration in 29 s when the /I-lactamase was synthesized at the basal level. For a similar biofilm of bacteria synthesizing enhanced levels of fl-lactamase ('derepressed'), the concentration of cefsulodin at the base was calculated to rise to 41% of the external concentration in about 50s and then remain at that level. This was despite the fact that cefsulodin is a poor substrate for this P-lactamase.

Extensive literature exists on spray development, mixing and combustion regarding engine modeling and diagnostics using single-component and model fuels. However, often the variation in data between different fuels, particularly relating to spray development and its effect on combustion, is neglected or overlooked. By injecting into a quiescent chamber, this work quantifies the differences in spray development from a multi-hole direct-injection spark-ignition engine injector for two single-component fuels (iso-octane and n-pentane), a non-fluorescing multi-component model fuel which may be used for in-cylinder Laser Induced Fluorescence experiments, and several grades of pump gasoline (with and without additives). High-speed recordings of the sprays were made for a range of fuel temperatures and gas pressures. It is shown that a fuel temperature above that of the lowest boiling point fraction of the tested fuel at the given gas pressure causes a convergence of the spray plumes. Increasing the fuel temperature increases this convergence, whilst an associated increased rate of evaporation tends to reduce the penetration of individual plumes. The convergence increases gradually with increasing fuel temperature until all plumes combine to form a single wider plume with a penetration rate greater than that of the individual plumes. When all plumes are converged to form a single plume along a central axis to all the plumes, any further increase in fuel temperature at the given gas pressure acts to increase the rate of evaporation of the fuel. At experiments up to 180 °C fuel temperature and down to 0.3 bar absolute gas pressure, none of the tested fuels were found to spontaneously vaporize; all observed spray formations being a gradual evolution. Increasing the gas pressure at any given fuel temperature, leads to an increase in the boiling temperature of all components of that fuel and, hence, diminishes these effects.

The blending of oxygenated compounds with gasoline is projected to increase because oxygenate fuels can be produced renewably, and because their high octane rating allows them to be used in substitution of the aromatic fraction in gasoline. Blending oxygenates with gasoline changes the fuels' properties and can have a profound affect on the distillation curve, both of which are known to affect engine-out emissions. In this work, the effect of blending methanol and ethanol with gasoline on unburned hydrocarbon and particulate emissions is experimentally determined in a spray guided direct injection engine. Particulate number concentration and size distribution were measured using a Cambustion DMS500. These data are presented for different air fuel ratios, loads, ignition timings and injection timings. In addition, the ASTM D86 distillation curve was modeled using the binary activity coefficients method for the fuel blends used in the experiments. In general, unburned hydrocarbon emissions were reduced at low load but increased at high load for the alcohol blends. The effect on particulate emissions was dependent on the operating point: for rich mixtures the accumulation mode number concentration and count median diameter were reduced with the oxygenate blends. However, blending gasoline with oxygenates also caused the nucleation mode number concentration to increase, particularly for M85. The distillation curve modeling showed that blending oxygenates affects the distillation curve much more than would be expected from a linear blending relationship: the front end volatility is reduced a little, whilst the mid range volatility is increased significantly, particularly for methanol blends.

Combustion in HCCI engines is a controlled auto-ignition of well-mixed fuel, air and residual gas. The thermal conditions of the combustion chamber are governed by chemical kinetics strongly coupled with heat transfer from the hot gas to the walls. The heat losses have a critical effect on HCCI ignition timing and burning rate, so it is essential to understand heat transfer process in the combustion chamber in the modelling of HCCI engines. In the present paper, a comparative analysis is performed to investigate the performance of well-known heat transfer correlations in an HCCI engine. The results from the existing correlations are compared with the experimental results obtained in a single cylinder engine. Significant differences are observed between the heat transfer results obtained by using Woschni, Assanis and Hohenberg correlations.