Biofilms and antibiotic therapy: is there a role for combating bacterial resistance by the use of novel drug delivery systems?

The conventional view of antibiotic resistance is one where bacteria exhibit significantly reduced susceptibility to antimicrobials in laboratory tests by mechanisms such as altered drug uptake, altered drug target and drug inactivation. Whilst these mechanisms undoubtedly make a major contribution to antibiotic failure in the clinic, the phenomenon of clinical failure in spite of sensitivity in laboratory tests is also well recognised. It is in this context that attention has focussed on bacteria growing as adherent biofilms, not only as the mode of growth of device-related infections associated for example with artificial joints and venous catheters, but also with other chronic infections such as those occurring in the respiratory tract. Growth as a biofilm almost always leads to a significant decrease in susceptibility to antimicrobial agents compared with cultures grown in suspension and, whilst there is no generally agreed mechanism for the resistance of biofilm bacteria, it is largely phenotypic. That is, when biofilm bacteria are grown in conventional laboratory suspension culture they become susceptible to antimicrobials. A number of elements in the process of biofilm formation have been studied as targets for novel drug delivery technologies. These include surface modification of devices to reduce bacterial attachment and biofilm development as well as incorporation of antimicrobials-again to prevent colonisation. Electrical approaches have been used either to release antimicrobials from device surfaces or to drive antimicrobials through the biofilm. Other technologies not specifically focussed on biofilms include aerosolized delivery of antibiotics to the lung and formulation into liposome and polymer-based vehicles. Liposomal systems have been widely studied, either to target antibiotics to the surface of bacterial biofilms, or by virtue of their property of being taken up cells of the reticuloendothelial system, to target antibiotics towards intracellular bacteria. Many polymer-based carrier systems have also been proposed, including those based on biodegradable polymers such as poly(lactide-co-glycolide) as well as thermoreversible hydrogels. Their contribution to the prevention or resolution of infection is reviewed.