Riddle of Biofilm Resistance

A biofilm is a population of cells growing on a surface and enclosed in an exopolysaccharide matrix. Biofilms are notoriously difficult to eradicate and are a source of many recalcitrant infections. The nature of bacterial biofilm resistance to antimicrobials is the subject of the present minireview. Pathogenic yeast such as Candida albicans also form recalcitrant biofilms, and this topic has recently been reviewed (5). Resistance is an ability of a microorganism to grow in the presence of an elevated level of an antimicrobial. In short, a strain for which the MIC is increased is resistant. By this conventional criterion, biofilm cells do not necessarily show increased resistance. With some exceptions, biofilm cells do not grow better than planktonic cells in the presence of a broad range of antimicrobials. This is evident from examination of susceptibility data in the biofilm literature (33). However, in most biofilm susceptibility studies, only survival of cells in a preformed biofilm rather than the ability of a biofilm to grow is recorded. Accordingly, the reported “resistance” describes an increased resistance of cells to killing. This is indeed what biofilms are good at: they are not easily eradicated by cidal antimicrobials. The ability of antimicrobials to inhibit biofilm growth indicates that they are able to diffuse through the biofilm and act normally against their targets. Why, then, do biofilm cells not die? This is the crux of the problem and the riddle that needs to be solved.