Drug Sensitivity Assay on Development of Biofilm in Non-Tuberculosis Mycobacteria

Abstract

The bacteria enclosed within the biofilm are extremely resistant to antibiotic treatments; such resistance can be explained by hypotheses, not necessarily limited to the following ones. First, the EPS secreted by biofilm bacteria, acts as a physical/chemical barrier, thus preventing penetration by antibodies or many antibiotics. The EPS is negatively charged and functions as an ion-exchange resin which is capable of binding a large number of the antibiotic molecules that are attempting to reach the embedded biofilm cells. Second, embedded biofilm bacteria are generally not actively engaged in cell division, are smaller in size and less permeable to antibiotics. We demonstrate here for the first time that biofilm cultures of a Mycobacterium are capable of growing at higher drug concentrations (i.e. have higher MICs) than suspension cultures. The MIC of selected drugs concentration that inhibited exponentially growing biofilm was found to be higher than the MIC for planktonic culture. The biofilm literature offers essentially three possible explanations for biofilm-associated drug resistance: (i) Slower biofilm growth, (ii) Decreased drug penetration into the bioflm due to architecture and/or extracellular matrix of the biofilm, (iii) Resistance mechanisms expressed specifically in biofilm bacilli but not in planktonic cells. The generation times of biofilm and planktonic M. smegmatis, and M. fortuitum cultures were found to be similar. Isoniazid shows specific activity of M. smegmatis and M .fortuitum biofilm cell growths. The work reported here shows that the bacterium is furthermore capable of differentiating into a multicellular, surface-attached form that is resistant to growth inhibition by antimycobacterials.