Objective - Biofilms behave like multicellular organisms, previously free-living unicellular organisms communicate to make group-level decisions in the film. This is an emergent property of biofilms not predicted by the behavior of individual bacteria. The properties and strength of the biofilm are determined by the adhesion of the cells to a substrate and the cohesion of the cells to each other. The biofilm formation is fixed in an outer polymeric substance made of an extracellular polymeric substance (EPS) matrix. The study was undertaken to determine the antibiotic susceptibility patterns of two unknown Bacillus species, isolated from biofilms found within sink drains. One Bacillus species (B1) exhibited resistance to ampicillin and the other Bacillus species (B2) was non-resistant to ampicillin and streptomycin.  

Materials and Methods - In the experiment, the Bacillus species grown in broth culture were inoculated individually and in combination with other species including Staphylococcus aureus and strains of Escherichia coli in a 96 well plate with M63 minimal media in a 5% CO2 environment for two days. Following incubation, resistance to ampicillin and streptomycin was assessed. 

Results- One of the Bacillus species (B2) gained resistance to streptomycin only after being incubated in the CO2 environment and remained non-resistant to ampicillin. The other species (B1) remained resistant to ampicillin and non-resistant to streptomycin throughout the experiment.   

Conclusions - The results suggest that the culture environment had an effect on the antibiotic susceptibility patterns of the Bacillus species perhaps by inducing or inhibiting the expression of antibiotic resistant genes, as has been seen in other microorganisms.