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D-amino acids are responsible for cell wall re-modelling in Staphylococcus and are capable of inhibition and mature biofilm disassembly. Staphylococcus aureus and Staphylococcus epidermidis are recognised as recurrent nosocomial pathogens and a common cause of biofilm-associated infections. The combination of amino acids used in the study consisted of d- and l- isomers of tyrosine, methionine, tryptophan and phenylalanine. A semiquantitative microplate crystal violet assay was used to assess the effect of amino acids on biofilm development. Biofilm viability staining using fluorescent microscopy was performed to assess the effect of the amino acid mixtures on biofilm development on submerged surfaces. None of the amino acids when tested individually or as a mixture could reduce biofilm formation. However, at the highest concentration tested 25 mmol 1−1 equimolar D-amino acid mixture of tryptophan, phenylalanine, tyrosine and methionine caused a considerable biofilm inhibition in three Staphylococcus strains. Microscopy analysis showed that initial surface attachment remained unaffected at 25 mmol 1−1 mixture of d-amino acids but bacteria did not proceed to form mature biofilms. This suggests inhibition of protein synthesis or a lack of polysaccharide extracellular adhesin formation as no aggregates were observed. The reported bioactivity of D-amino acid on biofilm development and disassembly has been conflictual. It has been established that D-amino acids are incorporated in the bacterial cell wall suggesting they play a role in the complexity of biofilm lifecycle. However, our study indicates that they play no direct role in the inhibition of biofilm formation in Staphylococcus.

Streptococcus mutans is the major cariogenic organism associated with Dental Caries, a widespread chronic disease of the oral cavity (Muras et al. 2018). It is associated with oral biofilm formation, production of organic acids, and has the capacity to out-compete non-cariogenic commensal species (Lemos et al. 2013). Recent studies carried out on fatty acids demonstrated effective antimicrobial activity against S. mutans (Hughes, 2014). This study will evaluate the activity of lipids on S. mutans biofilm formation and proposes that dietary constituents may be used as a natural therapy to maintain oral hygiene. Clinical isolate S. mutans 3014 D5929 was exposed to various concentrations of lipid for 24 h. Crystal violet assay was performed for quantification of biofilm biomass. Fluorescent microscopy using SYTO® 9 and Alexa Fluor® 647-labelled dextran conjugate was performed to visualise biofilm formation pre- and post-exposure to MCO. Biofilm biomass was reduced for all lipid concentrations. Fluorescent microscopy indicated a significant reduction in bacterial cell number and a lack of structural biofilm upon exposure to fatty acid mixtures when compared to control. The research demonstrated that lipid does have S. mutans based antimicrobial and antibiofilm capabilities.