Fig. 3. UBI-3 retains specificity and sensitivity for bacterial labelling. (A) Methionine (UBI-2) and norleucine (UBI-3) variants label bacteria with the same fluorescence intensity. Panels show bacteria labelled with either UBI-2 or UBI-3 with quantification of fluorescence (values normalised to UBI-2) (Bars represent mean, error bars represent standard error of mean, n ≥ 3, ns = not significant, scale bar = 10 μm). (B) Quantification from confocal images of UBI-3 (10 μM) imaging a panel of bacteria (statistical analyses shown compared to unlabelled bacteria, n ≥ 3 for each bacteria, *** = p < 0.001). (C) UBI-3; labels bacteria but not isolated human neutrophils (blue arrows). Dotted box enlarged in lower panel and line plot of fluorescence from green channel shown with yellow representing background, red indicating neutrophil area and blue representing single bacteria demonstrating labelling of bacteria but not neutrophils, scale bar = 10 μm. [MRSA: methicillin-resistant Staphylococcus aureus, MSSA: methicillin-sensitive Staphylococcus aureus].