Bacteria form a variety of shapes and sizes to survive different environments. This phenomenon is assumed to be important for infection, however its impact is still largely unknown. One shape change is filamentation; where bacteria stop dividing and grow into long “spaghetti-like” cells. A number of infections involve filamentation including the well-studied urinary tract infections (UTIs). While filamentation has been observed in UTIs, how it allows bacteria to successfully survive the immune response is not well understood.
Most phagocytosis studies focus on one parameter of the target being important for macrophage engulfment, such as particle size or geometry. We therefore studied a range of parameters with rigorous controls by employing different antibiotics used to treat UTIs to tightly manipulate the size and shape of the Escherichia coli strain UTI89. We initially used end-point assays to quantify the engulfment ability of macrophages and found that engulfment depends on a hierarchy between several parameters; size, shape and surface all contribute. We further investigated the interaction between macrophages and antibiotic-induced filaments using microscopy analysis to quantify engulfment dynamics of rods and filaments. In support of our end-point assay results, we found that engulfment of antibiotic-induced filaments is overall less efficient. Microscopy also allowed us to classify fully-engulfed vs partially-engulfed bacteria across time points, which revealed a difference in the timing of engulfment and an inability of macrophages to fully engulf filaments. Similar experiments using filaments from a human in vitro bladder model were performed and the engulfment of these filaments differs to that of antibiotic-induced filaments. Thus, filamentation due to different environmental stimuli affects engulfment, supporting our hypothesis that it’s a complex interplay of target parameters, and not just biophysical changes, that contribute to engulfment of filaments.
With several strains of E.coli now resistant to current antibiotics, our work identifies the importance of bacterial morphology during infections and may provide new ways to prevent or treat these infections via immune modulation or antimicrobials.