Bacterial swarming reducesProteus mirabilisandVibrio parahaemolyticuscell stiffness and increases β-lactam susceptibility

Swarmer cells of the gram-negative pathogenic bacteria Proteus mirabilis and Vibrio parahaemolyticus become long (>10-100 μm) and multinucleate during their growth and motility on polymer surfaces. We demonstrate increasing cell length is accompanied by a large increase in flexibility. Using a microfluidic assay to measure single-cell mechanics, we identified large differences in swarmer cell stiffness of (bending rigidity of P. mirabilis , 9.6 × 10 −22 N m 2 ; V. parahaemolyticus , 9.7 × 10 −23 N m 2 ) compared to vegetative cells (1.4 × 10 −20 N m 2 and 3.2 × 10 −22 N m 2 , respectively). The reduction in bending rigidity (~3-15 fold) was accompanied by a decrease in the average polysaccharide strand length of the peptidoglycan layer of the cell wall from 28-30 to 19-22 disaccharides. Atomic force microscopy revealed a reduction in P. mirabilis peptidoglycan thickness from 1.5 nm (vegetative) to 1.0 nm (swarmer) and electron cryotomography indicated changes in swarmer cell wall morphology. P. mirabilis and V. parahaemolyticus swarmer cells became increasingly sensitive to osmotic pressure and susceptible to cell wall-modifying antibiotics (compared to vegetative cells)—they were ~30% more likely to die after 3 h of treatment with minimum inhibitory concentrations of the β-lactams cephalexin and penicillin G. The adaptive cost of swarming is offset by the increase in cell susceptibility to physical and chemical changes in their environment, thereby suggesting the development of new chemotherapies for bacteria that leverage swarming for colonization of hosts and survival. Importance Proteus mirabilis and Vibrio parahaemolyticus are bacteria that infect humans. To adapt to environmental changes, these bacteria alter their cell morphology and move collectively to access new sources of nutrients in a process referred to as ‘swarming’. We found that a change in the composition and thickness of the peptidoglycan layer of the cell wall makes swarmer cells of P. mirabilis and V. parahaemolyticus more flexible (i.e., reduced cell stiffness) and increases their sensitivity to osmotic pressure and cell-wall targeting antibiotics (e.g., β-lactams). These results highlight the importance of assessing the extracellular environment in determining antibiotic doses and the use of β-lactams antibiotics for treating infections caused by swarmer cells of P. mirabilis and V. parahaemolyticus .