Spatial proximity dictates bacterial competition and expansion in microbial communities.

In microbial communities, bacteria can inhibit or facilitate each other by altering their shared environment. Most studies of these interactions have focused on well-mixed environments, leaving spatial effects underexplored. Here, we show that in an antibiotic-treated community, bacterial spread depends on a facilitation mechanism that only emerges in spatial settings. The facilitating species enables the community's range expansion but is then suppressed to a minority, making it a hidden initiator of the expansion. Focusing on two pathogens, immotile Klebsiella pneumoniae and motile Pseudomonas aeruginosa, we found that both tolerate a β-lactam antibiotic, with Pseudomonas being more resilient and dominating in well-mixed cultures. During range expansion, however, the antibiotic inhibits Pseudomonas' ability to spread unless it is near Klebsiella-which creates a clear zone by degrading the antibiotic, at the expense of its own growth. As Pseudomonas spreads, it competitively suppresses Klebsiella. Our modeling and experimental analyses reveal that this facilitation operates at a millimeter scale. We also observed similar facilitation by a Bacillus species isolated from a hospital sink, in both pairwise and eight-member bacterial communities with its co-isolates. These findings suggest that spatially explicit experiments are essential to understand certain facilitation mechanisms and have implications for surface-associated microbial communities like biofilms and for polymicrobial infections involving drug-degrading immotile and drug-tolerant motile bacteria.