Urban Road Dust as a Microbial Hotspot: Spatial Variation of Bacterial Communities and Metal Associations in the Philippines.

Urban road dust is a reservoir and transport medium for diverse microorganisms, yet its bacterial communities in tropical environments remain poorly understood. This study examined the spatial variation of road dust bacterial communities across three Philippine metropolitan areas: Metro Manila, Metro Cebu, and Metro Cagayan de Oro, spanning the country's three principal island groups. Dust from nine sites was analyzed using 16 S rRNA gene metabarcoding (V3-V4), with community composition, diversity, predicted functional potential (PICRUSt2), and metal associations evaluated; contamination was assessed using pollution indices (PLI, degree of contamination, and ecological risk). Proteobacteria and Actinobacteria dominated all sites, and a small core of genera: Paracoccus, Acinetobacter, Kocuria, and Deinococcus, was consistently detected. Alpha diversity did not differ among cities (Kruskal-Wallis, p > 0.05), whereas community composition did (PERMANOVA: R² = 0.466, p = 0.004), with distinct metropolitan clustering. This decoupling indicates that local conditions shape which taxa dominate rather than overall diversity, consistent with environmental filtering of a shared regional pool. Alkanindiges reached its highest relative abundance at Metro Cebu's commercial core. In Metro Cagayan de Oro, Paracoccus increased toward the most trace-metal-laden site (D3) and correlated positively with cobalt and nickel; with elevated predicted denitrification in this area, this points to a possible pollution-responsive signal. Metro Cagayan de Oro also showed a pronounced higher relative abundance of Chloroflexi, marking it as a distinctive, transitional urban environment. Pollution indices indicated baseline-to-moderate contamination across the sampled roadsides. Predicted function paralleled taxonomy (R² = 0.467). These findings establish a baseline for tropical road-dust microbiota and support integrating microbial assessment into urban environmental monitoring.