Floating macroplastics concentrate culturable human pathogens but do not amplify their antibiotic resistance in a...

Marine macroplastics may act as reservoirs and fomites for viable, potentially human-pathogenic bacteria (PHPBs), but quantitative evidence remains limited, especially in tropical ecosystems exposed to PHPBs contamination. Using 113 macroplastic samples and 50 paired seawater samples collected from two sites over two seasons in the Toliara reef ecosystem (Madagascar), a region lacking wastewater management system, we aimed to quantify viable PHPBs and compare their concentration, community composition, and phenotypic antibiotic resistance. PHPBs were quantified by selective culture, isolates were identified by MALDI-ToF (Matrix Assisted Laser Desorption Ionization - Time of Flight), and antibiotic susceptibility was assessed using standardized clinical disk-diffusion tests. Macroplastics harbored far higher PHPB loads than seawater (mean 9.2 × 10³ vs 9.6 CFU/g) and supported more diverse and compositionally distinct assemblages that did not significantly variate across sites and seasons. Communities were dominated by Vibrio alginolyticus and Vibrio harveyi (72% of total concentration), followed by Bacillus cereus (19%) and Micrococcus luteus (6%). Several enteric and opportunistic taxa were detected at lower relative abundances (<1%) but at measurable concentrations (Enterococcus spp., Klebsiella pneumoniae, Escherichia coli). Phenotypic antibiotic resistance profiles differed between macroplastics and seawater, but multiresistance levels remained high and comparable (mean MAR index = 0.43 on macroplastics). These findings suggest that plastics may constitute an important and potentially amplified pathway for exposure to resistant and viable pathogens, particularly in tropical coastal systems influenced by human activity.