Numbers and types of bacterioplankton proliferating in blood samples mixed with water of various salinity levels were examined to determine the characteristics of species associated with salinity. Water samples (total n = 88) were collected from the midstream of two rivers (freshwater; n = 10; salinity <0.05%), from around their estuaries (areas of freshwater, n = 20, salinity <0.05%; areas of brackish water, n = 20, salinity <0.05-3.1%; areas of marine water beyond the mouths of the rivers, n = 28, salinity 2.4-3.3%), and from the coast (areas of marine water; n = 10; salinity 3.3-3.5%). Freshwater bacteria were identified in 41 of 42 blood samples mixed with water at ≤1.3% salinity, and the genus Aeromonas, which is universally distributed in freshwater environments, was predominant. Marine bacteria were identified in all of 46 blood samples mixed with water at ≥1.8% salinity, and most comprised the genera Vibrio and Photobacterium that are universally distributed in seawater environments. Aeromonas was undetectable in all blood samples mixed with brackish or sea water at ≥1.8% salinity although they are detectable even in seawater environments. Thus, the present results showed that bacterioplankton capable of proliferating in human blood reflects the salinity of water.
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