Abstract
Bath toys pose an interesting link between flexible plastic materials, potable water, external microbial and nutrient contamination, and potentially vulnerable end-users. Here, we characterized biofilm communities inside 19 bath toys used under real conditions. In addition, some determinants for biofilm formation were assessed, using six identical bath toys under controlled conditions with either clean water prior to bathing or dirty water after bathing. All examined bath toys revealed notable biofilms on their inner surface, with average total bacterial numbers of 5.5 × 106 cells/cm2 (clean water controls), 9.5 × 106 cells/cm2 (real bath toys), and 7.3 × 107 cells/cm2 (dirty water controls). Bacterial community compositions were diverse, showing many rare taxa in real bath toys and rather distinct communities in control bath toys, with a noticeable difference between clean and dirty water control biofilms. Fungi were identified in 58% of all real bath toys and in all dirty water control toys. Based on the comparison of clean water and dirty water control bath toys, we argue that bath toy biofilms are influenced by (1) the organic carbon leaching from the flexible plastic material, (2) the chemical and biological tap water quality, (3) additional nutrients from care products and human body fluids in the bath water, as well as, (4) additional bacteria from dirt and/or the end-users’ microbiome. The present study gives a detailed characterization of bath toy biofilms and a better understanding of determinants for biofilm formation and development in systems comprising plastic materials in contact with potable water.
Highlights
Unwanted microbial growth in the built environment is frequently reported
We examined and compared biofilms that were established in new, identical bath toys under controlled conditions was confirmed by optical coherence tomography (OCT) analysis on selected bath toys, revealing heterogeneous biofilm shapes and thicknesses both within and between individual toys, ranging up to 100 μm (Fig. 1b, Figure S2 for additional images)
Based on these data we argue that the combination of four main factors impacted the magnitude and composition of bath toy biofilms, namely (1) the flexible plastic material and (2) the bath water quality that is further influenced by (3) chemical additives from washing products and the user, plus (4) biological contamination by the user’s microbiome and the environment
Summary
Unwanted microbial growth in the built environment is frequently reported. Bathroom conditions in particular are known to promote biofilm formation and growth due to moderately high temperatures and increased humidity.[1,2] In this regard, unwanted microbial growth has been reported, e.g., for basins, bath tubs, and drains,[3,4] as well as for shower fixtures[5,6,7] and shower curtains.[8] In the same environment, bath toys, best known for socalled “rubber ducks”, present an interesting junction between potentially vulnerable end-users and several determining factors for such growth, namely (1) low-quality polymeric material, (2). Synthetic polymeric materials in contact with potable water adsorb some organic matter from the water,[9] and release substantial amounts of organic carbon through migration, leakage, leaching, and/or permeation, including, e.g., plasticizers, stabilizers and antioxidants.[10,11,12,13] A fraction of this organic carbon is biodegradable and offers microorganisms a significant source of assimilable organic carbon (AOC).[14,15,16] This AOC in turn promotes microbial growth and biofilm formation[16,17,18,19] and influences the microbial community composition.[5,20,21] Flexible polymeric (i.e., plastic) materials, which are typically used in the production of bath toys, are known for excessive carbon leaching and unwanted biofilm formation and growth.[16,20]
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