Aim: To evaluate possible source of nutrients for bacterial growth within polyurethane (PU) foam of used cot mattresses as determinants of bacterial population density. Methods and results: Used infant mattresses (n = 30) were analysed for bacteria capable of degrading colloidal PU and for aqueous soluble chemical components (aromatic amines, ammonium ions, phosphates and protein). Mattress type (waterproof cover vs exposed PU foam at the infant-head region), mattress age and previous use by another child were evaluated as factors that could influence the measured parameters. The levels of protein extracted from PU foam were (i) significantly (P = 0.0019) higher for mattresses lacking a waterproof cover at the infant-head region and (ii) positively correlated with both culturable bacterial population densities of the PU foams and extent of growth of Staphylococcus aureus on aqueous leachates. No statistically significant (P > 0.05) associations between other measured parameters and mattress type/use factors were identified. Conclusions: Infant use of cot mattresses with exposed PU foam leads to accumulation of proteins within the PU, which can promote bacterial growth. Significance and Impact of the Study: The study provides a mechanistic explanation for increased levels of bacteria associated with exposed PU of cot mattresses. In the context of the common bacterial toxins hypothesis for the sudden infant death syndrome (SIDS), this could explain the lowered risk of SIDS associated with use of a waterproof cover above the mattress.