Faecal indicator bacteria on indoor floors linked to exterior sidewalk contamination in New York City

Although sources of faecal pathogens abound in urban environments, faecal bacteria on city streets remain largely unquantified. The extent to which faecal pathogens transfer between outdoor and indoor environments is also poorly understood. We use defined substrate analysis to quantify enterococci and coliform faecal indicator bacteria (FIB) on interior floor surfaces in a New York City institutional building, on shoe soles of building occupants and on exterior sidewalks. Enterococci and faecal coliforms are ubiquitous on interior floors, varying with foot traffic and floor surface type. Mean enterococci counts are highest in the heavily trafficked building entryway compared with less trafficked interior locations, and counts are significantly higher on carpets than on adjacent uncarpeted flooring. Enterococci and faecal coliforms appear on shoe soles of building occupants as well. Susceptibility of shoe sole bacteria to antimicrobial agents is commensurate with that of cultivated E. coli, E. faecalis and S. aureus strains. In pooled rainwater on city sidewalks, enterococci average 31,000 per 100 mL. Overall, our evidence indicates ubiquity of FIB on sidewalks, a translocation pathway via shoe soles and accumulation on indoor floor surfaces, particularly carpeted areas. These findings present ample opportunity for human exposure to faecal pathogens in the indoor environment.