Microbial cross-transmission and particle modelling in building drainage systems

In the event of a defect in the Building Drainage System (BDS), a cross-transmission routeexists from one part of a building to any other part via drainage airflows. A series ofinvestigations establish the conditions necessary for this route to be effective. A 2 storeydrainage stack was constructed with an inlet on floor 1 and a chamber with an extract fan,containing a removable WC, on floor 2. Airflow analysis and smoke visualisation tests haveshown that airflows can traverse a system, and that cross-transmission of airflows occur.Laboratory investigations recorded the extent to which microbes are transported on theairflows found in building drains. Overnight, liquid cultures of the wild-type bacterial strainPseudomonas putida KT2440 were prepared and diluted to between 106-109 colony formingunits (cfu) per ml-1. Six litres of diluted culture was introduced into the test rig to simulate asingle WC flush for each experiment. Results showed airflow and velocities consistent withthose experienced in real systems. Post- experimental sampling resulted in growth of a singlemorphotype on Pseudomonas isolation agar (PIA) which matched the appearance of theintroduced P. putida strain. Use of tryptone soya agar (TSA) and PIA contact plates in thechamber and on inner toilet surfaces also provided supporting evidence of the transmission(and deposition) of the introduced bacterium throughout the drainage system withPseudomonas specific colonies developing only in post experimental samples. Finally, inexperiments including a partial toilet trap, colonies of the inoculum were successfullyrecovered from the pre-sterilized toilet trap solution. The results confirm that thehypothesized cross-transmission route exists and that it presents a credible risk to publichealth.