Microbial burden in public transport: Detection and prevention of bacterial contaminants based on an in vitro bacterial community

Abstract

Passenger cabins of public transport represent environments in which humans reside for short or long periods. Together with them, microorganisms they carry shape the public transport microbiome. Among the microbiome, mostly human-associated microbes exist, that are unharmful or opportunistic for healthy humans. However, particularly vulnerable groups of the population with weakened immune system are at higher risk for severe infections. The confined and, especially during rush hours, crowded space poses an environment that facilitates microbial transmission. Measures that reduce the spread of potential pathogens should be addressed to interfere with airborne and surface transmission, the two main pathways of microbial transmission in such spaces. To contribute to the research of microbial burden in public transport, this thesis addressed approaches to detect and reduce bacterial contaminants. For that, the train cabin microbiome was monitored and investigated by regular sampling through one year. Based on present data, most abundant and relevant genera of the public transport microbiome were identified to develop an in vitro bacterial community. It includes surrogates of high abundant or healthcare relevant genera, namely Pseudomonas, Staphylococcus, Streptococcus, Propionibacterium, Corynebacterium, Sphingomonas, Flavobacterium, Burkholderia and Enterococcus. All nine species were biosafety level 1 organisms and were the main tool to test microbial interventions in this thesis. The aim was to create a reference bacterial community, that can more realistically represent effects of potential microbial interventions compared to single model organisms, that are mostly used in research. After a first characterization of the in vitro bacterial community, two main interventions were investigated: (I) Antibacterial surface coatings based on copper and copper-aluminum alloys, and (II) Reduced bioaerosol propagation through an air curtain. As a third microbial countermeasure, thermal inactivation of members of the bacterial community was investigated, with the perspective of reducing microbial burden in the passenger cabin air. Characterization of the developed public transport in vitro bacterial community showed that the DNA remained largely intact after various stress treatments. The bacterial community was further used for the detection of bioaerosols, using both particle-based measurements and assessments of bacterial survival. This enabled the identification of a maximum air curtain effectivity of 66 % in reducing particle and bacterial concentrations. The importance of using not only single species for investigations was highlighted by the observed protective effect of the in vitro bacterial community during the experiments on antibacterial surfaces. The novel surfaces with a copper content of 79 at.% and 100 at.% demonstrated their antibacterial potential after wet contact killing. Thermal inactivation of members of the in vitro bacterial community and a bacteriophage also proved to be an effective microbial countermeasure, with all of the tested microorganisms exhibiting no detectable survival after exposure to 85 °C for 2 s. Overall, this thesis highlights the importance of incorporating a broader range of test organisms to investigate their detection and strategies to reduce bacterial contaminations. The in vitro bacterial community was successfully established and applied to multiple research questions, demonstrating its potential as a reference for studies on bacterial detection and microbial intervention.