Oxidative Stress Contributes to Bacterial Airborne Loss of Viability

Stability of influenza A virus in droplets and aerosols is heightened by the presence of commensal respiratory bacteria

Aerosol transmission remains a major challenge for control of respiratory viruses, particularly those causing recurrent epidemics, like influenza A virus (IAV). These viruses are rarely expelled alone, but instead are embedded in a consortium of microorganisms that populate the respiratory tract. The impact of microbial communities and inter-pathogen interactions upon stability of transmitted viruses is well-characterized for enteric pathogens, but is under-studied in the respiratory niche. Here, we assessed whether the presence of five different species of commensal respiratory bacteria could influence the persistence of IAV within phosphate-buffered saline and artificial saliva droplets deposited on surfaces at typical indoor air humidity, and within airborne aerosol particles. In droplets, presence of individual species or a mixed bacterial community resulted in 10- to 100-fold more infectious IAV remaining after 1 h, due to bacterial-mediated flattening of drying droplets and early efflorescence. Even when no efflorescence occurred at high humidity or the bacteria-induced changes in droplet morphology were abolished by aerosolization instead of deposition on a well plate, the bacteria remained protective. Staphylococcus aureus and Streptococcus pneumoniae were the most stabilizing compared to other commensals at equivalent density, indicating the composition of an individual's respiratory microbiota is a previously unconsidered factor influencing expelled virus persistence.IMPORTANCEIt is known that respiratory infections such as coronavirus disease 2019 and influenza are transmitted by release of virus-containing aerosols and larger droplets by an infected host. The survival time of viruses expelled into the environment can vary depending on temperature, room air humidity, UV exposure, air composition, and suspending fluid. However, few studies consider the fact that respiratory viruses are not alone in the respiratory tract-we are constantly colonized by a plethora of bacteria in our noses, mouth, and lower respiratory system. In the gut, enteric viruses are known to be stabilized against inactivation and environmental decay by gut bacteria. Despite the presence of a similarly complex bacterial microbiota in the respiratory tract, few studies have investigated whether viral stabilization could occur in this niche. Here, we address this question by investigating influenza A virus stabilization by a range of commensal bacteria in systems representing respiratory aerosols and droplets.

Adenosine Monophosphate as a Metabolic Adjuvant Enhances Antibiotic Efficacy against Drug-Resistant Bacterial Pathogens

Global bacterial infections are on the rise, and drug resistance to bacteria is gradually rendering existing antibiotics ineffective. Therefore, the discovery of new strategies is urgently needed. Cellular metabolism is a key factor in the regulation of bacterial drug resistance, which cannot be separated from the utilization of energetic substances, suggesting that energetic substances may be associated with bacterial drug resistance. In this study, we found that adenosine monophosphate (AMP) can enhance the bactericidal effect of gentamicin against gentamicin-resistant Staphylococcus aureus. This synergistic effect can be generalized for use with different antibiotics and Gram-positive or Gram-negative bacteria. We also validated that the mechanism of AMP reversal of antibiotic resistance involves enhancing the proton motive force via the tricarboxylic acid cycle to increase antibiotic uptake. Simultaneously, AMP increases oxidative stress-induced cell death. This study presents a strategy for adopting low-dose antibiotics to control drug-resistant bacteria, which is important for future drug development and bacterial control.

Ambient carbon dioxide concentration correlates with SARS-CoV-2 aerostability and infection risk

An improved understanding of the underlying physicochemical properties of respiratory aerosol that influence viral infectivity may open new avenues to mitigate the transmission of respiratory diseases such as COVID-19. Previous studies have shown that an increase in the pH of respiratory aerosols following generation due to changes in the gas-particle partitioning of pH buffering bicarbonate ions and carbon dioxide is a significant factor in reducing SARS-CoV-2 infectivity. We show here that a significant increase in SARS-CoV-2 aerostability results from a moderate increase in the atmospheric carbon dioxide concentration (e.g. 800 ppm), an effect that is more marked than that observed for changes in relative humidity. We model the likelihood of COVID-19 transmission on the ambient concentration of CO2, concluding that even this moderate increase in CO2 concentration results in a significant increase in overall risk. These observations confirm the critical importance of ventilation and maintaining low CO2 concentrations in indoor environments for mitigating disease transmission. Moreover, the correlation of increased CO2 concentration with viral aerostability need to be better understood when considering the consequences of increases in ambient CO2 levels in our atmosphere.

Airborne survival and stress response in Listeria monocytogenes across different growth temperatures

In the food industry, ensuring food safety during transportation and storage is vital, with temperature regulation preventing spoilage. However, airborne contamination through foodborne pathogens remains a concern. Listeria monocytogenes, a psychrotolerant foodborne pathogen, has been linked to various foodborne outbreaks. Therefore, understanding how its airborne characteristics depend on the growth temperature is imperative. As a result, when the L. monocytogenes was floated in air for 30 and 60 min, the surviving population of 15 °C-grown L. monocytogenes that was suspended in air and attached on the surface was significantly higher than L. monocytogenes grown at 25°C and 37 °C. The fatty acid analysis revealed a significantly higher proportion of shorter chain fatty acids in L. monocytogenes grown at 15 °C compared to those grown at 37 °C. Under aerosolization, L. monocytogenes encountered osmotic and cold stresses regardless of their growth temperature. Transcriptomic analysis showed that stress response related genes, such as oxidative and cold stress response, as well as PTS system related genes were upregulated at 15 °C, resulting in the enhanced resistance to various stresses during aerosolization. These results provide insights into the different responses of aerosolized L. monocytogenes according to the different growth temperatures, highlighting a critical factor in preventing airborne cross-contamination.

Dependence on relative humidity in the formation of reactive oxygen species in water droplets

Water microdroplets (7 to 11 µm average diameter, depending on flow rate) are sprayed in a closed chamber at ambient temperature, whose relative humidity (RH) is controlled. The resulting concentration of ROS (reactive oxygen species) formed in the microdroplets, measured by the amount of hydrogen peroxide (H2O2), is determined by nuclear magnetic resonance (NMR) and by spectrofluorimetric assays after the droplets are collected. The results are found to agree closely with one another. In addition, hydrated hydroxyl radical cations (•OH-H3O+) are recorded from the droplets using mass spectrometry and superoxide radical anions (•O2-) and hydroxyl radicals (•OH) by electron paramagnetic resonance spectroscopy. As the RH varies from 15 to 95%, the concentration of H2O2 shows a marked rise by a factor of about 3.5 in going from 15 to 50%, then levels off. By replacing the H2O of the sprayed water with deuterium oxide (D2O) but keeping the gas surrounding droplets with H2O, mass spectrometric analysis of the hydrated hydroxyl radical cations demonstrates that the water in the air plays a dominant role in producing H2O2 and other ROS, which accounts for the variation with RH. As RH increases, the droplet evaporation rate decreases. These two facts help us understand why viruses in droplets both survive better at low RH values, as found in indoor air in the wintertime, and are disinfected more effectively at higher RH values, as found in indoor air in the summertime, thus explaining the recognized seasonality of airborne viral infections.

An assessment of the airborne longevity of group A Streptococcus

Group A streptococcus (GAS) infections result in more than 500 000 deaths annually. Despite mounting evidence for airborne transmission of GAS, little is known about its stability in aerosol. Measurements of GAS airborne stability were carried out using the Controlled Electrodynamic Levitation and Extraction of Bioaerosols onto a Substrate (CELEBS) instrument. CELEBS measurements with two different isolates of GAS suggest that it is aerostable, with approximately 70 % of bacteria remaining viable after 20 min of levitation at 50 % relative humidity (RH), with lower survival as RH was reduced. GAS airborne viability loss was driven primarily by desiccation and efflorescence (i.e. salt crystallization), with high pH also potentially playing a role, given reduced survival in bicarbonate containing droplet compositions. At low enough RH for efflorescence to occur, a greater proportion of organic components in the droplet appeared to protect the bacteria from efflorescence. These first insights into the aerosol stability of GAS indicate that airborne transmission of these respiratory tract bacteria may occur, and that both the composition of the droplet containing the bacteria, and the RH of the air affect the duration of bacterial survival in this environment. Future studies will explore a broader range of droplet and air compositions and include a larger selection of GAS strains.

Observations on phenomenological changes in Klebsiella Pneumoniae under fluidic stresses

In the present work, experiments are conducted to understand the consequence of stresses generated by flowing fluid on the bacterial morphology and virulence in microfluidic channels. We consider Klebsiella pneumoniae (KP, a clinical isolate), an ESKAPE pathogen, to be the model bacteria responsible for blood stream infections, bacteremia, including pneumonia, urinary tract infections and more. Four different stress conditions are generated by changing the flow rate and channel geometry subsequently altering the shear rate and stressing time (τ). We observe significant changes in the structural aspects of the stressed bacteria. With an increase in stressing parameters, the viability of the bacterial sample deteriorated. Most importantly, these stressed samples proliferate much more than unstressed samples inside the RAW264.7 murine macrophages. The results shed light on the complex relationship between flow stresses and bacterial virulence. Furthermore, the bacterial samples are challenged with ciprofloxacin to see how they behave under different stress conditions. The observations presented in the present study can be extended to model deadly diseases including bacteremia using organ-on-a-chip technology and to understand bacterial pathogenicity under realistic environments.