Microbes in Beijing’s Smog

When we think of smog and what’s in it, what probably comes to mind first is the chemical pollutants. But what about the microbes? More and more we’re discovering the importance of the complex web of microbes that are all around – and inside of – us. Which brings us to a study published earlier this week from China – researchers were able to identify 1315 different species of microbes from the dense smog in Beijing. These were mostly harmless bacteria, but several pathogenic and allergenic microbes were found in the smog as well.

While the chemical pollutants in smog have been extensively explored, microbes have not been characterized much before, even though they make up to 25% of smog’s particulate matter (PM). If you want to examine what’s in smog, it’s hard to find a better place than Beijing – in January of 2013, when the researchers took their smog samples, levels were >20-fold higher than the World Heath Organization’s guidelines (specifically, there was >500 micrograms of PM per cubic meter [µg/m³]). This was during a severe smog event that caused widespread respiratory irritation (primarily coughing) and respiratory disease-related cases. Based on the microbial analysis reported in the recent study, some of the microbes in the smog likely contributed to these medical conditions.

The researchers filtered their smog samples so that they could analyze the microbes by size. Specifically, one group consisted of microbes <2.5 micrometers (µm) in diameter, while the larger group was made up of microbes 2.5 µm to 10 µm in diameter. (Generally, when a person breathes in PM, these larger particles get stuck somewhere in their head, while the smaller ones can travel closer to their lungs.) Using DNA sequencing, in both size groups bacteria were found to be the most abundant microbes (81% to 86%), followed by eukaryotes (13% to 18%), archaea (0.8%), and viruses (0.1%). The researchers also correlated their results with the microbes' “normal” habitats – it was found that most of the bacteria and archaea were terrestrial (about 70%), some were fecal (about 15%), fewer naturally lived in freshwater (5% to 10%), and even fewer were marine-dwellers (<5%). But what’s more interesting is the microbe species identified. As mentioned before, this study identified 1315 microbes by species, which is an impressive accomplishment, and is necessary to verify whether the microbes are harmless or pathogenic/allergenic. Fortunately, most of the identified species were found to be harmless, but some potentially harmful ones were identified too. Specifically, in the smog samples Streptococcus pneumoniae (a bacteria that causes pneumonia), Aspergillus fumigatus (a fungal allergen and respiratory pathogen in people who are immunodeficient), and human adenovirus C (which causes respiratory tract infections in children) were all detected. And the proportion of adenovirus C (and fecal-related microbes) was found to increase with increased pollution levels.

It would definitely be interesting to repeat this type of study in other cities to compare the results. How do cities compare that have more average levels of pollution? This study also makes one wonder if the habitats we typically associate these microbes with (e.g., whether they’re terrestrial, fecal, etc.) is truly representative of their global distribution, or if it is just representative of the areas we’ve studied the most. (For example, it may be more common to find some of these “terrestrial” bacteria in aerosols, but because we haven’t studied aerosol habitats nearly as much as terrestrial ones, we’re unaware of this.)

So next time you’re taking a deep breath, you can consider all the biological critters that you may have just sucked down your airways.

For further reading:

• Chen Cao et al.’s article “Inhalable Microorganisms in Beijing’s PM_2.5 and PM₁₀ Pollutants during a Severe Smog Event”
• Jef Akst’s article “Microbial Smog“
• Teisha J. Rowland’s blog post “The Importance of ‘The Little Guys:’ Our Gut Microbiome”
• Teisha J. Rowland’s book Biology Bytes: Digestible Essays on Stem Cells and Modern Medicine