Bacteria the size of viruses

Early in the spring of 2020, SARS CoV-2, the infectious agent responsible for the novel Coronavirus (COVID-19) spread across the globe. In an instant, the global conversation shifted to a small virus undetectable to the naked eye. While the spread of viruses has brought discussions about the dispersal and filtering of small 'invisible' microbes to the forefront of community discussion, the concept of filtering and collecting microbial biomass has long been an important and valuable topic with industrial and biotechnical applications.

Figure 1. Images of small bacteria and viruses identified from groundwater at the contaminated Y-12 site in Oak Ridge, TN (photo credit: Putt, et al., 2019)

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The microbial size-fraction is seemingly out of sight and out of mind until we get sick, eat spoiled food, or see bacterial colonies growing before our eyes. These organisms are fractions of millimeters in diameter with viruses approximately 20-400 nanometers in diameter and archaea measured across a larger range from 10-1500 nanometers. Bacteria are typically larger ranging from 200-2000 nanometers, but advancements in the study and collection of DNA and high power imaging equipment like transmission electron microscopes allow for the investigations and discoveries of smaller and smaller bacteria. Small bacteria similar in size to viruses, like those shown in the transmission electron micrograph in Figure 1 are estimated to make up nearly 90% of the bacterial community in some aquatic systems (Proctor et al., 2018), and have even been identified in highly contaminated groundwater (Tian et al., 2020). Therefore, the investigation of these small bacteria goes beyond a general interest in their novelty, and instead asks what is their function, why are they so prolific, and what applications do we have for small bacteria? Investigations into small bacteria have been a focus of potential biotechnical applications for several decades (Lappin-Scott & Costerton, 1992), but recently there has been a growing focus on their active participation as a silent partner in carbon and nutrient cycling (Castelle et al., 2018;Tian et al., 2020). The small size of these bacteria not only makes them able to pass through small filter pores, it may allow for them to be preferentially mobilized and dispersed by flow and dispersion forces rather than by active chemotaxis (Herrmann et al., 2019; Tian et al., 2020). As we learn more about this group of small bacteria, it becomes increasingly clear that capturing, cultivating, and studying these small bacteria may be a thread by which we can improve our understanding of global nutrient cycling and better understand the complexity of natural ecosystems.

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