In casual conversations, usually when speaking with friends about bacterial diversity, I mention that only a tiny minority of the Earth's bacteria cause human disease. In that context, I used to mention that the number of such pathogens was small, a few dozen, perhaps around 100. In using such "off the cuff" estimates I was not alone. But, oh my, was I off, way off! A paper from 2022, by Bartlett et al. entitled "A comprehensive list of bacterial pathogens infecting humans" puts the number at 1513 and growing rapidly, as can be gleaned from the figure.
Are bacteria rapidly evolving pathogenicity? Should this be cause for concern? Not really. Inspection of the paper quickly provides numerous explanations for the large and growing number of bacteria infecting humans. For one, molecular tools for bacterial identification are now widely used in clinical laboratories, leading to the identification of many new bacterial species present at infected sites. Then there is the authors' definition of a pathogen: "We designate a bacterial species as pathogenic to humans if it has been isolated from a human either at the site of symptomatic infection or in association with a toxin-mediated illness acting at another site." Importantly, they distinguish between "putative" pathogens if there are fewer than three known cases and "established" pathogens if there are three or more known cases. Three! In short, even an extremely low frequency of infection still confers a bacterium the label pathogen. OK. But a cursory glance at the list of 1513 identified bacterial species reveals the surprising presence of familiar bacteria that are consumed by humans, for example Lactobacillus acidophilus (yogurt) and Bacillus subtilis (natto). Also included is Cutibacterium acnes, which despite its name is not clearly defined as the etiological agent of acne, as Christoph pointed out last Monday. These are just a few of the surprising pathogens on the list.
So yes, there are 1513 named bacterial species that have been associated with human infection. This is indeed an important list to have. But I suspect that if we were able to produce a histogram of the number of infected humans and plot those 1513 species in descending order, more than 90% of infections would be the result of a much fewer number (perhaps around 100?), with the rest providing a very long tail of bacterial species that cause very low numbers of infections. I may be wrong here again; a far as I know, there is no such histogram though it would be an interesting analysis.
The very high number of bacterial species in this list, with many of them not usually associated with human infections, brings up the question of what is the meaning of pathogen? In the end, I side with the concept put forth by Arturo Casadevall and Liise-anne Pirofski, in their article with the provocative title: "Microbiology: Ditch the term pathogen." While they admit that "the term pathogen is unlikely to go away," they remind us that disease is always the result of microbe-host interactions and that context is everything. "Disease is as much about the host as it is about the microbe." In short, unlike "a rose is a rose is a rose," a pathogen can be a pathogen but need not always be a pathogen.