Disease-Causing Bacteria Creatively Manipulate Hosts to Increase Chance of Survival

Researchers from the NYU Langone Medical Center in New York have discovered how Streptococcus pneumoniae (S. pneumoniae) which are responsible for the deaths of almost 500,000 children under the age of 5, across the world each year, are able to stealthily move from person to person, and the answer may surprise you.

S. pneumoniae are the culprits behind most pneumonia cases, and they can also cause “ear infections, sinus infections, meningitis, and bacteremia,” according to the Centers for Disease Control and Prevention (CDC). The bacteria “are spread through coughing, sneezing, and close contact with an infected person.”

Those who are most susceptible to the bacteria include individuals who are > 65 years of age, children younger than 2, those who have weakened immune systems, and those have “diabetes, heart disease, lung disease, HIV/AIDS, or people who smoke cigarettes or have asthma.” (Of note: news outlets have recently reported that former President George WH Bush, who is 92, and the former First Lady, Barbara Bush, 91, have been hospitalized with pneumonia.)

In their research study, published in the journal, Cell Host & Microbe, the researchers from Langone detail how S. pneumoniae spreads between hosts and why it utilizes a destructive method to accomplish this.

Through experimentation in mouse models, the researchers found that a host’s nasal secretions are increased by a pore-forming cytotoxin (pneumolysin) produced by S. pneumoniae. This increase in mucus enables cells in the mucous membranes to “expel the bacteria from the body.” The bacteria then travel in the nasal secretions on to the next host. According to the researchers, when they genetically altered S. pneumoniae so that it was unable to produce pneumolysin, the bacteria was unable to spread to the next host.

Speaking on their findings, Jeffrey Weiser, MD, chair of the Department of Microbiology at NYU Langone stated in a press release, "Factors that allow for the host-to-host transmission of disease-causing bacteria have not been thoroughly investigated by the field as a means of prevention. Our findings provide evidence of the tool used by these bacteria to spread, which promises to guide the design of new kinds of countermeasures."

Because healthy hosts do not expel contents from their airways in a regular manner, as they do for the contents of their gut, nor do airways supply the same readily available nutrients as the gut, the researchers state that bacteria need to "get creative" in both finding nutrients and leaving the host (for another). As such, the bacteria create the toxin which essentially “drill[s] holes (pores) into cells to get at the nutrients inside them” and allows the bacteria to survive long enough to infect the host. While the host’s immune system attacks the bacteria, the toxin simultaneously triggers the increase in secretions in which the bacteria can reside, and make their way to a new host, potentially one with a weaker immune system.

Past work has shown that S. pneumoniae “spreads more effectively when a person also has the flu,” where their immune system is weakened. Indeed, in mouse models, it has been shown that the increased secretions created by an influenza viral infection aid S. pneumoniae in overcoming “population constraints that come with remaining in one host.” In this current study, the researchers were able to show, for the first time, pneumococcal transmission in the absence of influenza viral infection.

Because the bacteria depend on their host to survive, experts have questioned why they would give off a toxin that is so destructive. According to the researchers, the benefits to the bacteria that come with increasing transferability outweigh the risks that come with attacking the food source (host).

With this in mind, Dr. Weiser stated, “Our study results argue that toxins made by bacteria are central mediators of transmission between hosts, which makes them attractive as a potential ingredient in vaccines, to which they could be added specifically to block transmission. There are precedents in using disarmed bacterial toxins, or toxoids, as vaccine ingredients, as with existing vaccines against diphtheria, tetanus and pertussis.”