Density matters when bacteria are in a league of their own

Legionnaires’ disease is a severe form of pneumonia, caused by the bacterium Legionella pneumophila (Lp). Although people infected with the bacterium can be easily treated with antibiotics, certain populations can be more vulnerable to the disease. Lp is naturally found in freshwater lakes and streams and rarely infects people. However, in man-made water-supply systems, it can grow and become hazardous if water is not properly maintained and small droplets of water containing the bacteria get into the air and are inhaled.

The bacterium has a complex life cycle. In one stage, it grows inside a water-borne amoeba. In another, it grows within slimy, complex layers made up of many different bacterial species called biofilms, which can form in water fountains and other water-handling systems. Within the biofilm, each species of bacteria tries to outgrow its neighbor, and success depends on a delicate balance of cooperation and competition between them.

As such, inter-bacterial conflict is ubiquitous within such competitive microenvironments. Although other research groups have shown evidence that Lp could antagonize the growth of neighboring bacteria on the same plate, it was unknown if surfactant played a direct or indirect role in bacteria inhibition. Using unbiased genetic approaches, Dr. Tera Levin, a postdoctoral research fellow, along with Brian Goldspiel, a former technician in Dr. Harmit Malik’s laboratory in the Basic Sciences division, identified the molecule produced by Lp that inhibited the growth of neighboring Legionella spp. Their work was published in a recent issue of the journal eLife.

Model for HGA activity in extracellular environments. Lp (orange) colonizes a surface and grows to form a microcolony. Once cells are at high density, they secrete abundant HGA (yellow). Through unknown mechanisms, high-density Lp are resistant to HGA’s effects (black outline), while low-density Lp or other Legionella species (blue) are inhibited by HGA and cannot invade the microcolony’s niche. Figure from publication

The authors found that the killer molecule was homogentisic acid (HGA), an intermediate in the catabolism of aromatic amino acids such as phenylalanine and tyrosine. The activity of HGA was dependent on its redox state: HGA was found to be only toxic under aerobic conditions.

Intriguingly, the authors also discovered that Lp itself is sensitive to HGA produced by neighboring, genetically identical Legionella cells. Furthermore, they also found that Lp cells are resistant to HGA under dense growth conditions. By this mechanism, Lp can avoid self-inhibition by producing and secreting HGA only during conditions when it is not susceptible to HGA. Of note, this study is the first to show that HGA has antimicrobial activity.

Given these dynamics, HGA may be deployed as a bacterial niche-protective strategy. “Our next steps are to pursue the unusual density-dependent susceptibility and resistance of L. pneumophila to HGA. Since we showed that the only known quorum sensing pathway in Legionella is not involved in susceptibility or resistance, I'm curious if this could be a useful handle to discover a new, unknown quorum sensing-like mode of regulation,” said Dr. Levin.

Levin TC, Goldspiel BP, Malik HS. 2019. Density-dependent resistance protects Legionella pneumophila from its own antimicrobial metabolite, HGA. Elife, May 28; 8. pii: 346086 [Epub ahead of print]

This work was supported by the National Institutes of Health and the Howard Hughes Medical Institute.