The French biochemist Jacques Monod (1910-1976), who won a Nobel Prize in 1965 for discovering how cells regulate the expression of genes, famously said, “What is true for Escherichia coli is true for the elephant.”
He meant the same fundamental mechanisms of molecular biology, such as protein synthesis, DNA replication, and cellular metabolism, are shared by both bacteria and elephants.
In his comment, Monod was taking a bird’s-eye view of biology. If you zoomed in enough, two bacterial cells with identical genomes — and even from the same colony — can be quite different from each other. One cell might express a particular gene at a high level while the other might express it at a low level or not at all. Each bacterium goes on to transmit its expression level to its offspring in a process called epigenetic inheritance. Such variations among otherwise identical cells and organisms is called bistability.
Bistability might be a survival strategy that single-celled organisms use to adapt to fluctuating environmental conditions.
Now, a team of researchers, led by scientists from the Helmholtz Centre for Infection Research in Germany, has reported that a gene in the bacterium Pseudomonas aeruginosa shows bistable expression.
This is important because P. aeruginosa is an opportunistic and deadly pathogen. Most burn victims succumb to secondary P. aeruginosa infections. It is a major cause of keratitis, an eye infection, and also infects urinary catheters. Hospital infections of P. aeruginosa are notoriously resistant to antibiotics. Medical researchers and doctors are keen to understand what makes the bacterium so deadly — and the new study opens a door towards this goal.
The researchers have reported that the differential expression of the glpD gene in individual cells of P. aeruginosa was associated with differences in the bacterium’s ability to cause disease, i.e. its pathogenicity properties. The findings were reported in Proceedings of the National Academy of Sciences.
Tracking gene expression
The genome of P. aeruginosa contains nearly 6,000 genes. When a gene is expressed, the cell makes corresponding RNA. These RNA serve as transcripts. Think of them as the gene’s working copies. The cell loads these RNA onto protein-making factories called ribosomes. There the information from the RNA is used to stitch amino acids together in different sequences to make the protein corresponding to that gene.
Many genes produce relatively few transcripts. Let’s call them low expression genes (LEGs). The high expression genes (HEGs) on the other hand produce many more transcripts — in some instances 100x to 10,000x more.
In general, there is less variation in transcript numbers when an HEG is expressed than when an LEG is expressed. While an LEG in one cell might express one transcript, in another cell it might express 30, so this is a range of 30x. But for HEGs, a 30x range is highly unlikely.
In the new study, the researchers analysed over 300 P. aeruginosa strains for all the transcripts that they made from their genes. They found that although the glpD gene was an HEG, the variation in its transcript counts P. aeruginosa cells made was way more than for the other HEGs. The finding hinted that the glpD gene might be an HEG in some cells and an LEG in others.
To explore this possibility, the team fused the DNA segment that regulated the expression of the glpD gene with a gene that makes the green fluorescent protein (GFP). Whenever the glpD gene was expressed, the cell would also make the GFP protein, which glows under light. This way, the researchers had a small, green light-bulb that lit up when the glpD gene was expressed. The team also used a more short-lived variant of GFP to make the signal more accurately reflect the gene’s real-time expression.
The fickle gene
Only a small fraction of cells with the fusion gene lit up and maintained their ‘on’ state over an extended period of time. More interestingly, when a cell that had already lit up divided into two, one of the daughter cells also lit up but the other didn’t. In some lineages, the team found that the ‘on’ ability was passed down to up to five generations.
In lineages that descended from a single cell that didn’t light up, i.e. was ‘off’, about 5-6% of the daughter cells randomly switched to ‘on’. And in half of these cases, the ‘on’ state was short-lived. In the other half, it persisted as if the cell had descended from an ‘on’ ancestor.
Finally, more cells were ‘on’ when glycerol was present. This was expected since P. aeruginosa bacteria use the glpD gene to make use of glycerol.
The moth larva test
The larvae of the greater wax moth (Galleria mellonella) are parasites of honeycombs. Scientists use the larvae as a model to study how bacterial infections develop and spread. In the new study as well, the researchers infected greater wax moth larvae with P. aeruginosa bacteria — but before they did, they removed the glpD gene first. The researchers found that these P. aeruginosa bacteria had a significantly reduced ability to kill the larvae compared to bacteria containing the intact gene.
This was a sign that the expression of the glpD gene was associated with the bacteria’s ability to infect.
The researchers hypothesised that the interaction of P. aeruginosa with the cells in the bodies of the mammals (including humans) it infects might also be associated with higher glpD expression. To test this, the team cultured mouse immune cells and infected them with P. aeruginosa bacteria containing the GFP fusion gene. Team members found that the bacteria that came in contact with macrophage cells of the mouse immune system fluoresced more brightly, meaning glpD was being expressed more.
Playing it safe
The variability in the expression levels of the glpD expression was evident even in small clusters of around 10-50 cells. Given that cells that produced more glpD transcripts were better at setting infections in motion suggested that an infection can be initiated by even a small group.
As the researchers wrote in their paper, “varying levels of glpD expression … might be a strategy … for the success of P. aeruginosa as an opportunistic pathogen”.
Axiomatically, a drug targeting this variation could prevent P. aeruginosa from being the scourge it currently is in hospitals.
D.P. Kasbekar is a retired scientist.
Published – August 21, 2025 05:30 am IST