Antibiotics Trigger Protein Sharing Among Bacteria, Aiding Persister Cells
Antibiotics Trigger Protein Sharing Among Bacteria, Aiding Persister Cells
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New research headed by scientists at Baylor College of Medicine suggests that when bacteria are under antibiotic attack, it is not “every man for himself.” The team developed a genetic system in Escherichia coli to track how the cells transferred proteins between them. The results indicated that bacterial populations work as a team to survive antibiotics, pooling their resources and helping quiescent or dormant cells survive. Using different techniques, including high-resolution imaging, the team found that antibiotic treatment induced the transfer of proteins between different E. coli strains, and between E. coli and other species of bacteria.
They discovered that antibiotics stimulate bacteria to differentiate into groups of what they describe as vesicle-producing, and protein-receiving cells, and that antibiotic “persisters” with reduced protein synthesis acquire proteins released by their neighbors. The discoveries may help to explain why some bacteria are hard to eliminate, and also point to potential future approaches to improve antibiotic effectiveness.
“Antibiotics are designed to kill bacteria or stop them from growing,” said Christophe Herman, PhD, professor of molecular and human genetics and of molecular virology and microbiology at Baylor. “Yet many times, antibiotics leave behind a small group of survivors. These survivors are not genetically resistant; instead, they temporarily shut down certain parts of their metabolism, entering a dormant-like state that allows them to endure treatment and later regrow. Understanding how survivors form and remain is a major challenge in fighting persistent infections.”
Herman is senior and co-corresponding author of the team’s published paper in Science,” (“Antibiotics stimulate protein transfer to persister cells,”) in which the team further explained, “Protein uptake enhanced the antibiotic persistence of recipient cells, revealing that vesicle exchange promotes bacterial survival during antibiotic treatment.”
Scientists have long known that bacteria can help each other resist antibiotics by sharing genes that provide antibiotic resistance. But as the authors pointed out, “Whereas horizontal gene transfer is known to spread antibiotic resistance genes, far less is understood about the mechanisms and effects of horizontal protein transfer.”
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Antibiotic treatment stimulates vesicle production, so for their current study, Herman and colleagues investigated whether bacteria could also directly share proteins. Previous studies had indicated that bacteria can share proteins, but the experimental evidence was not clear. “To directly measure horizontal transfer, we constructed a genetic system in Escherichia coli consisting of a donor and a recipient strain.”
First author Alice X. Wen, a Baylor McNair Scholar in the Medical Scientist Training Program (MD/PhD), working in the Herman lab, further explained, “To detect protein transfer, we designed a sensitive system using the bacterium Escherichia coli. We engineered one group of bacteria (donors) to make a special enzyme called Cre, and another group of the same bacteria (recipients) to contain a genetic ‘switch’ that could only flip if Cre protein entered the recipient.”
Using this system, investigators discovered that when donor and recipient bacteria were grown together, protein transfer occurred but was rare under normal conditions. In contrast, when the bacteria were exposed to low, non-lethal levels of antibiotics, protein transfer increased by thousands of times. “We then investigated how proteins were moving from one cell to another,” Wen said. “We found that the transfer still occurred when donor cells were removed, leaving behind only the liquid in which they had grown. This ruled out direct cell-to-cell contact and pointed to something released into the environment.”
By combining biochemical techniques and advanced microscopy, the team discovered that the proteins were transported by tiny membrane vesicles. These structures, which look like tiny bubbles, are made of bacterial membrane that pinch off from cells and float freely. “Bacterial membrane vesicles, which contain proteins, have been proposed as mediators of horizontal protein transfer,” they pointed out. “Additionally, antibiotic treatment stimulates vesicle production.”
Looking closer at their experimental system, the team found that the recipient cells showed strong signs of dormancy—these cells slowed down protein production, reduced their metabolism, and activated genes associated with persistence, such as HipA. “Recipient cells with high HipA activity were more likely to take up protein-carrying vesicles and survive antibiotic treatment,” Wen said. “When HipA was removed, both protein uptake and survival dropped.”
Protein transfer also helped dormant bacteria survive exposure to lethal antibiotic doses after vesicle transfer; that is, exposing cells to an increased concentration of vesicles before antibiotic treatment led to increased survival. “Protein uptake enhanced the antibiotic persistence of recipient cells, revealing that vesicle exchange promotes bacterial survival during antibiotic treatment,” the authors stated. The results suggested that transferred proteins helped dormant cells endure stress while their own protein production was shut down. “Uptake of key proteins, such as ribosomal components, metabolic enzymes, or DNA repair factors, from active neighbors may help persisters endure proteome-damaging stress despite reduced protein synthesis.”
Herman said, “Our study shows that antibiotics cause a genetically identical group of bacteria to differentiate into two distinct groups: donor cells that respond by releasing protein-filled vesicles, and recipient cells that become dormant but capable of taking up proteins from incoming vesicles, which helps them survive,” Herman said. “This teamwork allows vulnerable members of a bacterial population to persist in the face of a potentially deadly antibiotic attack.”
The researchers are interested in identifying the proteins in vesicles that contribute to recipient persistence. Understanding donor-recipient interactions among bacteria opens new doors in the fight against chronic and persistent infections. In conclusion, the authors stated that their work “… reveals that antibiotics stimulate the differentiation of bacteria into distinct groups of vesicle-producing and protein-receiving cells, which allows antibiotic persisters with decreased protein synthesis to acquire proteins secreted from active neighbors. New strategies to eliminate persisters could be developed by inhibiting or hijacking horizontal protein transfer.”
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