How phages spread Salmonella virulence genes

A global study uncovers the hidden role of viruses in driving bacterial evolution and highlights a surprising bacterial gene that helps fight back.

A recent study co-authored by the Catalan Institute for Water Research (ICRA-CERCA), together with researchers from China and Finland, and published in the journal iMeta, reveals how viruses known as bacteriophages, or simply “phages,” are quietly reshaping the genetic landscape of Salmonella enterica, a major cause of foodborne illness worldwide.

By transporting and spreading virulence genes across ecosystems and continents, these viral vectors may be enhancing the pathogen’s ability to infect, persist, and evolve. At the same time, the study uncovers a powerful defense mechanism within Salmonella itself, offering fresh hope for the development of phage-safe therapeutic strategies.

This collaborative research analyzed more than 466,000 Salmonella enterica genomes and over 5,000 viral sequences from global databases. The findings show that phages act as global shuttles for genes that enhance bacterial virulence, including fliC, iacP, mgtB and misL. These genes are involved in bacterial motility, colonization, and host invasion.

“Phages are not just passive passengers in microbial ecosystems,” says Dr. Mingming Sun, senior author of the study and professor at Nanjing Agricultural University. “They are active players in bacterial evolution, and in this case, they are helping to spread genes that can make Salmonella more dangerous.”

The research team used phylogenetic analysis to uncover strong genetic similarities between virulence genes found in phages and those present in Salmonella, offering clear evidence of horizontal gene transfer. These exchanges are not merely theoretical. Many phages carry genetic elements that, once inside a bacterial host, can be transcribed and translated using the host’s cellular machinery, raising concerns about their role in amplifying bacterial virulence.

Perhaps the most unexpected finding comes from within Salmonella itself. The researchers identified a regulatory gene, csrA, that functions as a natural defense mechanism. When overexpressed, csrA interferes with the phage life cycle by enhancing the expression of the cI repressor gene. This reduces the activation and release of prophages, thereby limiting the spread of virulence genes across bacterial populations.

“We were surprised by how effective csrA is at suppressing prophage activity,” says Dr. Jose L. Balcazar, co-author of the study and senior researcher at ICRA-CERCA in Spain. “It’s as if the bacteria have evolved an internal brake to protect themselves from becoming more virulent.”

Implications for public health and food safety

The study carries significant implications for global public health and food safety. As phage therapy is being explored as an alternative to antibiotics, ensuring that therapeutic phages do not carry virulence genes is becoming critically important. The researchers emphasize the need to screen rigorously or engineer phages to minimize the risk of horizontal gene transfer.

In parallel, understanding the role of bacterial regulatory genes such as csrA could open new avenues for controlling the spread of virulence traits. This knowledge may support the development of microbial strategies to limit pathogen evolution and transmission in environments such as food production systems.