Biological and comparative genomic characterization of Staphylococcus aureus phage SJP49-2 and its effects on biofilm biomass

In the world of food safety, a persistent enemy hides in plain sight: Staphylococcus aureus. This bacterium is a common traveler on meat and dairy products, capable of causing illness not just by multiplying, but by releasing toxins that remain dangerous even after the bacteria themselves are killed. The problem is compounded when these bacteria form biofilms, which are slimy, protective communities that cling to surfaces like stainless steel or plastic in processing plants. These communities act like a fortress, shielding the bacteria from standard cleaning chemicals and making them incredibly difficult to remove. Because these bacteria can survive in such tough conditions, scientists have been looking for a more precise way to fight them, turning their attention to nature's own predators: viruses that infect bacteria, known as bacteriophages. These viruses are highly specific, hunting down only their bacterial targets while leaving other cells unharmed, offering a potential biological tool to clean up food production environments without the harsh chemicals that sometimes fail against these resilient biofilms.

Researchers in Xinjiang, China, recently set out to test a specific virus they had isolated, named SJP49-2, to see if it could effectively hunt down Staphylococcus aureus and dismantle these protective biofilms. They began by examining the virus under a microscope, revealing a structure typical of its kind: a geometric head containing its genetic material and a long, flexible tail used to attach to bacteria. To test its hunting range, the team exposed the virus to a collection of 92 different strains of the bacteria. The virus proved to be a skilled hunter, successfully attacking and destroying 64 of those strains, a success rate of nearly 70 percent. This broad ability to infect different versions of the bacteria suggested it could be a useful tool for dealing with the varied strains found in real-world food settings.

The team then put the virus through a series of stress tests to see how well it could survive the conditions it might encounter in a food processing plant. They exposed it to a wide range of temperatures and acidity levels. The virus held up well in neutral environments and at room temperature, but it struggled significantly when heated above 50 degrees Celsius, losing most of its ability to infect bacteria after an hour at higher temperatures. This finding is crucial for food safety applications, as it suggests the virus is best suited for use in cooler environments or as a final step after heat treatment, rather than as a tool to be added during cooking. It also showed that the virus could survive in a wide range of pH levels, remaining active in environments that are slightly acidic to slightly alkaline, which covers many conditions found in food processing.

Next, the researchers investigated how the virus performed against the bacteria in two different states: floating freely in liquid and locked inside a biofilm. When the bacteria were floating, the virus stopped them from multiplying effectively, especially when introduced in high numbers. The real test, however, came with the biofilms. The team first tried to prevent the biofilms from forming by adding the virus at the start. In these cases, the virus reduced the amount of slimy biomass that built up by nearly 80 percent compared to untreated samples. They also tested whether the virus could break down biofilms that had already formed and hardened. This was even more successful; when the virus was applied to established biofilms, it reduced the remaining biomass by nearly 95 percent. This indicates that the virus is not just good at stopping the bacteria from building a fortress, but is also capable of dismantling one that is already in place.

To see if these results held up in a real food setting, the researchers placed the bacteria on sterile cubes of beef and treated them with the virus. They observed the bacterial counts over eight hours. In the untreated beef, the bacteria multiplied rapidly. In the beef treated with the virus, the number of bacteria was significantly lower at the end of the experiment, and the rate at which they grew was much slower. The most effective treatment, where the virus was introduced in the highest ratio to the bacteria, kept the bacterial count more than two log units lower than the control group. This means the virus successfully limited the growth of the bacteria on the meat surface, preventing them from reaching the high numbers seen in the untreated samples.

Finally, the team looked inside the virus to understand its genetic makeup. They sequenced its entire genome, which is a relatively small circle of DNA containing 69 genes. By comparing this genetic code to that of 19 other similar viruses, they found that while the virus shares a core set of genes with its relatives, it also carries a unique combination of other genes that make it distinct. The genetic analysis suggested that this virus has the potential to enter a dormant state inside the bacteria, a trait known as temperance, rather than immediately killing the host. However, despite this genetic potential, the virus demonstrated strong antibacterial activity in their experiments. The researchers also scanned the virus's genes for any known danger signals, such as toxins that could harm humans, and found no evidence of such threats in the standard databases they checked, though they noted that some gene names in the data were inconsistent and required careful interpretation.

The study concludes that SJP49-2 is a promising candidate for controlling Staphylococcus aureus in food-related environments. It effectively kills the bacteria, prevents them from forming protective slime, and can break down existing slime layers, all while showing a safety profile that does not include known human toxins. While the virus is sensitive to high heat, its ability to work in a beef matrix and its broad range of activity against different bacterial strains suggest it could be a valuable addition to the toolkit used to keep food safe. The researchers emphasize that their work provides a solid foundation for further study, particularly in understanding how this virus interacts with its host and how its unique genetic features might be harnessed for better food safety strategies.