New York, NY—For decades, microbiologists have debated how many bacteria are needed to fight off an infection. Some researchers believed that a large number of bacteria were necessary to overwhelm the invader, while others argued that a small number of highly specialized bacteria could do the job.

Now, a team of scientists at The Rockefeller University has resolved this debate, showing that the number of bacteria required to fight off an infection depends on the type of infection.

"Our findings provide a new understanding of how the immune system combats bacterial infections," said Paul W. Ewald, head of the Laboratory of Evolutionary Systems Biology at Rockefeller. "This knowledge could lead to the development of new therapies for treating bacterial infections."

The study, published in the journal Nature Immunology, focused on two types of bacterial infections: pneumonia and sepsis. Pneumonia is an infection of the lungs, while sepsis is a life-threatening infection of the bloodstream.

The researchers found that a large number of bacteria were necessary to fight off pneumonia. This is because pneumonia is caused by a type of bacteria that is able to quickly multiply and spread throughout the lungs. In order to clear the infection, the immune system needs to produce a large number of antibodies and other immune cells to kill the bacteria.

In contrast, the researchers found that a small number of bacteria were sufficient to fight off sepsis. This is because sepsis is caused by a type of bacteria that is not able to multiply as quickly as the bacteria that causes pneumonia. As a result, the immune system does not need to produce as many antibodies and other immune cells to clear the infection.

The researchers also found that the number of bacteria required to fight off an infection varied depending on the host. This is because the immune system of each individual is different. Some people have a stronger immune system than others, and this can affect how well they are able to fight off an infection.

The findings of this study have implications for the treatment of bacterial infections. For example, the study suggests that a large number of antibiotics may be necessary to treat pneumonia, while a small number of antibiotics may be sufficient to treat sepsis. This could help to reduce the risk of antibiotic resistance, which is a growing problem.

"Our study provides a new framework for understanding how the immune system combats bacterial infections," said Ewald. "This knowledge could lead to the development of new therapies for treating bacterial infections and reducing the risk of antibiotic resistance."