The role of innate immunity, antibiotics, and bacteriophages in the course of bacterial infections and their treatment

Critical to our understanding of infections and their treatment is the role the innate immune system plays in controlling bacterial pathogens. Nevertheless, many in vivo systems are made or modified such that they do not have an innate immune response. Use of these systems denies the opportunity to examine the synergy between the immune system and antimicrobial agents. In this study, we demonstrate that the larva of <i>Galleria mellonella</i> is an effective in vivo model for the study of the population and evolutionary biology of bacterial infections and their treatment. To do this, we test three hypotheses concerning the role of the innate immune system during infection. We show that i) sufficiently high densities of bacteria are capable of saturating the innate immune system, ii) bacteriostatic drugs and bacteriophages are as effective as bactericidal antibiotics in preventing mortality and controlling bacterial densities, and iii) minority populations of bacteria resistant to a treating antibiotic will not ascend. Using a highly virulent strain of <i>Staphylococcus aureus</i> and a mathematical computer-simulation model, we further explore how the dynamics of the infection within the short term determine the ultimate infection outcome. We find that immune activation in response to high densities of bacteria leads to a strong but short-lived immune response which ultimately results in a high degree of mortality. Overall, our findings illustrate the utility of the <i>G. mellonella</i> model system in conjunction with established in vivo models in studying infectious disease progression and treatment.