Pathogenic Potential of Hemolytic and Non-Hemolytic Plant Growth Promoting Bacteria in Galleria mellonella tests.

Abstract

Bacterial hemolysis indicates virulence in humans with partial or total lysis of red blood cells. In this study, Galleria mellonella larvae were used to evaluate the human virulence of 31 fungal pathogen antagonist bacteria, which previously showed alpha and gamma hemolysis in in vitro blood agar tests. Galleria mellonella was chosen to evaluate virulence due to technical advantages such as low cost, availability, results in 2-3 days, absence of ethical issues, and functional similarities with the mammalian and insect immune systems. We hypothesized that antagonistic bacteria that showed partial lysis (α) and no lysis (ɣ) activity in the blood agar test would exert non-virulence effects in the in vivo test with Galleria. Among the 31 bacterial strains, three gamma (ɣ) hemolysin-producing bacteria (Pseudomonas aeruginosa PaMR4, Rhizobium pakistanense RpCR1, and Staphylococcus saccharolyticus SC4) showed virulence, and the other twenty-eight (α and ɣ hemolytic) showed non-virulent effects. The Galleria test suggests that ɣ hemolysin-producing bacteria may be pathogenic to humans, even if they cannot lyse red blood cells (RBCs). Therefore, it is essential to perform additional virulence tests on bacteria that previously showed α and ɣ hemolysis. In this study, the 31 bacterial strains used showed different responses to antibiotics. Some Gram-positive strains showed resistance to 23 antibiotics, and Gram-negative strains showed resistance to nine antibiotics. Antagonistic bacteria without hemolytic activity (ɣ hemolysis) could still be pathogenic to humans, so it is advisable to perform additional tests, such as the Galleria mellonella virulence test, and if virulence is confirmed in these tests, they should not be used as biological control agents for plant disease management.