138 Musser et al. Archives of Insect Biochemistry and Physiology Archives of Insect Biochemistry and Physiology 58:138–144 (2005) © 2005 Wiley-Liss, Inc. DOI: 10.1002/arch.20031 Published online in Wiley InterScience (www.interscience.wiley.com) Evidence That Caterpillar Labial Saliva Suppresses Infectivity of Potential Bacterial Pathogens Richard O. Musser, 1 * Hyeog S. Kwon, 1 Spencer A. Williams, 1 C. James White, 1 Michael A. Romano, 1 Scott M. Holt, 1 Shay Bradbury, 1 Judith K. Brown, 2 and Gary W. Felton 3 Salivary enzyme, glucose oxidase (GOX) from the caterpillar Helicoverpa zea, catalyzes the conversion of glucose to gluconic acid and hydrogen peroxide. Because hydrogen peroxide has well-known antimicrobial properties, we examined whether caterpillar labial saliva could reduce the infectivity of bacterial pathogens. We examined the effects of caterpillar saliva on the growth of two bacteria species Serratia marcescens and Pseudomonas aeruginosa. Wells formed in LB agar contained a solution of salivary gland extract (Sx) and glucose, GOX and glucose, Sx only, GOX only, or glucose only. After 18 h of incubation, the diameter of cleared bacteria was measured. Wells treated with only GOX, Sx, or glucose showed no measur- able area of clearing, while wells treated with GOX with glucose or Sx with glucose had considerable clearing. To determine if saliva could provide protection to caterpillars in vivo, a surgery was performed on caterpillars that prevented the secretion of labial saliva. Caterpillars were fed a diet containing either no added bacteria or treated with high levels of S. marcescens or P. aeruginosa. Caterpillars that could not secrete saliva had significantly higher levels of mortality when feeding on diet treated with either bacterium than caterpillars that could secrete saliva when feeding on equal levels of bacteria-treated diet. Our evidence demonstrates for the first time that insect saliva in situ can provide protection against bacterial pathogens and that the salivary enzyme GOX appears to provide the antimicrobial properties. Arch. Insect Biochem. Physiol. 58:138–144, 2005. © 2005 Wiley-Liss, Inc. KEYWORDS: bacteria; pathogen; saliva; induced defenses 1 Department of Biological Sciences, Western Illinois University, Macomb, Illinois 2 Department of Plant Sciences, Center of Insect Science, University of Arizona, Tucson, Arizona 3 Department of Entomology, Pennsylvania State University, University Park, Pennsylvania Paper presented at the 51st Annual Meeting of the Entomological Society of America, October 2003. Symposium entitled Insect Saliva: An Integrative Approach. Contract grant sponsor: Illinois Department of Agriculture; Contract grant sponsor: University Research Council, Western Illinois University; Contract grant sponsor: Center for Insect Science, University of Arizona. *Correspondence to: Richard O. Musser, Department of Biological Sciences, Western Illinois University, Macomb, IL 61455. E-mail: ro-musser@wiu.edu INTRODUCTION Insect saliva has numerous functions, such as di- gestion, water balance, circumventing animal-host defenses, maintenance of mouth parts, pheromone production, pathogen transmission, anti-predator defense, and is suspected to have anti-microbial properties (Brough, 1983; Ribeiro, 1995; Felton and Eichenseer, 1999). Only recently has it been determined that herbivore saliva can suppress in- duced herbivore defenses of plants analogous to saliva from blood-feeding arthropods suppressing the defensive response of their vertebrate hosts (Kahl et al., 2000; Musser et al., 2002a,b, 2005a,b; Na and Chenzhu, 2004). Glucose oxidase (GOX), the salivary component of the caterpillar Helico- verpa zea (Boddie), was the first characterized sup- pressor of induced plant defenses (Musser et al., 2002a). However, insect herbivore survival is not only dependent on circumventing plant defenses, but must also avoid the detrimental effects of insect pathogens. Saliva of mammals and other organ- isms is widely known to have antimicrobial prop-