<div>Host-pathogen interactions are the result of long term evolutionary processes due to the conflicting goals of the host and the infections pathogens in their quest for survival, creating an interplay of co-evolution as various adaptation are acquired by one and then in turned adapted to by the other. Selection of the host’s antimicrobial strategies and the resultant adaptations of infectious microorganisms leads to the development of complex and dynamic relationships ranging from symbiotic to commensal to pathogenic. In an effort to understand the selective process and identify unique mechanisms of antimicrobial defense, sera from 18 species (7 invertebrate, 11 vertebrate) were tested for antimicrobial potential against 20 Gram-negative and 11 Gram-positive bacteria. <i>Alligator mississippiensis</i> sera exhibited the strongest inhibitory potential. A transposon mutagenesis screen performed on the resistant bacterium <i>Serratia marcescens</i> identified several genes, including <i>glnL</i>, as necessary for defense. The <i>glnL</i> gene encodes for the sensory histidine kinase/phosphatase NtrB, controlling the expression of regulatory genes in response to nitrogen limitation. Attenuated growth of the Tn::<i>glnL</i> mutant in the presence of alligator serum and minimal media was rescued with nitrogen supplementation, suggesting the existence of a mechanism for nitrogen limitation as an antimicrobial strategy in alligator sera. Utilization of a <i>Drosophila melanogaster</i> oral model of infection showed that <i>glnL</i> is required for <i>S. marcescens</i> virulence, and nitrogen supplementation rescued the phenotype, as measured by fly mortality and bacterial cfu recovery. S. marcescens, an environmentally ubiquitous Gram-negative bacterium, is an opportunistic pathogen in several species, including alligators and Drosophila. Subsequent <i>in vitro</i> testing of the antimicrobial potential of invertebrate hemolymph utilizing the Tn::<i>glnL</i> mutant showed a nitrogen-dependent growth inhibition of species in the order Dipteria. Combined, these results support a model of evolutionary convergence of nitrogen limitation as an antimicrobial mechanism. This work not only identifies a novel antimicrobial strategy that could be used in the development of therapeutics, and a novel virulence factor in <i>S. marcescens</i>, but has broad mplications for bacterial management and can provide insight into the evolutionary history of host-pathogen interactions.</div>