Plants are resistant to most potentially pathogenic microbes. Frequently, resistance results from defenses activated upon recognition of “non-self.” Invasion of a variety of pathogens, including Gram-negative bacteria, into plants is betrayed by the presence of pathogen-associated molecular patterns (PAMPs). Plants challenged by a non-pathogenic bacterial strain or a purified PAMP often form cell wall modifications called papillae. These cell wall thickenings, which can be observed in the electron microscope, can more easily be visualized by staining for the component molecule callose and using fluorescent microscopy. We describe a method to measure callose following infiltration of leaves of Arabidopsis thaliana, a model organism for basic and applied research on plant biology, with pathogenic and non-pathogenic strains of Pseudomonas syringae pv. tomato or a purified bacterial PAMP. We also detail a method to measure the growth of bacteria infiltrated into leaves of Arabidopsis. These methods can be used to understand the interactions between pathogen and host plant.