Light micrograph of Friedlander's bacillus (Klebsiella pneumoniae), a pathogenic bacterium that often causes pneumonia. The Enterobacteriaceae are a large family of Gram-negative bacteria that includes, along with many harmless symbionts, many of the more familiar pathogens, such as Salmonella, Escherichia coli, Yersinia pestis, Klebsiella, and Shigella. Other disease-causing bacteria in this family include Proteus, Enterobacter, Serratia, and Citrobacter. Magnification: 500x at 35mm.