Serratia marcescens is short and rod shaped. It is a facultative anaerobe, meaning that it can grow in either the presence of oxygen (aerobic) or in the absence of oxygen (anaerobic). Primarily it uses fermentation as the means of gathering energy and has enzymes (superoxide dismutase, catalase or peroxides) that protect it from reactive oxygen species, allowing it to live in oxygenated environments. Serratia marcescens is a gram negative bacterium. Gram negative bacteria have a thin cell wall made of a single layer of peptidoglycan that is enclosed by an outer membrane. The outer membrane has lipopolysaccharides (LPS), which are a special kind of phospholipid composed of fatty acids that are attached to a glucosamine phosphate dimer. One glucosamine is then attached to a core polysaccharide that extends to the O polysaccharides[18]. The outer membrane also serves as a means to regulate the uptake of nutrients and the exclusion of toxins. The protein pores and transporters found in the envelope layers vary in selectivity