ABSTRACT
Escherichia coli is a short, gram-negative bacillus. Most strains are motile.The organism is a facultative anaerobe and grows readily on most laboratory media, producing a characteristic fetid odor. Serologic identification is based on the antigen specificity of the O antigens. To date, more than 150 group O (somatic cell wall), at least 50 H (flagellar), and a similar number of K (capsular) antigens have been identified. Clinical studies indicate that certain O group antigens are more likely to be incriminated in disease than other strains; this finding suggests a correlation between pathogenicity and antigenic composition. Strains that are rich in K antigens are more resistant to destruction by antibody, complement, and phagocytosis and thus are more likely to circumvent host defense mechanisms than are other strains.
