Pathogenicity of enteric Gram-negative bacteria
The Enterobacteriaceae are a large family (>20 genera and 100 species) of facultatively anaerobic Gram-negative bacilli that are easily cultured, reduce nitrate and ferment glucose. The wide diversity of named species is in part because they are easy to grow and study in the laboratory, but also because they are capable of causing a wide variety of clinical syndromes. Habitat and transmission The Enterobacteriaceae are almost ubiquitous organisms. They are found in: • the normal flora of animals and humans (forming a major component); • water; • soil. Transmission is both from other animals or humans and from the inanimate environment. Many infections arise from the body’s normal flora when opportunities are provided by medical, surgical or other therapy Pathogenicity 1) Capsules Many species produce extracellular capsular polysaccharides (e.g. Klebsiella spp., Escherichia coli and Salmonella typhi). S. typhi possesses a capsule or Vi (virulence) antigen and a vacci