MICROBIOLOGY

• Aerobic Gram-negative rod (Fig), non-motile, non-spore-forming), family Enterobacteriaceae.
• Four groups: subdivided into serotypes and subserotypes based upon lipopolysaccharide O antigenic repeats.
  
  • Group A (S. dysenteriae, see separate module for discussion of Group A infections, often cause of pandemics arising from natural and man-made disasters)
  • Group B (S. flexneri, leading cause in low- and middle-income countries)
  • Group C (S. boydii, relatively uncommon cause)
  • Group D (S. sonnei, the leading diarrhea cause in high-income countries)
    • An outbreak in the UK and other European countries with XDR S. sonnei. The risk may be MSM, acquired as STD[1][24].
  • >46 serotypes mean low protective immunity develops when exposed to new infections to prevent the next.
• In clinical microbiology laboratories, MacConkey or EMB agar yields the best growth in addition to Shigella-Salmonella agar.