A 2-year-old male with no past medical history presented to the emergency department with fever and 2 days of bloody diarrhea. Stool cultures were sent to the laboratory. A Gram stain of the specimen showed the morphology seen in Figure 1. On the 5% sheep blood agar plate, the predominant organism had colonies that appeared flattened and spreading (Figure 2A). On MacConkey agar the colonies were noted to be non-lactose fermenting (Figure 2B). A Hektoen enteric (HE) agar was used as a differential and selective media to differentiate Salmonella from Shigella. On the HE agar the colonies were clear with a green appearance due to the color of the agar (Figure 2C).
Shigella is a bacterium in the Enterobacteriaceae family and is a Gram-negative rod that is facultatively anaerobic. It is non-motile, a non-spore former, and does not ferment lactose. There are four species of Shigella that are associated with subgroups A-D. Our isolate was identified as Shigella sonnei, which is the most common species in the U.S. and comprises subgroup D. The other subgroup/species correlations are listed in Table 1. The slide agglutination antisera test is used to aid in serogrouping. The suspected colony is mixed on a slide with antisera that contains specific antibodies to Shigella. If clumping (agglutination) occurs, it is considered a positive result for the specific subgroup. The organism was identified as Shigella sonnei by slide agglutination antisera testing. In addition, Shigella has certain biochemical properties that aid in further identification and confirmation.
Table 1: Shigella sp. determination by serogroup
Shigella is one of the most common causes of bacterial gastroenteritis and is often associated with poor sanitation and overcrowded conditions. Transmission occurs through routes such as: fecal-oral and person to person contact. Of note, only a small amount of the bacteria (as low as 10 organisms) is required to cause disease. Hemolytic-uremic syndrome is a complication that may occur with shiga-toxin producing Shigella (the most commonly associated is S. dysenteriae). Shigella has demonstrated antibiotic resistance and therefore does undergo susceptibility testing.
Nataro JP, Bopp CA, Fields, PI, Kaper JB, Strockbine, NA. 2015. Escherichia, Shigella, and Salmonella, p 603-626. In Jorgensen J, Pfaller M, Carroll K, Funke G, Landry M, Richter S, Warnock D (ed), Manual of Clinical Microbiology, Eleventh Edition. ASM Press, Washington, DC.
-Valerie Juarez, M.D., 3rd year Anatomic and Clinical Pathology resident, UT Southwestern Medical Center
-Erin McElvania TeKippe, Ph.D., D(ABMM), is the Director of Clinical Microbiology at Children’s Medical Center in Dallas Texas and an Assistant Professor of Pathology and Pediatrics at University of Texas Southwestern Medical Center.