Microbiology Case Study: 40 Year Old Male with A Diabetic Foot Ulcer

Clinical Presentation and History

The patient is a 40 year old male with a past medical history of type 2 diabetes mellitus with significant neuropathy and hypertension with a past surgical history of right metatarsal osteomyelitis. He presents to hospital with fever, right ear pain, headache, two episodes of diarrhea and redness and blistering to the right 3rd metatarsal. Upon examination he was noted to have a 1 cm ulceration on the right 3rd toe on the dorsal aspect associated with redness and edema. He was therefore assessed as having diabetic foot ulcer with possible osteomyelitis for which blood cultures were performed.

Laboratory Identification

Gram stains performed on the positive blood culture broth showed gram negative rods (Image 1). In our institution initial positive blood cultures are tested by the Verigene System (Luminex Corp., Austin, TX), which allows for rapid identification of common bacterial pathogens causing blood stream infections (Escherichia coli, Klebsiella oxytoca, Klebsiella pneumonia, Pseudomonas aeruginosa, Acinetobacter spp., Citrobacter spp., Enterobacter spp., and Proteus spp.) along with detection of several resistance genes (CTX-M, IMP, KPC, NDM, OXA, VIM) within ~ 3 hours. In this case, no targets on the Verigene panel were detected. Simultaneously, the specimen was plated onto blood, chocolate and MacConkey agars where the organism grew robustly on all three plates (Image 2). The MacConkey agar showed the organism to be a non-lactose fermenter. Once the organism adequately grew on these agar plates, final species identification was performed on the automated MALDI-TOF instrument which showed Salmonella species. To appropriately type the organism, Salmonella latex agglutination testing was performed which identified Salmonella species Group B (Non-typhoidal). Of note, multiple blood cultures from this patient were positive for Salmonella species, Group B.

Image 1. Gram stain of blood culture broth containing gram negative rods.
Image 2. Growth of the organism on chocolate, 5% sheep blood, and MacConkey agars.

Discussion

Salmonella is a gram negative, flagellated facultative anaerobic, non-lactose fermenting bacilli. The taxonomy and nomenclature of salmonella organisms are quite complex however the most widely used classification scheme is the Kauffman-White which is updated yearly by the WHO. Currently, members of the 7 Salmonella subspecies can be serotyped into one of more than 2500 serotypes (serovars) according to antigenically diverse surface structures: somatic O antigens (the carbohydrate component of lipopolysaccharide [LPS]) and flagellar (H) antigens.

Nontyphoidal salmonellae are a major cause of diarrhea worldwide. In the United States, non-typhoidal salmonellosis is one of the leading causes of foodborne disease. Salmonella enteritidis and Salmonella typhimurium are among the most frequently isolated organisms. Salmonella is most commonly associated with ingestion of contaminated poultry, eggs, and milk products. Salmonella gastroenteritis typically occur within 8 to 72 hours following exposure, however lower bacterial doses can prolong the incubation period. Although Salmonella typically causes diarrheal diseases including gastroenteritis and enteric fever, however there are rare instances where hematogenous involvement leads to bacteremia, osteomyelitis or endovascular infections.

In this case the source of Salmonella-related bacteremia is still a mystery. The presumed source was osteomyelitis, but the patient’s subsequent toe amputation revealed minimal osteomyelitis and rare fungal organisms.

References

  1. Procop, Gary W. et al (2017). Koneman’s Color Atlas and Textbook of Diagnostic Microbiology. 7th edition. Philadelphia, PA.
  2. Hohmann, Elizabeth L. (2018). Nontyphoidal salmonella: Gastrointestinal infection and carriage. Uptodate.com. Retrieved on November 14, 2019. https://www-uptodate.com/contents/nontyphoidal-salmonella-gastrointestinal-infection-and-carriage

-Anna-Lee Clarke-Brodber, MD is a 3rd year AP/CP resident at University of Chicago (NorthShore). Academically, Anna-Lee has a particular interest in Cytopathology. In her spare time she enjoys hanging out with her family.

-Erin McElvania, PhD, D(ABMM), is the Director of Clinical Microbiology NorthShore University Health System in Evanston, Illinois. Follow Dr. McElvania on twitter @E-McElvania. 

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