Microbiology Case Study: A 60 Year Old with Non-Healing Wound

Clinical history

A 60 year old patient with a past medical history of type II diabetes mellitus, right Charcot foot, and cirrhosis presented to the emergency department with altered mental status and several days of subjective fevers, as well as a 2 month history of right lateral malleolar non-healing ulcer which had subacutely increased in size and volume of drainage.

The patient’s spouse reported the wound had been showing purulent discharge for 3 weeks. Nine days before presentation, the patient had seen a foot and ankle specialist for evaluation of his Charcot foot and the ulcer. Radiographs were taken at this time, but no treatment was initiated. Worsening of the wound was associated with an episode of long travel, after which the patient reported being able to see bone.

Infectious disease noted that the patient had a pet corgi.

On exam, the patient was febrile with dry mucous membranes and oriented only to person. MRI showed evidence of possible osteomyelitis. The patient subsequently underwent a right below the knee amputation.

Laboratory findings

Gram smear of a sample taken from the patient’s ankle wound in the emergency department showed many neutrophils, moderate gram positive cocci and moderate gram negative bacilli, with intraleukocytic organisms seen. Growth was observed on blood and chocolate agar plates, but there was no growth on the MacConkey plate. The organisms were identified as few Pasteurella multocida, few vancomycin resistant Enterococcus faecalis, and few usual skin flora.

Image 1. Gram stain of the sample taken from the patient’s ankle wound.

Blood cultures drawn in the emergency department were positive at 10 hours in both bottles, and again on planting showed growth on blood and chocolate agar, but no growth on MacConkey. The organism was identified as P. multocida, consistent with that which grew from the ankle wound culture.

The patient underwent a right below the knee amputation, and anaerobic cultures taken from the right foot again grew P. multocida.

Discussion

Pasteurella multocida is a nonmotile gram negative bacillus which is part of the normal oropharyngeal flora in domestic dogs and cats. It is a facultative anaerobe, positive for oxidase, catalase, and indole. It grows on chocolate and blood agar, forming small, gray, non-hemolytic colonies. It does not typically grow on MacConkey agar.

P. multocida is classically associated with a zoonotic soft tissue infection in humans who suffer bite wounds from a pet, as well as licking of any broken skin by a pet. These infections have a characteristic rapid onset and intense inflammatory response, and can progress to necrotizing fasciitis. Cases of Pasteurella osteomyelitis can be associated with significant wound infections. Conditions such as diabetes, liver dysfunction, and organ transplantation can predispose patients to Pasteurella bacteremia.

Pasteurella spp. are susceptible to beta-lactam antibiotics in most cases, and since Pasteurella wound infections are usually polymicrobial, recommended treatment is broad-spectrum such as amoxicillin-clavulanate. In isolated Pasteurella infections, first line treatment is penicillin, although there are some that favor testing isolates from sterile sites for the presence of beta-lactamase production, and treating those infections with ampicillin-sulbactam, pipercillin-tazobactam, or ceftriaxone. (Weber)

References

  1. Weber, David J., and Sheldon L. Kaplan. “Pasteurella infections.” UpToDate, Wolters Kluwer, 15 June 2018, http://www.uptodate.com/contents/pasteurella-infections?search=pasteurella%20treatment&source=search_result&selectedTitle=1~25&usage_type=default&display_rank=1#H14. Accessed 4 Feb. 2020.

-Tom Koster, DO is a 1st year Anatomic and Clinical Pathology Resident at the University of Vermont Medical Center.

-Christi Wojewoda, MD, is the Director of Clinical Microbiology at the University of Vermont Medical Center and an Associate Professor at the University of Vermont.

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