A 58 year old female presented to the emergency department with complaints of a fever (reaching 102.9°F) and headache with associated nausea and vomiting for the past 24 hours. Her past medical history was significant for a resection of a recurrent hemangiopericytoma by the neurosurgery service three weeks prior. The patient also noted clear drainage from this surgical site which had begun 5 days ago. Other symptoms noted at presentation included decreased appetite and dehydration. She denied back & neck pain, photophobia or stroke and seizure-like symptoms. Her vital signs were all within normal limits. On physical exam, a healing surgical wound was noted in the posterior auricular area with clear drainage, but no blood or exudates were visualized. She had no tenderness when her spine was palpated and neurologic exam showed a left sided facial droop and tongue deviation which were noted previously and attributed to her multiple central nervous system surgeries. Complete blood count (CBC) showed a mild increase in white blood cells and anemia. An external ventricular drain was placed and cerebral spinal fluid (CSF) was sent to the microbiology lab for culture. Blood cultures and a swab from the surgical wound were also collected.
Image 1. Gram stain of the cytospin CSF showed many acute inflammatory cells and numerous Gram negative bacilli (1000x).
Image 2. Growth of large, glossy, reddish-orange colonies on sheep blood agar (image taken after 72 hours of incubation).
Image 3. Growth of large, deep red colonies on MacConkey agar (image taken after 72 hours of incubation).
Gram stain of the CSF showed numerous acute inflammatory cells and many Gram negative bacilli (Image 1). Culture of the CSF and wound swab showed large, glossy red colonies on sheep blood and MacConkey agars (Images 2 & 3). Analysis of the colony by matrix assisted light desorption ionization time of flight mass spectrometry (MALDI-TOF MS) identified the organism as Serratia marscescens.
Serratia marscescens is a facultative Gram negative bacillus that is a member of the Enterobacteriaceae family. S. marscesens is ubiquitous in the environment and the most frequent and clinically important species in the genus. Although S. marscesens usually doesn’t cause infection in healthy individuals, it is notorious for colonizing and causing infections in hospitalized patients, particularly those who are immunocompromised, in intensive care units (especially intubated patients) and those with indwelling catheters. While respiratory infection are most common, S. marscesens has also been implicated in numerous other opportunistic infections such as urinary tract infections, wound infections and septicemia. Brain abscesses and meningitis are less common. S. marscesens has been implicated as the cause of outbreaks in hospitals and can often be traced back to pieces of medical equipment including nebulizers, bronchoscopes, laryngoscopes and contaminated solutions. Person to person transmission is also recognized and thought to be predominantly transmitted via direct contact.
In the laboratory, S. marscesens can be identified by its characteristic non-diffusible red pigment, prodigiosin. Care should be taken when interpreting the lactose reaction on MacConkey agar, as the red pigment may be confused with a positive reaction, while S. marscesens is known to be lactose negative. As a member of the Enterobacteriaceae family, S. marscesens is able to ferment glucose, reduce nitrate to nitrite and has a negative oxidase reaction. A unique feature of this genus is that all Serratia spp. produce three proteolytic enzymes: lipase, gelatinase, and DNase. Commercial systems, including MALDI-TOF MS, are helpful in the identification of S. marscesens as well.
Treatment of Serratia marscescens infections can be difficult due to various antimicrobial resistance mechanisms, such as expression of extended spectrum beta lactamases (ESBLs), AmpC cephalosporinases and carbapenemases, exhibited by the organism. In the case of our patient, she was empirically started on vancomycin and piperacillin-tazobactam and taken to surgery for wound wash out, removal of hardware and repair of CSF leak. Her antibiotics were changed to meropenem and gentamicin. She was discharged to a rehabilitation facility and received meropenem for a total of 6 weeks.
-David Marbury, MD, is a 3rd year Anatomic and Clinical Pathology resident at the University of Mississippi Medical Center.
-Lisa Stempak, MD, is an Assistant Professor of Pathology at the University of Mississippi Medical Center in Jackson, MS. She is certified by the American Board of Pathology in Anatomic and Clinical Pathology as well as Medical Microbiology. Currently, she oversees testing performed in both the Chemistry and Microbiology Laboratories. Her interests include infectious disease histology, process and quality improvement and resident education.