Microbiology Case Study: A 58 Year Old Female with Shortness of Breath, Fever, and Chills

Case History

A 58 year old female presented to the emergency department with a chief complaint of shortness of breath, fevers and chills since the previous day. Her past medical history is significant for chronic obstructive pulmonary disease, hypertension, and borderline personality disorder. Vitals signs were significant for an oxygen saturation of 88%. Physical examination of the patient was difficult as the patient became increasingly agitated, however, the patient appeared in no acute distress with moist mucous membranes, anterior lung fields were clear to auscultation, there were no cardiac murmurs, and examination of their skin revealed no rashes or lesions.

Laboratory Findings

Laboratory tests were significant for a lactate level of 2.5 with a white blood cell count and complete metabolic panel within normal limits. Chest x-ray did not show evidence of consolidation or interstitial infiltrates. Urinalysis was within normal limits. One set of blood cultures was also drawn during this initial encounter. The patient became increasingly agitated after initial examination and was discharged with some laboratory tests pending. After incubating for 20 hours, the aerobic blood culture bottle flagged positive for bacterial growth, with gram stain demonstrating a gram negative coccobacillus and a rapid Verigene identification of Acinetobacter. The patient came back to the emergency department the next day with stable vital signs and unremarkable complete blood count and chest x-ray. The patient was started on meropenem which was switched to ciprofloxacin two days later, after bacterial antibiotic susceptibility results showed susceptibility to carbapenems, amikacin, amp/sulbactam, ceftazidime, ciprofloxacin, gentamin and tobramycin.

Image 1. Gram negative coccobacilli characteristic of Acinetobacter on Gram stain.
Image 2. Acinetobacter morphology on MacConkey agar.


Acinetobacter is a genus of gram negative bacteria, with some genospecies identified as human pathogens including species in the A. calcoaceticus-A. baumannii complex (ACB) which are difficult to differentiate by phenotypic characteristics. Species in the ACB include genospecies 1 (A. calcoaceticus), genospecies 2 (A. baumannii), genospecies 3, and genospecies 13TU.

In the laboratory, Acinetobacter appear as non-pigmented mucoid, domed colonies with a smooth surface on growth media. Acinetobacter are non-motile, aerobic, catalase positive, oxidase negative, indole negative bacteria. Acinetobacter are also non-glucose fermenters and do not utilize lactose.

Out of the ACB genospecies, A. baumannii is considered the most significant pathogen, causing 80% of nosocomial infection. A. baumannii is an environmental bacteria which inhabits soil and water. In hospital settings, A. baumannii can survive on environmental surfaces for extended periods of time and is resistant to desiccation and cleaning solutions. The most common settings in which A. baumannii infections occur are within intensive care units where there are immunocompromised patients utilizing medical devices such as ventilators or catheters which are surfaces A. baumannii frequently colonizes. Not surprisingly, sites where these medical devices preside are the most common sites of infection for A. baumannii including the respiratory tract (hospital acquired pneumonia), bloodstream infections, and wound infections. Interestingly, A. baumannii wound infection have also been seen at a high prevalence in wartime and disaster victims. A. baumannii has been recovered in 63% of wounds from soldiers in Iraq and Afghanistan and 20% of wounds from victims after a tsunami in 2004.

 Importantly, A. baumannii can be resistant to several classes of antibiotics including fluroquinolones (DNA topoisomerase mutations), aminoglycosides (transposons), beta lactams (AMP C beta lactamase), and carbapenems (OXA carbapenemase), making infections with multidrug resistant organisms challenging to treat. In this case, the microbe had an OXA carbapenemase but was susceptible to carbapenems. In addition, this patient’s relatively benign presentation and normal laboratory results raise the question of whether this bacteria was causing a bloodstream infection or was simply a skin colonizer which grew after being inoculated into the blood culture media. Acinetobacter, in addition to colonizing hospital equipment and surfaces is a common colonizer of the skin as well as respiratory tract of patients on respiratory ventilators. Thus, Acinetobacter can be inadvertently cultured in blood and sputum samples, making correlation of the patient’s clinical symptoms and signs with culture results very important.   

-Liam Donnelly, MD 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.

Microbiology Case Study: A Male in his Early 20s with Generalized Body Aches

Clinical History

An African American male in his early 20s presented to the emergency department (ED) with complaints of a sore throat, headache, generalized body aches, and fatigue for the past week. He also noted intermittent fever and chills as well as some nausea with a decrease in his appetite. He had been seen multiple times in the ED recently for similar symptoms. His past medical history was non-contributory and he noted no significant travel or exposure history with the exception of attending a local party 10 days ago. His temperature was 100.5°F and vitals were otherwise normal. His physical exam was normal with the exception of dry mucous membranes indicating mild dehydration. Initial laboratory testing showed a leukopenia (white blood cell count of 1.5 TH/cm2) with 39% lymphocytes and rapid antigen testing for group A Streptococcus, influenza, and infectious mononucleosis were negative. The patient was admitted for further work up due to the prolonged nature of his symptoms.   

Laboratory Identification

Results from additional infectious disease testing are in the table below.

This pattern of results is most consistent an acute HIV infection.


Human immunodeficiency virus (HIV) is an enveloped, single stranded RNA virus which belongs to the family Retroviridae. HIV is most commonly sexually transmitted via body fluids such as blood, semen, and vaginal secretions directly contacting mucosa membranes. HIV can also be transmitted due to needle stick injuries, blood transfusions, and transplacentally from infected mother to fetus or by breast feeding. Acute HIV illness presents as a mononucleosis-like syndrome with fever, pharyngitis, arthralgias, malaise, and weight loss. During this acute illness, the HIV RNA viral load is extremely high. After a period of clinical latency, which on average is approximately 10 years, there is a deterioration of the immune system, the CD4 count drops, and the patient is at risk for opportunistic infections and neoplastic diseases.

Based on the 2014 CDC/APHL guidelines, the initial screening test for HIV is an antigen-antibody combination assay. These immunoassay based tests detect the p24 antigen and antibodies to HIV-1 and HIV-2 (see image below). By testing for the p24 antigen in addition to HIV antibodies the time to a positive patient result is decreased (window period) as p24 is one of the first viral proteins to appear, even before antibodies are present.    

If the antigen-antibody test is repeatedly positive, the second step in the testing algorithm is an antibody differentiation assay. This test has taken the place of the Western blot and Western blot is no longer recommended in the diagnosis of HIV. If the antibody differentiation test is positive, the diagnosis of HIV-1 or HIV-2 is confirmed. As this step only detects the presence of antibodies, the differentiation test will be negative in an acute HIV infection.

If there is a discrepancy between the first two steps in the testing algorithm or an indeterminate result is obtained, the final step involves nucleic acid amplification testing (NAAT) to detect viral RNA. Viral RNA is the first HIV-1 specific marker to appear following infection. In the case of an acute or untreated long term infection, the viral load can approach levels up to 100 million copies.  

When additional history was obtained from our patient, he said he was sexually active with a new male partner in the past few weeks and did not use protection. He stated he had been treated with Chlamydia in the past. Further testing for CD4 count, other opportunist & sexually transmitted infections, and HIV genotype testing was performed and outpatient HIV care was arranged for the patient. 

-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. She is the Director of Clinical Pathology as well as the Microbiology and Serology Laboratories. Her interests include infectious disease histology, process and quality improvement, and resident education.

Microbiology Case Study: An 8 Month Old Female with Vomiting and Diarrhea

Clinical History

An 8-month-old female presented to the pediatric emergency department (ED) due to vomiting and diarrhea for the past 10 days.  Per mother, the baby has had a fever and 6-8 episodes of diarrhea & 2-3 episodes of vomiting each day. On the day of admission, the mother noted the diarrhea was like mucous and contained blood and the baby was unable to tolerate anything by mouth. Past medical history was not significant and sick contacts included a sibling with a recent viral illness. In the ED, the baby had a fever of 103.1°F and was tachycardic. On physical exam, the baby was weak & lethargic with dry mucous membranes and a capillary refill of 2-3 seconds. Mom noted the baby did not produce tears when she cried and had decreased urinary output for the past 24 hours, consistent with dehydration. Initial labs revealed an elevated white count of 20.0 TH/cm2 and a C-reactive protein of 6.60 mg/dL, suggestive of an infectious process. Blood, urine, and stool cultures were sent to the microbiology laboratory and the baby was received IV fluids and ceftriaxone. 

Laboratory Identification

Blood culture signaled positive after 36 hours of incubation on the automated instrument and revealed gram negative rods.

Image 1. Whitish gray colonies on sheep blood and MacConkey agars after 48 hours of incubation at 35°C in ambient air.
Image 2. “Bull’s eye” colonies with a pink center and white outer edge on cefsulodin-Irgasan-novobiocin (CIN) agar after 48 hours of incubation at 22°C in ambient air.

MALDI-TOF mass spectrometry identified the isolate as Yersinia enterocolitica. The urinalysis was positive and the urine culture grew >100,000 CFU/ml of Escherichia coli. Stool culture was negative for Salmonella, Shigella, E. coli O157:H7, Aeromonas, and Plesiomonas. Antigens for Shiga toxin and Campylobacter jejuni were both negative. Stool culture for Y. enterocolitica was not ordered. A multiplex PCR panel for gastrointestinal pathogens also identified Y. enterocolitica


Yersinia enterocolitica is a member of the Enterobactericeae family and when transferred via the fecal oral route, can cause gastroenteritis, terminal ileitis, and mesenteric lymphadenitis, particularly in young children, the elderly, and immunocompromised patients, who consume raw or undercooked pork, chitterlings, or drink unpasteurized milk products. Because Y. enterocolitica can survive and multiple at refrigerated temperatures, prepackaged lunchmeats and packed red blood cells can be common sources for infection as well. Rarely, septicemia can result from migration of the organisms into the lymph nodes and then the blood.

Stool, blood, and lymph node cultures are often submitted to the microbiology laboratory for the detection of Y. enterocolitica. The organism is a gram-negative rod that can grow well on routine media such as sheep blood, chocolate, and MacConkey agars at 22°C and 35°C in ambient air. When there is a clinical concern for gastroenteritis caused by Y. enterocolitica, a selective media such as cefsulodin-Irgasan-novobiocin (CIN) agar should be added to the stool culture to enhance isolation. Y. enterocolitica grows as “bull’s eye” colonies with a pink center and surrounding clear to white border on CIN agar.

The organism ferments glucose & sucrose, is positive for catalase & urease, and is oxidase negative. Y. enterocolitica is able to be identified by manual and automated biochemical systems, such as API 20E and Vitek as well as MALDI-TOF mass spectrometry. Culture independent multiplex PCR panels for the diagnosis of gastrointestinal syndromes are gaining popularity due to sensitivity & improved turnaround times; however, reimbursement and the necessity for the isolated organism for susceptibility testing and typing the in the case of outbreak investigations continue to be items of concern.

The majority of cases of Y. enterocolitica gastroenteritis do not require antimicrobial treatment. In the case of severe disease and those that are immunocompromised or with systemic disease should receive treatment with a fluorquinolone or trimethoprim sulfamethoxazole. While Y. enterocolitica produces beta lactamases, it is still uniformly susceptible to extended spectrum cephalosporins as well.    

In the case of our patient, she received 8 days of ceftriaxone and was transitioned to oral trimethoprim sulfamethoxazole and discharged home to finish the 21-day course of antibiotics due to bacteremia from Y. enterocolitica. Mother was counseled to fully cook pork products before feeding to the baby.    

-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. She is the Director of Clinical Pathology as well as the Microbiology and Serology Laboratories. Her interests include infectious disease histology, process and quality improvement, and resident education.

Microbiology Case Study: A 60 Year Old Woman with Endometrial Cancer and Uterine Bleeding

Case History

A female in her 60s with no significant past medical history presented to the emergency department with continued abnormal uterine bleeding for the past two months. She reported never entering menopause. Patient complained of recent unintentional weight loss, shortness of breath, urinary frequency, and new-onset night-time fever and severe fatigue. Physical exam was notable for large uterus and left lower quadrant abdominal tenderness on palpation. Sterile speculum exam revealed a friable tissue extruding from the uterus and malodorous mixture of thin white purulent discharge and blood. Endometrial biopsy, tissue and blood cultures were collected. Labs were notable for normocytic anemia and leukocytosis. Urine analysis was suggestive of urinary tract infection. Pelvic ultrasound showed an enlarged uterus, thickened endometrium with mixed echogenicity. The patient was admitted to the hospital for work up of potential gynecological malignancy and was started on empiric antibiotic therapy for possible urinary tract infection.

The patient’s tissue Gram stain was notable for 2+ WBCs (PMNs) and 4+ gram negative rods. The predominant organism grew on 5% sheep blood and chocolate agars but there was no growth on the MacConkey agar (Image 1).

Image 1. Pasteurella multocida grows on blood (A) and chocolate (B) agar plate.
Image 2. Gross cross section of the friable fungating endometrial lesion. (A) Microscopic pathological findings consistent with undifferentiated carcinoma. (B)


The predominant organism was identified by MALDI-TOF MS as Pasteurella multocida. Given the microbiological findings, patient was further questioned but she denied having pet at home or contact with animals. The patient underwent a surgery to remove her uterus, both ovaries and Fallopian tubes. The gross pathological examination of her uterus revealed an exophytic endometrial lesion with extensive area of necrosis surrounding the lesion (Image 2A). Final microscopic pathological diagnosis was significant for a highly malignant and invasive tumor of the uterus (Image 2B) and active infection of her left Fallopian tube.

Here we present a rare case of a P. multocida intrauterine infection lacking history of animal exposure in the context of gynecologic malignancy. P. multocida is a small, non-motile, non-spore forming, aerobic and facultative anaerobic gram negative coccobacillus that is associated with animal exposure. [1] This organism is found in the digestive and respiratory tract of domesticated animals (cats, dogs, cattle, sheep etc.) as well as wild animals (lions, buffaloes, panthers etc.). The suggested hypothesis for transmission of Pasteurella to human involves close contact with the infected animal’s oral cavity. Thus, the most likely route of infection in humans is through direct animal bites or scratch, depositing the bacteria on compromised skin or if an infected animal lick on an exposed human mucosal surface [1, 2]. In the general population, P. multocida causes local infection that may progress to abscess formation, but bacteremia is uncommon. However, elderly patients and those with associated immunosuppressed states from chronic disease or cancer are at higher risk of more complicated severe infections [2].

The presented case highlights the importance of clinical microbiology in the setting of obtaining detailed history and conducting physical examination.  Uncommon cases of P. multocida infection secondary to pet scratches or bite inducing genital infection and causing tubo-ovarian abscess have been reported. Similarly, rare cases of immune-compromised P. multocida bacteremia without history of animal contact have been described. The most likely explanation is the re-activation of previously asymptomatic colonization of the respiratory and gastrointestinal tract in the context of immunosuppression. As such, patients presenting with systemic Pasteurella infection without a documented animal exposure should undergo extensive evaluation in order to exclude the presence of an underlying immunodeficiency [1, 3-4].


  1. Weber DJ., Wolfson JS., Swartz MN., Hooper DC. Pasteurella multocida infections – report of 34 cases and review of literature. Medicine. 1984;63(3):133-54.
  2. Raffi F, Barrier J, Baron D, Drugeon HB, Nicolas F, Courtieu AL. Pasteurella multocida bacteremia- report of 13 cases over 12 years and review of the literature. Scand J Infecti Dis. 1987; 19:385-93.
  3. Kimura K., Hagiya H., Yamamoto N., Yoshida H., Akeda Y., Nishi I., Tomono K. Pasteurella multocida multiple intrapelvic abscesses in a young woman with uterine cervical cancer. J Infect Chemother (2018).
  4. Lukban JC., Baker MS. Pasteurella multocida isolation from a tuboovarian abscess. A case report. J Reprod Med 1995;40:603-5.

– Pouya Jamshidi, MD is a 1st year anatomic pathology resident at University of Chicago (NorthShore). Academically, Pouya has a particular interest in neuropathology and cellular basis of neuroplasticity. In his spare time, Pouya enjoys listening to classical music and opera.

-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. 

Microbiology Case Study: A 54 Year Old Woman with Fever

Patient History

A 54 year old woman is seen by her oncologist with complaints of rigors and fevers for the past two months. Her past medical history is significant for Stage IV Follicular Lymphoma. Previous treatment included rituximab and bendamustine that was completed a year prior. Currently her therapeutic plan is to receive rituximab maintenance therapy every two months (her most recent dose was 2 days prior to this visit) via a port. She has two dogs, both of which had a diarrheal illness three months prior. She has chickens and spent the summer RV camping around New England. She has not had diarrhea or noticed any rashes. Her travel history includes a trip to Europe  4 years prior. A blood culture was collected due to her fevers and rigors and was found to be positive 29 hours post incubation.

Gram stain of the blood culture bottle showed small, faintly staining gram negative, curved rods (Image 1). The patient was started on ciprofloxacin and referred to the infectious disease clinic. After 3 days of incubation, small slightly mucoid yellow gray colonies grew on 5% sheep blood and chocolate agar (Image 2). There was no growth on the MacConkey agar. Initial Gram stain revealed similar gram negative rods to the Gram stain performed on the blood culture media. MALDI-TOF identified the organism as Campylobacter jejuni. The patient was asked to stop taking ciprofloxacin and start azithromycin. She also had her port removed on this day. Four days after her oncology visit, the patient’s blood was negative for organisms. Her fever and rigors resolved as well. Susceptibility testing showed the organism to be resistant to ciprofloxacin and susceptible to azithromycin.

Image 1. Gram stain from the blood shows faintly staining curved gram negative rods.
Image 2. Growth on chocolate agar after 3 days (Not in a microaerophilic environment).


Campylobacter jejuni is a small, gram negative, curved rod. It is the most common cause of bacteria-mediated diarrheal disease globally (1). In immunocompromised patients, it can cause a variety of extraintestinal diseases: septicemia, meningitis, septic arthritis, and endocarditis (2). Infection can also lead to Guillain-Barre syndrome. This is an autoimmune disease of the peripheral nerves that is thought to be due to antigenic cross reactivity between the surface lipopolysaccharides of the bacteria and the patient’s peripheral nerve gangliosides (3). Of patients diagnosed with Guillain-Barre syndrome, 20-40% will have had a history of Campylobacter jejuni infection (2).

Campylobacter jejuni grows best at 42°C in a microaerophilic environment (5% O2,10% CO2, and 80% N2). It displays a darting motility in broth and will not grow in 3.5% NaCl. Almost all are oxidase and catalase positive. On Campy-BA, a selective blood agar plate for Campylobacter jejuni, colonies will be peach colored. This media is made up of a Brucella agar base, sheep red blood cells, and various antibiotics that suppress the growth of normal fecal flora(2).

Macrolides (erythromycin, azithromycin) are considered the treatment of choice, however it has been reported that resistance approaches 1.7% (1 and 2). Fluoroquinolones like ciprofloxacin can be used, however resistance is higher due to widespread use of this drug in medical and veterinarian practices and agricultural businesses.


  1. Yang W, Zhang M, Zhou J, Pang L, Wang G, Hou F. The Molecular Mechanisms of Ciprofloxacin Resistance in Clinical Campylobacter jejuni and Their Genotyping Characteristics in Beijing, China. Foodborne Pathog Dis. 2017;14(7):386-392.
  2. Tille P. Bailey & Scott’s Diagnostic Microbiology. Fourteenth Edition. Elsevier;2017.
  3. Murray P. Medical Microbiology. Seventh Edition. Elsevier; 2013.

-Angela Theiss, MD is a 3rd 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.

Microbiology Case Study: A 14 Year Old Female with 3 Day History of Abdominal Pain

Case History

A 14 year old Caucasian female was transferred to the pediatric emergency department from an outside hospital due to a 3 day history of abdominal pain. The pain was concentrated in the right lower quadrant and was accompanied by nausea and two episodes of vomiting. Her vital signs were normal and physical exam demonstrated tenderness and guarding upon palpation of the right lower quadrant of the abdomen. Her white blood cell count was elevated at 21.1 TH/cm2 and showed 91% neutrophils. Abdominal ultrasound was unremarkable and CT scan was inconclusive for appendicitis. The patient was taken to surgery for an exploratory laparotomy and her appendix was removed.

Laboratory Identification

Image 1. Microscopic review revealed a female nematode with characteristic cephalic inflations, numerous eggs in the uterine cavity, and an intestinal cavity at the posterior end (H&E, 20x).
Image 2. Numerous oval shaped eggs with a thick shell that are slightly flattened on one side were observed (H&E, 200x). The eggs measured 55 x 25 um in dimension.

Gross examination of the appendectomy specimen showed an unremarkable appendix that measured 5.7 cm in length by 0.7 cm in diameter. There was no evidence of perforation. On sectioning, a small, white, “worm-like” structure (0.6 x 0.1 cm) was identified at the tip of the appendix and submitted for histologic examination. Microscopic review identified a female nematode with many eggs characteristic of Enterobius vermicularis. There was no acute inflammatory process identified upon microscopic review of the appendix.


Enterobius vermicularis, commonly referred to as pinworm, is a nematode infection that frequently presents as perianal itching in young children or those living in crowded settings, with symptoms most prominent in the evening and night time. Adult female worms reside in the cecum of the large intestine and migrate to the perianal area during the night to lay eggs, resulting in irritation. Often, infections can be asymptomatic as well. E. vermicularis is one of the most common helminthic infections in the United States.  

Humans are the only known host of E. vermicularis and become infected by ingesting embryonated eggs from feces or handling contaminated materials such as clothing, bed linens or from bathroom surfaces. Pinworm has a direct lifecycle and the larvae hatch in the small intestines and develop into adult worms that occupy the colon. It takes about one month from ingestion of infective eggs for E. vermicularis eggs then to be shed on the perianal folds.

Laboratory identification of E. vermicularis is usually made by using a piece of scotch tape or an adhesive paddle applied to the perianal skin in the morning and then visualizing the eggs microscopically. The eggs of pinworm are oval in shape & are flattened on one side with a thick capsule and measure between 50-60 x 20-30 um in size.On occasion, the eggs can be seen on pap smears as well. E. vermicularis worms can sometimes be visualized during colonoscopy, gastrointestinal & pelvic surgeries, and are capable of being identified by histology. Histologic sections of adult E. vermicularis worms usually show prominent lateral alae on the outer surface, testis or ovaries depending on the sex of the worm, and the intestinal tract. In gravid female worms, the characteristic eggs are numerous and can be helpful in the identification. 

Treatment options of an E. vermicularis infection include an initial dose of albendazole, mebendazole,or pyrantel pamoate followed by a second dose two weeks later to prevent possible reinfection. Family members and other close contacts may be treated as well ensure eradication. In the case of our patient, her post-surgical course was uneventful and her white blood cell count trended down to 7.0 TH/cm2 after surgery. She was discharged home the following day. 

-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. She is the Director of Clinical Pathology as well as the Microbiology and Serology Laboratories.  Her interests include infectious disease histology, process and quality improvement, and resident education.

Microbiology Case Study: A 31 Year Old Man with History of IV Drug Use

Case History

The patient is a 31 year old man with a history of intravenous drug use with last reported use nine months previous, who reports low back pain. The patient’s symptoms started as a mild pain and progressively worsened over two weeks to the point that he was unable to stand or ambulate. He also developed intermittent radiation of pain to the bilateral lower extremities and associated symptoms of chills and diaphoresis. Blood cultures were sent. MRI showed an epidural abscess at the level of L5-S1. The patient underwent lumbar spinal decompression surgery, and intra-operative cultures were sent for evaluation.

Laboratory Identification

Gram smear of blood cultures showed gram negative bacilli (Image 1). Culture of the abscess specimen and blood cultures showed growth on chocolate, blood, and MacConkey agar; growth on MacConkey plates did not show lactose fermentation (Image 2,3). MALDI-TOF identified this organism as Serratia marcescens.

Image 1. Gram stain of blood culture showing gram negative rods.
Image 2. Blood agar showing large colonies without hemolysis.
Image 3. MacConkey agar showing non lactose fermenting colonies.


Serratia marcescens is a motile, facultatively anaerobic, gram negative bacillus of the Enterobacteriaciae family. Some strains of Serratia produce a distinctive brick red pigment, prodigiosin (Image 4), although non pigmented strains are frequently isolated from human infection sites. Serratia marcescens is one of the few Enterobacteriacea that produces DNAse, lipase, and gelatinase. It does not usually ferment lactose. This species is widely present in the environment, including in animals, insects, plants, water, and soil, but unlike other Enterobacteriaciae species it is not a typical component of normal human fecal flora.

Image 4. Colonies of Serratia marcescens producing red pigment. Photo from the CDC Public Health Image Library (https://phil.cdc.gov/Details.aspx?pid=10544).

Eight species of Serratia have been found to cause infections in humans. Of these, >90% are caused by Serratia marcescens (1). This is a rare cause of infection in immunocompetent hosts but can cause opportunistic nosocomial infections, especially following invasive procedures such as such as intravenous catheterization, respiratory intubation, and urinary tract manipulations. The most common infections caused by Serratia marcescens are urinary tract infections, pneumonia,surgical wound infections, eye infections, and bacteremia. Multiple hospital outbreaks of Serratia have been reported, with sources of infection including tap water, soap, blood transfusion products, and injected medications (2). It has also been described as a cause of endocarditis in injection drug users (3).

Serratia is intrinsically resistant to ampicillin, ampicillin-sulbactam, and 1st and 2nd generation cephalosporins due to an inducible, chromosomal AmpC beta-lactamase. Resistance to later-generation cephalosporins may be induced through exposure to these antibiotics, despite not being detected on initial antibiotic susceptibility tests. Thus, susceptibility testing is misleading and thirdgeneration cephalosporins (such as ceftazidime, ceftriaxone, and cefpodoxime) should be avoided for the treatment of Serratia species regardless of in vitro susceptibility.


  1. Laupland KB, Parkins MD, Gregson DB, Church DL, Ross T, Pitout JD. Population-based laboratory surveillance for Serratia species isolates in a large Canadian health region. Eur J Clin Microbiol Infect Dis. 2008; 27: 89–95.
  2. Mahlen SD. Serratia infections: from military experiments to current practice. Clin Microbiol Rev. 2011; 24:755.
  3. Mills J., Drew D. Serratia marcescens endocarditis: a regional illness associated with intravenous drug abuse. Ann Intern Med. 1976; 84:29–35.

-Erica Worswick is a pathology student fellow 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.