Hematopathology Case Study: A 64 Year Old Man with Widespread Lymphadenopathy

Case history

A 64-year-old, previously healthy man presented with a history of cervical and axillary lymphadenopathy of unknown duration. He did not endorse night sweats, weight loss, or fever. Radiologic examination (CT chest and MRI abdomen) revealed numerous enlarged mediastinal, peritracheal, periaortic, periportal and retroperitoneal lymph nodes. He underwent excisional biopsy of a 3.5 cm axillary lymph node.


Microscopic Description

Histologic examination of the node revealed distortion of nodal architecture by a proliferation of neoplastic-appearing follicles. Follicles were distinct from one another, and closely packed. In areas the follicles were present back-to-back. Follicular centers were comprised of mostly small, cleaved centrocytes and showed no obvious zonation. There was loss of tingible body macrophages.


Immunohistochemical analysis revealed CD20-positive B cells in a follicular pattern. The germinal centers revealed an underlying follicular dendritic meshwork highlighted by staining for CD21. Interestingly, while the germinal centers demonstrated immunopositivity for BCL-6, there was minimal to absent CD10 staining on follicular B cells. Analysis of BCL-2 staining revealed only few cells to be positive within the follicular centers, consistent with resident follicular helper T cells (Th cells). Equivalent numbers of CD3 and CD5 positive T cells were noted in the interfollicular zones. The Ki-67 proliferation index was estimated at 15-20% within follicular centers. Flow cytometric phenotyping demonstrated a lambda light chain restricted clonal B-cell population expressing CD20, CD19 and, FMC7. These neoplastic B-cells were negative for CD5 and CD10 expression.


The morphologic features were consistent with Follicular Lymphoma; however the phenotype (BCL-2 negativity in follicular centers) was unusual for this diagnosis. Fluorescence in situ hybridization (FISH) was negative for an IgH/BCL-2 fusion; however, a BCL-6 rearrangement at the 3q27 locus was detected in 70% of the cells.  Taken together, a diagnosis of Follicular Lymphoma with a BCL-6 rearrangement was given.


Follicular lymphoma (FL) is a germinal center derived B-cell neoplasm. The majority of cases exhibit the pathognomonic translocation t(1418)(q32; q21). This translocation leads to overexpression of the anti-apoptotic BCL-2 protein, which can be detected by immunohistochemistry on germinal center B cells. Lymphoma cells are usually positive for germinal center origin markers BCL-6 and CD10 and do not co-express CD5. As exhibited in this case, FL can exhibit biologic heterogeneity and may not express these typical markers. The follicular proliferation with absence of germinal center zonation and tingible body macrophages as seen in this case represents classic morphology of follicular lymphoma but aberrant phenotypic markers [and absence of t(14;18)] may be a pitfall in this diagnosis.

FL with lacking of CD10 expression, BCL-2 expression, and t(14;18) translocation and harboring only BCL-6 positivity with 3q27 rearrangement is rare. Only few such cases have been reported in the literature. Published data reveals that the hallmark t(14;18) translocation is absent in about 10-15% of FL. The majority of these cases are negative for BCL-2 expression, and 9-14% of them demonstrate BCL-6 rearrangement (3q27 locus). While BCL-6 rearrangement can be present in both the usual t(14;18) harboring FL, and also in cases without t(14;18), the latter is rare. Interestingly, studies have shown BCL-6 rearrangements to be more frequent in in BCL-2 rearrangement negative FL – which is evidence of the anti-apoptotic role of non-rearranged BCL-6 in certain microenvironments.

One third of t(14;18) negative FL are also reported to have rare or negative expression of CD10. Morphologically, this subtype has been shown to have significantly larger follicles than  their t(14;18)-positive counterparts, but the distinction may not be obvious in all cases. Some of these cases are shown to have a component of monocytoid B cells. This findings can be problematic in differentiating these FL cases from marginal zone lymphoma (MZL) that can also harbor BCL-6 rearrangements and lack t(14;18), CD10 and BCL-2 positivity. Absence of prominent marginal zone proliferation, BCL-6 protein expression and characteristic genetic alterations present in MZL, such as trisomies 3, 7, and 18 can help differentiating MZL from t(14;18)-negative FL.

This case highlights the importance of morphologic evaluation of a excisional biopsy tissue, and FISH studies to help identify the rare t(14;18) negative FL. While the reported cases are few, there is no published difference in prognosis or survival when compared to t(14;18)-positive FL. As such, it is not clear whether the follicular lymphoma grading scheme applies to t(14;18)-negative FL; however, no significant grading difficulties or differences have been reported.


  1. Jardin F, Gaulard P, Buchonnet G, et al. Follicular lymphoma without t(14;18) and with BCL-6 rearrangement: a lymphoma subtype with distinct pathological, molecular and clinical characteristics. Leukemia. 2002;16:2309–2317
    2. Leich E, Salaverria I, Bea S, et al. Follicular lymphomas with and without translocation t(14;18) differ in gene expression profiles and genetic alterations. Blood. 2009;114(4):826-834.



Aadil Ahmed, MD is a 3rd-year anatomic and clinical pathology resident at Loyola University Medical Center. Follow Dr. Ahmed on Twitter @prion87.


-Kamran M. Mirza, MD PhD is an Assistant Professor of Pathology and Medical Director of Molecular Pathology at Loyola University Medical Center. He was a top 5 honoree in ASCP’s Forty Under 40 2017. Follow Dr. Mirza on twitter @kmirza.

Microbiology Case Study: A 73 Year Old Male with Fever, Lethargy, and Chills

Case History

A 73-year-old man presents to his primary care provider during the height of a bad influenza season with fever, lethargy, and chills. Symptoms started 24 hours prior to presentation. A rapid influenza rapid test was performed in the physician’s office and the result was negative for influenza A and B. What is the most likely cause of this man’s illness?


Influenza…but how can that be?


Rapid antigen testing has been the mainstay for influenza testing since the 1980’s. These tests detect influenza A and B viral nucleoprotein antigens in respiratory specimens, giving a qualitative “positive” or “negative” result. Antigen testing was developed to shorten the turnaround time to results for common respiratory viruses influenza and respiratory syncytial virus (RSV), with an assay run time of approximately 15 minutes compared to the several days it takes for influenza detection by viral culture. Rapid antigen testing is very easy to perform, allowing CLIA-waived testing to be performed at point-of-care.

Unfortunately, rapid antigen testing has poor sensitivity. The most comprehensive analysis found the sensitivity of rapid antigen testing to be around 60% in adults and slightly higher (although still not good) in children. Due to the poor sensitivity, the CDC recommends only employing rapid antigen testing when the prevalence of influenza in the community is >10%…but why does the prevalence matter? Knowing the prevalence of a disease in your population allow you to calculate the positive and negative predictive value.

Positive and negative predictive values answer the question, “What is the chance that my positive test result means my patient has the disease (PPV) or what is the chance that my negative test result means my patient does not have the disease (NPV).” You can calculate the PPV or NPV of any assay by knowing the sensitivity and specificity of an assay along with the prevalence of disease in the community (Figure 1).

Figure 1. Calculation of positive and negative predictive values.

Positive and negative predictive values fluctuate with the amount of disease seen in a community. For example, if testing for polio in the United States, where the virus has been eradicated, a positive test result by any method is far more likely to be a false-positive than a true-positive result. This is due to the low positive predictive value (PPV) of a positive test result in the setting of non-existent polio. The converse is true for negative predictive values (NPV). In the height of influenza season, a negative test result for influenza in a patient with signs and symptoms of influenza disease is more likely to be a false-negative than a true-negative result.

For influenza rapid antigen testing, the PPV is highest when influenza activity in the community is high (positive test result is likely to indicate influenza infection) and the PPV is lowest when influenza activity is low in the community is low such as in summer, when a positive influenza test result is most likely to be a false-positive result.

Conversely, NPV is highest when influenza activity is low in a community, and a negative test result is most likely indicating that the patient does not have influenza infection. NPV is lowest when influenza activity in a community is high, and a negative test result is more likely to indicate a false-negative result in a patient with influenza infection.

The specificity of rapid antigen assays is tied to the circulating influenza viral subtypes in a given season, and is generally quite high. Sensitivity and specificity do not change due to the prevalence of disease in the community, unlike positive and negative predictive values.



  1. Centers for Disease Control and Prevention (CDC) website on influenza testing (https://www.cdc.gov/flu/professionals/diagnosis/clinician_guidance_ridt.htm)
  2. Altman Douglas G, Bland J Martin. Statistics Notes: Diagnostic tests 2: predictive values BMJ 1994; 309 :102
  3. Chartrand C, Leeflang MM, Minion J, Brewer T, Pai M. Accuracy of Rapid Influenza Diagnostic Tests: A Meta-analysis. Ann Intern Med. 2012;156:500–511.doi: 10.7326/0003-4819-156-7-201204030-00403


-Erin McElvania, PhD, D(ABMM), is the Director of Clinical Microbiology NorthShore University Health System in Evanston, Illinois.

Microbiology Case Study: A 51 Year Old Female with New Onset Progressive Weakness

Case History

A 51 year old female with a past medical history for migraines was otherwise healthy up until 6 weeks ago when she began to notice progressive weakness, myalgia and new onset spontaneous lower extremity bleeding. She was evaluated by the internal medicine service and was found to be profoundly thrombocytopenic. A further workup consisting of a bone marrow biopsy revealed findings that were consistent with high-grade (Burkitt’s) lymphoma. She was initiated on chemotherapy. Two days after initiating chemotherapy she became profoundly pancytopenic with recurrent fevers. Additionally, she had worsening erythema and pain in her right buttocks and left thigh. Despite the usage of broad-spectrum antibiotics, her symptoms worsened. Two sets of blood cultures were drawn and the anaerobic bottles of both sets flagged positive after 15 hours.

Lab Identification

Gram stain revealed gram positive rods, some of which did not retain the crystal violet stain but all appeared box car shaped. This organism only grew under anaerobic conditions. On the anaerobic blood plate, the organism swarmed the media.

Image 1. Gram stain from a positive blood bottle showing gram positive rods (100x oil immersion).
Image 2. Anaerobic growth on blood agar showing a few colonies with a lawn of growth.

By use of mass spectrometry, the MALDI-TOF positively identified the organism as Clostridium septicum.


Clostridium septicum is a gram positive, highly motile, spore-forming organism that grows best under anaerobic conditions. It is found ubiquitously in soil and at a low prevalence rate in the human gastrointestinal tract. C. septicum is best known for its ability to cause neutropenic enterocolitis and “atraumatic” (spontaneous) gas gangrene which is in contrast to “traumatic” gas gangrene caused by C. perfrigens .2 Both neutropenic enterocolitis and atraumatic gas gangrene are commonly seen in association with a malignancy usually hematologic or gastrointestinal in nature. Neutropenic entercolitis is mostly commonly seen in patients that are undergoing chemotherapy treatment. A combination of mucosal injury by the cytotoxic drugs, profound neutropenia, and impaired host defense allows for edema and necrosis of the bowel wall by microorganisms.1 This then allows for hematogenous dissemination of gut flora which includes C. septicum to more distal sites. It is also possible to have weakness in the mucosal lining from mass effect alone without any preceding neutropenia also allowing for hematogenous dissemination. In animal models C. septicum has been shown to be much more virulent than C. perfrigens requiring 300x fewer organisms to have the same lethal effect.2 This lethal infection even with appropriate treatment has a mortality rate of 60%.3

To diagnose C. septicum a gram smear will show gram positive rods with occasional rare sub terminal or terminal spores. They can often appear pleomorphic. They grow under anaerobic conditions and may start out as a single solid colony but usually swarm the plate after 24 hours growth. A more conclusive diagnosis can be made on the MALDI-TOF using mass spectrometry. Effective treatment requires both debridement of infected sites and appropriate antibiotics. The Infectious Diseases Society of America (IDSA) guidelines for skin and soft tissue infections recommend the use of high dose IV penicillin and IV clindamycin.4 Clindamycin is a protein synthesis inhibitor and is believed to aid in preventing toxin synthesis.



  1. Urbach DR, & Rotstein, OD. Typhlitis. Cancer J Surg 1999; 42(6):3 415-419.
  2. Srivastava I, Aldape MJ, Bryant AE, Stevens DL, Spontaneous C. septicum gas gangrene: A literature review, Anaerobe (2017).
  3. Larson CM, Bubrick MP, Jacobs DM, West MA. Malignancy, mortality, and medicosurgical management of Clostridium septicum infection. Surgery. 118(4):592–597
  4. Stevens DL, Bisno AL, Chambers HF, et al. Executive Summary: Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections: 2014 Update by the Infectious Diseases Society of America. Clinical Infectious Diseases 2014; 59:147–159.


-Noman Javed, 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: 6 Year Old Male with Meningitis

Case History

A 6 year old male presented to the emergency department with a concern for ventriculo-peritoneal shunt (VP) malfunction. His past medical history is significant for myelomeningocele and hydrocephalus since birth. On arrival, symptoms included high fever (102.7°F), headaches and swelling at the VP shunt catheter site in the neck. Over the past week, his mother also noted nausea, vomiting and diarrhea. CT scan of the head revealed increased size of the 3rd and lateral ventricles which was concerning for either a VP shunt malfunction or infection. Lab work showed a white count of 13.5 TH/cm2 and elevated CRP values suggestive of an infection/inflammatory process. He was taken to surgery for VP shunt removal and placement of an external ventricular drain (EVD). Intra-operatively, purulent yellow material was noted at both the proximal and distal ends of the catheter. Cerebrospinal fluid (CSF) was sent for Gram stain and bacterial culture. He was started on vancomycin and ceftriaxone.

 Laboratory Identification

Image 1. CSF Gram stain prepared from the cytospin showed many white blood cells and Gram positive bacilli (100x oil immersion).
Image 2. Gram stain from the liquid media culture showing gram positive bacilli (100x oil immersion).
Image 3. Small, grayish colonies with a narrow zone of beta hemolysis grew on blood agar after 48 hours incubation in a 35°C incubator with 5% CO2.

Bacterial cultures collected from a shunt tap and intra-operatively both showed short gram positive bacilli on Gram stain (Image 1&2). The organism grew on blood and chocolate agars as small, gray colonies with a narrow zone of beta-hemolysis when observed closely (Image 3) after incubation at 35°C in CO2. The isolate was positive for catalase and showed a “tumbling motility.” MALDI-TOF MS identified the isolate as Listeria monocytogenes.


Listeria species are gram positive bacilli that grow as facultative anaerobes and do not produce endospores. The major human pathogen in the Listeria genus is L. monocytogenes and it is found in soil, stream water, sewage & vegetable matter and may colonize the gastrointestinal tract of humans and animals.

The most common mode of transmission is ingestion of contaminated foods, in particular, raw milk, soft cheeses, deli meats and ice cream. L. monocytogenes’ ability to grow at cold temperatures (4°C) permits multiplication in refrigerated foods. In a healthy adult, it causes an influenza like illness and gastroenteritis. Pregnant women are especially susceptible to disease and neonates infected in utero can develop granulomatosis infantiseptica which can lead to miscarriage, stillbirth or premature delivery. Elderly or immunocompromised can present with a febrile illness, bacteremia and meningitis (20-50% mortality).

In the microbiology laboratory, L. monocytogenes is usually identified via blood, CSF or placental bacterial cultures. It grows well on standard agars and after overnight incubation, the small, gray colonies show a narrow zone of beta hemolysis on blood agar. L. monocytogenes is positive for catalase & esculin and the CAMP test demonstrates block like accentuated hemolysis. It has characteristic tumbling motility at room temperature and an umbrella shaped motility pattern in semi-solid agar.  Automated methods of identification provide reliable species level differentiation on the majority of current platforms.

Susceptibility testing should be performed on isolates from normally sterile sites. Ampicillin, penicillin, or amoxicillin are given for L. monocytogenes, and gentamicin is often added for its synergistic effect in invasive infections. Trimethoprim-sulfamethoxazole and vancomycin can be used in cases of allergy to penicillin. Cephalosporins are not effective for treatment of listeriosis.

In the case of our patient, after L. moncytogenes was identified, his antibiotic therapy was changed to ampicillin and gentamicin. Antibiotics were administered for 3 weeks before the placement of a new VP shunt. On further questioning, his mother revealed his diet consisted heavily of hot dogs and soft cheeses. She was educated on how to prevent subsequent infections prior to discharge.



-Jaspreet Kaur Oberoi, MD, is a 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. She is the director of the Microbiology and Serology Laboratories. Her interests include infectious disease histology, process and quality improvement and resident education.

Microbiology Case Study: An 89 Year Old with an Infected Wound

Case History

An 89-year-old gentleman presented with a cough, subjective fever, and potential wound infection on his forehead. He has a past medical history notable for hyperlipidemia, coronary artery disease, chronic congestive heart failure, past STEMI, history of gastric lymphoma, diabetes, Parkinson disease, and stage III chronic kidney disease. Two months prior to presentation, he fell down the steps of his apartment and suffered a laceration of his forehead that ultimately required surgical repair. He was discharged home with wound care instructions. He was doing relatively well up to 5 days prior to presentation when he started to develop a small, soft, erythematous lesion on his forehead with swelling and non-purulent appearing serous drainage at the site of repair. After it persisted, he presented to the emergency department where he underwent a small incision and drainage. A specimen was collected in a sterile syringe and sent to the microbiology laboratory for culture.

Image 1. Gram stain from a fluid culture illustrating filamentous branching gram-positive bacilli (100x, oil immersion).
Image 2. Modified Kinyoun stain from a fluid culture illustrating filamentous branching modified acid fast bacilli (100x, oil immersion).
Image 3. Chocolate agar illustrating chalky white colonies.

The organism was confirmed as Nocardia abscessus/asitica by a reference laboratory.


Nocardia is a genus of aerobic, weakly Gram-positive, catalase-positive, rod shaped bacteria that are modified acid-fast and form beaded branching filaments. They grow slowly on commonly used nonselective culture media, with colonies becoming evident in 3-5 days, and have the ability to grow in a wide temperature range. Colony morphology is variable, with colors ranging from white to orange and texture ranging from smooth to chalky. They are saprophytic organisms that are commonly found in soil, organic matter, and in the oropharynx as normal flora. Virulence factors for Nocardia include catalase, superoxide dismutase, and cord factor. Cord factor prevents lysosomal fusion with the phagosome, thus inhibiting phagocytosis by macrophages.

There are more than 80 species of Nocardia causing various forms of human disease, the most pathogenic of which are: N. asteroides, N.braziliensis, N.caviae, N.nova and N.abscesses. Symptoms can range from a localized lung or cutaneous infection to disseminated disease. Most infections involve the lung initially following inhalation of the organisms which commonly spread to extrapulmonary sites with the disease being more severe and likely to disseminate in immunosuppressed patients. In addition, the skin can be a site of primary infection through traumatic inoculation in immunocompetent patients.

Primary cutaneous infections occur in 5% of cases and manifest in one of the following ways: lymphocutaneous infection, mycetoma, superficial cellulitis, or localized abscesses. The most commonly involved sites for cutaneous infections are the extremities, though infection can affect any area, including the head and neck.

Treatment for primary cutaneous nocardiosis includes antimicrobial therapy and surgical irrigation and drainage when appropriate. Though antibiotic therapy is recommended, spontaneous resolution can occur without treatment. Trimethoprim-sulfamethoxazole is used most frequently for nocardiosis with the usual duration of therapy being 2-3 months in those cutaneous disease.


  1. Wilson JW. Nocardiosis: Updates and Clinical Overview. Mayo Clinic Proceedings. 2012;87(4):403-407. doi:10.1016/j.mayocp.2011.11.016.
  2. Vijay Kumar GS, Mahale RP, Rajeshwari KG, Rajani R, Shankaregowda R. Primary facial cutaneous nocardiosis in a HIV patient and review of cutaneous nocardiosis in India. Indian Journal of Sexually Transmitted Diseases. 2011;32(1):40-43. doi:10.4103/0253-7184.81254.
  3. Lee TG, Jin WJ, Jeong WS, et al. Primary Cutaneous Nocardiosis Caused by Nocardia takedensis. Annals of Dermatology. 2017;29(4):471-475. doi:10.5021/ad.2017.29.4.471.
  4. Outhred, A.C., Watts, M.R., Chen, S.CA. et al. Nocardia Infections of the Face and Neck. Curr Infect Dis Rep 2011 Apr;13(2):132-40. doi: 10.1007/s11908-011-0165-0.


-Clayton LaValley, MD is a 2nd 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.


Lymphocytosis Can Be Anything

Case History

A 63 year old patient presented with a high white cell count of 108 K/uL and thrombocytopenia of 110 K/uL.

Peripheral smear examination revealed marked lymphocytosis with presence of numerous small to medium sized lymphoid cells with round to oval nuclei, clumped nuclear chromatin and variable amount of cytoplasm, some with cytoplasmic projections. As the features were consistent with a lymphoproliferative disorder peripheral blood was sent for flow cytometry.



Based on the morphology the differential diagnosis included B-cell lymphoproliferative disorders such as marginal zone lymphoma, hairy cell leukemia/variant, or less likely chronic lymphocytic leukemia and/or mantle cell lymphoma.

Flow cytometry revealed presence of clonal B-cells expressing CD19, CD20, Cd11c, CD103 and FMC-7. The cells were negative for CD5, CD10, and CD25.

The phenotype together with the morphology and CBC findings were diagnostic of hairy cell leukemia variant.


Hairy cell leukemia variant ( HCL-v) is a B-cell lymphoproliferative disorder that resembles classic hairy cell leukemia but exhibits variant cytological and hematological features such as leukocytosis and also shows variant immunophenotype including absence of CD25, CD123 and/or annexin A1.

HCL-v is about one tenth as common as HCL (hairy cell leukemia) with an annual incidence of approximately 0.03 cases per 100,000 population. There is slight male preponderance. Patients with HCL-v typically present with leukocytosis with an average WBC of 30 K/ul and /or thrombocytopenia.

The 5 year survival rate is around 50-60%. Most patients require therapy which can range from splenectomy to combination chemotherapy with Rituximab.



  1. WHO classification of Tumors of Haematopoietic and Lymphoid Tissues; IARC 2017


-Neerja Vajpayee, MD, is the director of Clinical Pathology at Oneida Health Center in Oneida, New York and is actively involved in signing out surgical pathology and cytology cases in a community setting. Previously, she was on the faculty at SUNY Upstate for several years ( 2002-2016) where she was involved in diagnostic work and medical student/resident teaching.

Microbiology Case Study: A 74 Year Old Female with Right Knee Swelling

Case History

A 74 year old female presented to the ED with a chief complaint of fever, right knee swelling and pain for three days. Past medical history was significant for a right total knee arthroplasty approximately 5 months prior, with no significant complications. Physical exam revealed the patient to be febrile (103 degrees Fahrenheit), a swollen right knee that was warm to the touch and erythema surrounding the surgical incision site. Routine labs were obtained while in the ED which revealed a leukocytosis with an elevated ESR and CRP. Imaging was ordered and showed a large joint effusion of the right knee with intact hardware. Arthrocentesis was performed which returned 80 cc of cloudy yellow fluid with no crystals identified by light microscopy, a nucleated cell count of 169,200/cmm of which 97% were neutrophils.

Laboratory Identification

The primary gram stain was reported as polys and gram negative bacilli present. Cultures revealed a pure moderate growth on sheep blood and chocolate agar with no growth on the MacConkey agar. Colony morphology on the sheep blood agar was smooth, gray with no hemolysis appreciated. The key biochemical and physiologic characteristics of the isolate included: positivity for indole, nitrate reduction, catalase, ornithine decarboxylase, and fermentation of mannitol and sucrose; negativity for urea and maltose fermentation.  The isolate was identified by MALDI-TOF as Pasteurella multocida. Upon further questioning, the patient admitted to living with two indoor cats but denied any recent history of bites or scratches.

Image 1. Chocolate agar with smooth gray colonies.


Pasteurella multocida is a non-motile, oxidase positive, small -gram negative bacilli capable of fermenting glucose. This organism is part of the normal flora of the gastrointestinal tract and nasopharynx of wild and domestic animals. Humans who have extensive exposure to animals may be found to have Pasteurella multocida as part of their upper respiratory tract flora. With no significant virulence factors, this organism is often viewed as an opportunistic pathogen which requires mechanical disruption of anatomic barriers as occurs with bite and scratch wounds from cats and dogs. Though most infections are associated with bites or scratched from animals, infection can occur with non-bite exposure to animals. The typical disease caused by Pasteurella multocida is a focal soft tissue infection following a bite or scratch. However, chronic respiratory infections in patients with preexisting chronic lung disease and heavy animal exposure, and bacteremia with metastatic abscess formation have been documented.

Biochemical characteristics can be utilized in identifying the different Pasteurella species. The key biochemical and physiologic characteristics for Pasteurella multocida include: positivity for indole, nitrate reduction, catalase, ornithine decarboxylase, and fermentation of mannitol and sucrose; negativity for urea and maltose fermentation.

The vast majority of these organisms are susceptible to penicillin, thus susceptibility testing is generally unnecessary. Additionally, soft tissue infections caused by animal bites are frequently polymicrobial and warrant use of therapeutics with a broader spectrum. However, should the need arise to perform susceptibility testing, the Clinical and Laboratory Standards Institute (CLSI) does provide break points for Pasteurella multocida.


  1. Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott’s Diagnostic Microbiology. Mosby; 2007.
  2. Koneman EW. Koneman’s Color Atlas and Textbook of Diagnostic Microbiology. Lippincott Williams & Wilkins; 2006.


-Justin Rueckert, DO is a 2nd 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.