Microbiology Case Studies: Babesia vs. Malaria

Patient History: Case 1

A 55 year old Asian woman presented to an emergency department in southern New England in September with complaints of a high fever with chills for the past 5 days. She noted feelings of excessive tiredness, muscle aches, and headache. She also described a decrease in appetite and nausea with vomiting and diarrhea. On physical exam, she was febrile (103.8°F) and scleral icterus was identified. Laboratory workup revealed findings suggestive of hemolysis including increased LDH (401 U/L) and increased unconjugated bilirubin (1.7 mg/dL), despite hemoglobin & hematocrit values in the normal range (13.7 g/dL & 39.3%, respectively). Elevated liver enzymes were also noted; AST 81 U/L and ALT 72 U/L. When questioned regarding traveling history, she reported a trip to Spain and Portugal 5 months earlier. Though she acknowledged living in a rural area of the Northeastern U.S. and indicated that her husband was diagnosed with Lyme disease one year earlier, she denied both recent time outdoors and arthropod or mosquito bites.

Patient History: Case 2

A 31 year old African American woman with a history of sickle cell trait presented to an emergency department in southern New England in September complaining of fevers of 5 days duration. She described being asymptomatic in the mornings followed by high spiking fevers with muscle aches and dull frontal headaches in the evenings. A physical exam revealed a fever (103°F), but no evidence of meningismus. Laboratory workup revealed a mild, microcytic anemia (hemoglobin & hematocrit: 10.7 g/dL & 32.5%, MCV: 76.3 fL), a decreased absolute lymphocyte count and increased band neutrophils. When questioned regarding recent travel, she reported having returned from Africa 10 days earlier. While abroad, she had primarily been in Nigeria’s capital, but she had also visited rural areas. She did not recall having been bitten by mosquitos, but she did not take any anti-malarial prophylaxis. Further, she denied both recent travel to the woods in the Northeastern U.S. and recent arthropod bites.

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Figure 1. Peripheral blood smear from patient 1 showing ring-like forms which contain a small amount of cytoplasm and a chromatin dot as illustrated by the arrows. Both intra-erythrocytic and extra-cellular forms are present. Platelets are denoted by arrowheads.

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Figure 2. BinaxNOW lateral flow assay from patient 1 is negative for the various Plasmodium spp.

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Figure 3. Peripheral blood smear from patient 2 showing ring forms and trophozoites within red blood cells as denoted by arrows. Inset illustrates a scattered gametocyte. Platelets are denoted by arrowheads.

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Figure 4. BinaxNOW lateral flow assay from patient 2 is positive for non-falciparum malaria species as indicated by the faint positive reaction denoted by the red arrow in the T2 region.

 

For patient 1, given the results from the peripheral blood (Figure 1), the negative BinaxNow results (Figure 2) and her lack of recent travel to malaria endemic regions, her illness was attributed to infection by Babesia spp. Further serologic testing was positive for Babesia microti. She was seronegative for Anaplasma phagocytophilum, Borrelia burgdorferi, Ehrlichia chaffeenesis. This finding was confirmed by PCR of her blood, which detected B. microti, but failed to detect B. duncani or B. divergens/MO-1. Approximately 3% of her red blood cells contained intracellular parasites.

For patient 2, her disease was most consistent with an infection by a non-falciparum species of malaria, including P. ovale, P. vivax or P. malariae, given her recent travel to Nigeria and advanced forms seen in the peripheral blood (Figure 3). Further speciation was uncertain due to low parasitemia levels (<1%) and the findings were unable to exclude a mixed infection with a low P. falciparum burden.

Discussion

The clinical and laboratory presentations of babesiosis and malaria are quite similar despite the fact that each infection is caused by a distinct and highly unique microorganism. As seen in the two cases above, both illnesses often begin insidiously with fevers, headache and muscle & joint aches. The non-specific nature of the patient’s symptoms results in an unclear etiology unless key elements of the patient’s history, including exposure to insect and arthropod vectors and travel or habitation in endemic areas, are provided.

Examination of thick and thin blood smears is useful in the diagnosis of these two diseases. While both organisms have a very similar sized lifecycle forms which selectively infect red blood cells and prompt hemolysis, there are a few useful distinguishing characteristics. In the case of babesiosis, which is transmitted by the Ixodes scapularis tick in the United States, there are small ring like structures, both within red blood cells and extra-cellularly. The diagnostic tetrad form, known as a Maltese Cross, is helpful if identified but is not frequently observed in human infections. No advanced forms or pigment is present. In the case of malaria, which is transmitted via the female anopheline mosquito, protozoa are only found within red blood cells and advanced forms, including schizonts or gametocytes, are helpful in further speciation, if present. Other features, such as size of the infected red cell, number of merozoites, level of parasitemia and gametocyte shape, are helpful in the morphologic assessment of the Plasmodium spp.

Due to the pathogenic severity of P. falciparum, it is important that the microbiology laboratory has the ability to make the diagnosis in real time across all shifts. The BinaxNOW is an FDA approved lateral flow assay that is simple to perform and provides rapid diagnostics, though it isn’t as sensitive as microscopy. The test is comprised of two antigens: one specific to P. falciparum (T1) and one antigen common to all Plasmodium spp. (T2). The test will be positive for levels of parasitemia greater than 5,000 parasites per microliter.

As utilized in the above cases, other various laboratory modalities can aid in the diagnosis of babesiosis and malaria, including serologic tests and PCR, however, these tests may not be available in STAT situations. Using a variety of tests and obtaining a thorough travel history, will help the provider arrive at the correct diagnosis of blood protozoa.

 

lavik

-JP Lavik, MD, PhD, is a 3rd year Anatomic and Clinical Pathology Resident at Yale New Haven Hospital.

Stempak

-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.

Minimal Inhibitory Concentrations and Antimicrobial Dosing: How are they related?

Microbiology laboratories use clinical breakpoints to categorize microorganisms as susceptible, intermediate, or resistant. These breakpoints help guide the selection of antimicrobial therapy having a high likelihood of achieving therapeutic success in patients. One in vitro marker of antimicrobial activity is the minimal inhibitory concentration (MIC), the lowest concentration of antibiotic that prevents visible growth of a standard bacterial inoculum. National committees, such as the Clinical Laboratories Standards Institute (CLSI), Food and Drug Administration (FDA), and The European Committee on Antimicrobial Susceptibility Testing (EUCAST) define clinical practice breakpoint MIC values for each bacterial genus. These defined values are determined using wild type value distributions in relation to what serum drug levels are achievable with standard antimicrobial dosing.

All drugs have individual pharmacokinetic properties such as absorption, volume of distribution, and rate of elimination. These factors contribute to what concentration of drug will be achieved at a certain site of infection. A good index of overall antibiotic exposure in a patient is the serum area-under-the-curve (AUC), which is influenced directly by the drug dose and clearance. Antibiotics also have pharmacodynamic properties, which relate to the drug’s effect on the microorganism over time. There are two main groups of pharmacodynamic characteristics seen with antimicrobial agents: time-dependent bactericidal action (Figure 1) and concentration-dependent bactericidal action (Figure 2). The clinical efficacy of an antibiotic is related to the relationship between pharmacokinetic/pharmacodynamic (PK/PD) parameters of a drug and the MIC of the specific organism. Bacterial strains with an increase in MIC may exhibit relative resistance by in vitro laboratory standards, but because there is no increase in the PK/PD parameters, the increased MIC can sometimes be overcome by altering dosing regimens to optimize the drug concentrations achieved.

For example, β-lactam antibiotics exhibit time-dependent killing activity, so dosing regimens which maximize duration of exposure to drug concentrations above the MIC of the organism are particularly effective for treating bacteria with this this class of antibiotics. Prolonged infusion times and smaller fractions of total daily doses given more frequently are two strategies through which this can be achieved. For drugs exhibiting concentration-dependent killing, such as aminoglycosides, dosing regimens can be optimized by giving a higher dose in order to achieve higher peak concentrations. The pharmacokinetic properties of drug can also be used to overcome elevated MICs for some organisms depending on the site of the infection. A good example of this would be urinary tract infections. Antibiotics that achieve high concentrations in the urine, such as aminoglycosides, can be used to successfully treat organisms with elevated MICs. Therefore, while healthcare providers utilize breakpoint MIC values to select antimicrobial regimens, understanding characteristics of an antimicrobial, including PK/PD parameters and tissue distribution, along with taking into account the site of infection and the MIC of the infecting organism, can provide the opportunity for optimization of antimicrobial dosing strategies.

fig1

Figure 1. For antibiotics which confer time-dependent antimicrobial activity, microbial killing is optimized when the concentration of antibiotic is above the MIC for as long of a time period as possible.

fig2

Figure 2. For antibiotics which confer concentration-dependent antimicrobial activity, microbial killing is optimized when a high peak concentration of antimicrobial is achieved.

 

References:

  1. Mouton JW, Brown DFJ, Apfalter P et al. The role of pharmacokinetics/pharmacodynamics in setting clinical MIC breakpoints: the EUCAST approach. Clin Microbiol Infect. 2012;18:E37-E45.
  2. Levison ME, Levison JH. Pharmacokinetics and pharmacodynamics of antibacterial agents. Infect Dis Clin North Am. 2009;23(4):791-vii.
  3. MacGowen AP. Role of pharmacokinetics and pharmacodynamics: does the dose matter? CID. 2001;33(suppl 3):S238-239.
  4. Martinez MN, Papich MG, Drusano GL. Dosing regimen matters: the importance of early intervention and rapid attainment of the pharmacokinetic/pharmacodynamic target. Antimicrob Agents Chemother. 2012;56(6):2795-2805.

 

burns

-Alaina Burns, PharmD, is a PGY-2 Pediatric Pharmacy Resident at Children’s Health, Children’s Medical Center in Dallas, Texas.

Facing CJD and Prion Diseases in the Lab

According to the National Institute of Health (NIH), Creutzfeldt-Jakob disease (CJD) is a rare, degenerative, fatal brain disorder that affects about one person in every one million people per year worldwide. In the United States there are about 300 cases per year. Some of us know the ailment better as “Mad Cow disease,” but that is only one form of this illness which is not caused by a virus or bacteria. CJD is a prion disease. A prion is a protein that exists in both a normal form, which is a harmless, and in an infectious form. The infectious form of the protein takes on a different folded shape, and once these abnormal proteins appear, they aggregate or clump together. Investigators think these prion aggregates may lead to the neuron loss and other brain damage seen in CJD. However, they do not know exactly how this damage occurs.

Since laboratory professionals may deal with specimens from possible CJD patients, we need to know how to properly handle them should such a situation arise. If the Operating Room calls your labs to process a brain biopsy specimen from a patient who was suspected of having a prion disease, would you know what to do? Can your lab do that? Should your lab do that?

Prions are dangerous, but CJD cannot be transmitted through the air or through touching or most other forms of casual contact. Prion transmission can occur, however, from contact with highly-infectious specimens. Brain tissue, eye tissue, and pituitary tissue are considered high-risk specimens, and contact with these should be avoided. When asked to handle a brain biopsy, medical staff and safety experts should work out a plan. For instance, a lab tech who is trained in Category A packaging could go to the OR, dress in fully protective PPE (including a body suit, gloves, and hood), and receive the specimen in the OR and package it there. The specimen is then ready for transport to the reference laboratory. If another department asks you to handle tissue samples from a suspected CJD patient, stop everything and escalate the issue immediately. Contact your medical director, your manager, or the safety officer and await further instructions.

There are other specimen types a lab might receive from a prion patient. Blood, serum, urine, feces, and sputum are considered no-risk specimens. Prions are not found in these types of specimens, and they may be handled and processed as usual.

The last category of specimens from prion patients is known as “low-risk.” These specimens include CSF, kidney, liver, spleen, lung, lymph nodes, placenta, and olfactory epithelium tissues. Of course the most common specimen a lab would see from this group is a spinal fluid, and labs do need to make sure they do not handle it as a normal specimen.

Lab staff should be notified when a specimen is going to be sent from a prion patient, particularly when a low-risk specimen like a CSF is on the way. Procedures should be in place, and it is recommended that such specimens have special labels on them to alert those of the potential risks.

There is no record of lab employees becoming infected with prions from handling low-risk specimens, but they must still be handled with care. All testing of low-risk specimens should be performed inside a Biological Safety Cabinet (BSC). Use disposable equipment as much as possible. For example, use disposable cups for stains or reagents where possible. Perform manual testing only; do not run low-risk specimens on automated analyzers as disinfection is not easily accomplished.

While using standard bleach solutions to disinfect surfaces is recommended after processing low-risk specimens, a lab spill of such a specimen is an entirely different matter, and this is why lab specimens should have special labeling. When a low-risk specimen spills, the area should be flooded with 2N Sodium Hydroxide (NaOH) or undiluted sodium hypochlorite (bleach). Remember, never mix bleach with formaldehyde as it produces a dangerous gas, so if a pathology specimen is spilled, only use NaOH. Leave the solution on the spilled material for one hour, then rinse with water. Place the spill materials into a sharps container so that they will be incinerated. If a spill of a low-risk CJD or prion specimen occurs, contact a manager, a medical director, or the safety officer immediately.

Laboratory professionals handle infectious specimens every day which is why it is so important that we utilize Standard Precautions. Wear PPE when working in the lab and treat all specimens as if they were infectious. It’s the only way to prevent a lab-acquired infection. If you see a co-worker not wearing gloves or a lab coat and working at a lab counter or computer, use coaching to remind them that those surfaces are potentially contaminated with pathogens, and they can be deadly. We can protect ourselves from low-risk prion disease (and other pathogens) with everyday PPE. If a specimen is processed in the lab and it is found later the patient was prion-positive, you do not want to be the one who wasn’t wearing PPE when you handled the specimens. The results will be potentially disastrous for you and your family.

Remember, if you receive a phone call that a CJD or prion specimen is being sent to the lab, escalate the situation immediately. Find out if your lab is able to receive and process that type of specimen. Protect yourself, and keep your lab safe from CJD and other infectious pathogens.

 

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-Dan Scungio, MT(ASCP), SLS, CQA (ASQ) has over 25 years experience as a certified medical technologist. Today he is the Laboratory Safety Officer for Sentara Healthcare, a system of seven hospitals and over 20 laboratories and draw sites in the Tidewater area of Virginia. He is also known as Dan the Lab Safety Man, a lab safety consultant, educator, and trainer.

 

Local is Global

The words “global health” usually triggers thoughts of exotic diseases in exotic locales. But, we should remember that “global” includes our own backyard! Public health and clinical laboratories and lab professionals in the US play an important role in global health efforts, just as labs and lab initiatives in remote, resource poor areas. Labs are important for healthcare at local, national, and international levels. Without labs, we risk antimicrobial resistance, spread of infectious diseases, environmental exposures, and inadequate management of chronic non-communicable diseases like diabetes.

Despite their significant role in healthcare, our labs at home face funding and staffing challenges. It is estimated that 7,000 medical technologist positions need to be filled annually, and only 6,000 are produced each year. The number of training programs have decreased by 15% since 1990. CMS has recently announced that a bachelor’s degree in nursing is equivalent to a degree in biological sciences required to perform high-complexity testing. While nursing education provides invaluable medical knowledge, it does not include in-depth scientific study of principles behind laboratory testing and technology.

Both clinical and public health labs in the US are facing financial challenges. Public health labs, especially, have functioned on minimal budgets for several years. With these challenges, maintaining status quo can be difficult let alone scaling up activities when needed for managing crises. We see this play out with the Zika virus. The CDC has already spent 87% of funding allotted for Zika. State public health labs are worried about their ability to continue to meet routine needs while scaling up to be able to perform Zika testing. The FDA recommendation for screening donated blood products puts additional burden on laboratories and blood banks.

The reason we don’t think of our own backyard when we hear “global health” is because we don’t have as many of the exotic diseases seen in other locales. This is in large part because we do have quality laboratory systems in place. While in the field, comments such as “I had no idea pathologists did this much” have been made to me. As lab professionals we need to advocate for laboratory medicine, at home and abroad.

 

Sarah Brown Headshot_small

-Sarah Brown, PhD, DABCC, is an Assistant Professor of Pediatrics and Pathology and Immunology at Washington University in St. Louis School of Medicine. She is passionate about bringing the lab out of the basement and into the forefront of global health.  

Antimicrobial Stewardship Down Under

If you’re an infectious disease/antimicrobial stewardship/microbiology geek, then the Australian blog AIMED is relevant to your interests. AIMED focuses on practical antimicrobial prescribing issues of relevance to hospital and community prescribers. It is supported by a local brains trust of General Practitioners, Pharmacologists, Pharmacists, Microbiologists and Infectious Disease Physicians. It also provides internet access to key Hunter New England resources for medical staff including guides to local antibiograms, infection control resources and personnel.

For those who don’t know, AIMED is an acronym for five principles that guide patient treatment with antimicrobials:

  • Antimicrobial selection and dosage
  • Indication for antimicrobial treatment
  • Microbiological assessment
  • Evaluate patient at 48-72 hours
  • Duration should be specified

If you’d like to learn more, check out their blog.

Stomatocytes: Inherited or Acquired?

A 40 year old patient presented with macrocytic anemia.

CBC Results:

  • WBC: 8.6 K/uL                     Normal
  • RBC: 3.60 M/uL                   Decreased
  • Hb : 13.1 g/dl                       Normal
  • MCV: 111.3 fl                        Increased
  • MCH: 36.4 pg                      Increased
  • MCHC:  32.7                        Normal
  • RDW:     15.5%                     Increased
  • Platelet: 360 K/uL              Normal

Review of peripheral smear showed several stomatocytes (Figures 1-3).

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Figure 1

 

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Figure 2
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Figure 3

Serum vitamin B12 and folate levels were normal and serum iron studies were consistent with anemia of chronic disease. Typically anemia of chronic disease is normocytic normochromic. Based on presence of numerous stomatocytes and macrocytosis (which can be seen in patients with hereditary stomatocytosis) it was recommended that patient be investigated for hereditary stomatocytosis and acquired causes of stomatocytosis.

STOMATOCYTES

“Stomatocyte” describes the appearance of red blood cells (RBCs) on the peripheral blood smear. Stomatocytes (also called hydrocytes) contain a mouthlike or slitlike pattern that replaces the normal central zone of pallor.

Mechanism of stomatocyte formation — When the normal biconcave disc becomes a uniconcave cup red blood cell (RBC) will appear as a stomatocyte on the peripheral blood smear. There are several mechanisms by which this change can occur:

  • In hereditary stomatocytosis (HSt), the mechanism of stomatocyte formation often involves changes in cell volume caused by reduced intracellular ion content.
  • In most cases of acquired stomatocytosis and rare inherited conditions that affect lipid metabolism formation often involves either a decrease in RBC membrane surface area or qualitative changes in the composition of the membrane lipid bilayer.

 

DISORDERS WITH STOMATOCYTES ON PERIPHERAL BLOOD

  1. Hereditary stomatocytosis (HSt) is an inherited autosomal dominant condition characterized by an excess of stomatocytes Patients have variable degrees of hemolysis and anemia. Hereditary stomatocytosis (HSt) is a rare disorder that presents with various degrees of hemolytic anemia and abnormal red blood cell (RBC) morphology. The genetic abnormalities responsible for these conditions remain incompletely characterized. Some of the defects associated with this condition involve membrane transporters such as Piezo1, Gardos, Rhesus antigen-associated glycoprotein, and the anion exchanger band 3.
  2. Several rare inherited defects affecting membrane lipid composition have been reported to have stomatocytosis on the peripheral blood smear.
  • Tangier’s disease
  • Rh null disease
  • Phytosterolemia
  1. Liver disease/medications. Stomatocytes can be seen with some acquired conditions such as chronic liver disease (most often due to alcoholism) or acute alcohol intoxication. The stomatocytosis with acute alcohol intoxication appears to be transient, and it may affect a significant proportion of RBCs. The mechanism is thought to be due to a reduction in RBC membrane surface area rather than an increase in RBC volume. Also, dministration of some medications can cause transient stomatocytosis. This was demonstrated in a study that demonstrated formation of stomatocytes upon exposure of RBCs to drugs like vinblastine and chlorpromazine. Intercalation of the drug into the inner half of the lipid bilayer may be responsible for creating the abnormal morphology.
  1. In some healthy individuals, stomatocytes occasionally can be found on the peripheral blood smear. This is thought to be due to a drying artifact; hence, it is important to evaluate several different areas of the peripheral smear before determining that a patient has circulating stomatocytes.

Hereditary stomatocytosis (HSt)

HSt can be completely asymptomatic or can present with chronic hemolytic anemia of varying severity. The age of presentation depends on the specific gene mutation, presence of other inherited conditions, and other environmental factors. The increasingly routine use of the complete blood count (CBC) in asymptomatic individuals has resulted in earlier diagnosis in some individuals who otherwise might never have come to medical attention. There does not appear to be a relationship between the degree of peripheral stomatocytosis on the blood smear and the severity of hemolytic anemia

The diagnosis of HSt is made by demonstrating the presence of anemia associated with the characteristic changes in RBC morphology (stomatocytosis) in conjunction with altered RBC indices and osmotic fragility. Genetic testing for PIEZO1 or Gardos channel mutations is confirmatory but not required.

The evaluation for HSt includes review of the complete blood count (CBC) and peripheral blood smear, which may show stomatocytes.The blood smear should be reviewed closely to ensure there are no abnormalities of white blood cells (WBCs) or platelets. The RBC indices typically show an increased mean corpuscular volume (MCV) in the range of up to 140 femtoliters (fL) and abnormally low or high mean corpuscular hemoglobin concentration.

 

Vajpayee,Neerja2014_small

-Neerja Vajpayee, MD, is an Associate Professor of Pathology at the SUNY Upstate Medical University, Syracuse, NY. She enjoys teaching hematology to residents, fellows and laboratory technologists.

 

An Emerging Trend: Multi-Drug Resistant Fungus

Over on her blog, Maryn McKenna discusses the latest emerging microscopic threat: drug-resistant fungal infections. It mentions the organism Candida auris and states: “The Centers for Disease Control and Prevention (CDC) is so concerned that it recently sent an alert to U.S. hospitals, even though only one possible case of the resistant fungus has been identified in the United States so far.” 

 

Is Renting Laboratory Equipment a Viable Option?

While equipment rental agency aren’t new–anyone who’s done a home renovation or taken a vacation is familiar when such services–I would guess that not many laboratory professionals would consider using one. A relatively new online equipment rental company called Kwipped is trying to change that.

You can read an article written by the company’s CEO here.

Browse the company’s laboratory offerings here.

What do you think? Could equipment rental be a viable option for the laboratory?

Carbapenem-Resistant Enterobacteriaceae Found in Rio de Janeiro’s Water

Recent studies conducted by Brazilian researchers found “super bacteria” in the waters where Olympic athletes will be competing. According to MercoPress, “The Brazilian group’s lead researcher, Renata Picao, said Rio’s “super bacteria” made its way into the city’s waterways through sewage from local hospitals, due to a lack of basic sanitation in the metropolitan area.”

A recent Lab Medicine podcast discusses laboratory testing for CRE. You can listen to it here.

Maryn McKenna writes extensively about antimicrobial resistance. You can watch to her recent TED talk (or read the transcript) to learn why the presence of CRE in Rio’s water is so concerning.

Microbiology Case Study: 2 Year Old with Fever and Bloody Diarrhea

Case

A 2-year-old male with no past medical history presented to the emergency department with fever and 2 days of bloody diarrhea.  Stool cultures were sent to the laboratory.  A Gram stain of the specimen showed the morphology seen in Figure 1. On the 5% sheep blood agar plate, the predominant organism had colonies that appeared flattened and spreading (Figure 2A).  On MacConkey agar the colonies were noted to be non-lactose fermenting (Figure 2B).  A Hektoen enteric (HE) agar was used as a differential and selective media to differentiate Salmonella from Shigella.  On the HE agar the colonies were clear with a green appearance due to the color of the agar (Figure 2C).

 

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Gram stain showing Gram-negative rods
shig2
Isolate growing on (A) 5% sheep blood, (B) MacConkey, and (C) Hectoen Enteric agars

Identification

Shigella is a bacterium in the Enterobacteriaceae family and is a Gram-negative rod that is facultatively anaerobic. It is non-motile, a non-spore former, and does not ferment lactose.  There are four species of Shigella that are associated with subgroups A-D.  Our isolate was identified as Shigella sonnei, which is the most common species in the U.S. and comprises subgroup D.  The other subgroup/species correlations are listed in Table 1.  The slide agglutination antisera test is used to aid in serogrouping.  The suspected colony is mixed on a slide with antisera that contains specific antibodies to Shigella.  If clumping (agglutination) occurs, it is considered a positive result for the specific subgroup. The organism was identified as Shigella sonnei by slide agglutination antisera testing.   In addition, Shigella has certain biochemical properties that aid in further identification and confirmation. 

Table 1:  Shigella sp. determination by serogroup

Serogroup Organism
A Shigella dysenteriae
B Shigella flexneri
C Shigella boydii
D Shigella sonnei

 

Clinical Significance

Shigella is one of the most common causes of bacterial gastroenteritis and is often associated with poor sanitation and overcrowded conditions.  Transmission occurs through routes such as: fecal-oral and person to person contact.  Of note, only a small amount of the bacteria (as low as 10 organisms) is required to cause disease.  Hemolytic-uremic syndrome is a complication that may occur with shiga-toxin producing Shigella (the most commonly associated is S. dysenteriae).  Shigella has demonstrated antibiotic resistance and therefore does undergo susceptibility testing.

 

References:

Nataro JP, Bopp CA, Fields, PI, Kaper JB, Strockbine, NA.  2015. Escherichia, Shigella, and Salmonella, p 603-626.  In Jorgensen J, Pfaller M, Carroll K, Funke G, Landry M, Richter S, Warnock D (ed), Manual of Clinical Microbiology, Eleventh Edition. ASM Press, Washington, DC.

 

-Valerie Juarez, M.D., 3rd year Anatomic and Clinical Pathology resident, UT Southwestern Medical Center

-Erin McElvania TeKippe, Ph.D., D(ABMM), is the Director of Clinical Microbiology at Children’s Medical Center in Dallas Texas and an Assistant Professor of Pathology and Pediatrics at University of Texas Southwestern Medical Center.