Microbiology MediaLab Exam Simulator

Sequence-based analysis is an alternate method used to identify bacteria and fungal isolates. This methodology uses rRNA genes (rDNA) and their intergenic regions as common target sites to identify these organisms. Which of the following rRNA gene target sites is used to identify fungal isolates?

28S rRNA is correct because this subunit is used to identify fungal organisms. Within the 28S rRNA gene, there are D1/D2 regions that are commonly used as target sites to identify fungi. 16S rRNA is incorrect. The bacterial subunit 16S rRNA is encoded by a 1500 bp gene. Amplification of the first 500 base pairs is sufficient for the identification of most bacteria including anaerobes and mycobacteria organisms. 25S rRNA and 14S rRNA are both incorrect answers as these are not target regions used to identify bacteria and fungal

Each of the following bacterial species works synergistically to produce bacterial vaginitis EXCEPT:

Although Streptococcus agalactiae may inhabit the vaginal canal, particularly during pregnancy, it is not associated with bacterial vaginitis; rather, may be a cause of postpartum and neonatal sepsis. Gardnerella vaginalis, Mobiluncus species, and various anaerobes, including Prevotella species, Porphyromonas asaccharolytica, Fusobacterium nucleatum and Peptostreptococcus species act in synergy to produce bacterial vaginitis.

Gram-positive cocci in clusters and tetrads were seen on a clinical Gram stain taken from an open wound. Which of the following would be the MOST appropriate preliminary report?

Considering the microbiologist's findings in this case, the report should state "Gram-positive cocci in clusters or tetrads". In reporting out Gram stains, one should not go beyond what objective observation will allow. In this case, the Gram-positive cocci are arranged in tetrads and clusters. However, one should stop short of naming staphylococci in an official report, as the large Gram-positive cocci in tetrads is also suggestive of Micrococcus species, for which human infections are rare. This prevents misleading information for healthcare providers.

Cyclospora species are classified as:

Cyclospora species are considered members of the Sporozoa since they are unicellular and have an apical complex. They develop in the gastrointestinal tract of vertebrates throughout their entire life cycle. Cyclospora cayetanensis involves only humans in their life cycle, oocysts are passed in the feces, and transmission is thought to be by the fecal-oral route. Amoeba includes organisms capable of movement by means of cytoplasmic protrusions called pseudopodia. Some examples include Entamoeba histolytica, Entamoeba dispar, and Blastocystis hominis. . Flagellates have specialized locomotor organelles called flagella, these long, thin cytoplasmic extensions vary in number and position. Some examples of flagellates include Chilomastix mesnili, Giardia duodenalis & Dientamoeba fragilis. Ciliates include organisms that move by means of cilia or short extensions of cytoplasm that cover the surface of the organism. An example of a ciliate that infects humans is Balantidium coli.

All of the following parasites may typically survive human stomach juices except:

Entamoeba gingivalis trophozoite typically resides in the gingival crevices in the gums. They typically do not infect the intestines, but if they are swallowed, the stomach juices would destroy them. There is no known cyst form for E. gingivalis. Cysts are noted by nature to be resistant to juices in the human stomach. The most common transmission of parasites is the ingestion of cysts in contaminated food or water. The trophozoite forms, or feeding forms, are much more delicate forms and are easily destroyed by stomach juices and the outside environment. Thus, all of the cyst forms would survive the stomach juices, where the trophozoite would be destroyed.

Baseline testing for latent Mycobacterium tuberculosis infection (LTBI) is recommended for all newly-hired healthcare workers. In which of these cases would a tuberculin skin test (TST) be repeated a second time, one to three weeks after the first test?

First test is negative and they haven't been tested previously. A negative TST is repeated 1 - 3 weeks after the first test if the individual being tested does not have a documented record of a negative test that occurred less than or equal to 12 months prior to new employment. A positive TST result should be followed up with additional testing to exclude TB disease such as chest X-ray and/or blood testing for TB disease to rule out LTBI (latent TB infection). IGRA is a blood test for determining exposure to the bacteria causing TB. A positive test is followed through similarly to a positive TST.

A bacterium was isolated as the cause of a urinary tract infection. The organism showed no lactose fermentation on MacConkey agar (MAC). On triple sugar iron (TSI) media, the organism produced an A/A with H2S. What organism is most likely observed?

From the information given, the organism in question does not ferment lactose but does ferment sucrose and glucose as determined from the MAC agar and TSI. The organism is also H2S positive. From the list of organisms, only Proteus vulgaris produces these reactions. Enterobacter aerogenes ferments lactose, sucrose, and glucose, but does not produce H2S. Proteus mirabilis is negative for lactose and sucrose fermentation, positive for glucose fermentation, and is H2S positive. Edwardsiella tarda is negative for lactose and sucrose fermentation, positive for glucose fermentation, and is H2S positive.

A patient grew out Mycobacterium tuberculosis (MTB) from an acid-fast bacilli (AFB) sputum culture. The physician called requesting information on which four primary drugs should be used for the treatment of MTB. Which of the following primary antitubercular agents would be recommended for treatment?

Isoniazid, Rifampin, Ethambutol, and Pyrazinamide is the correct answer. To prevent resistant strains of MTB, four primary antitubercular drugs should be used for a total of 8 weeks. All four of the drugs listed are the primary drugs of choice in the initial treatment of MTB. However, if susceptibility has been determined for isoniazid, rifampin, and pyrazinamide, then ethambutol can be discontinued. After 8 weeks, isoniazid and rifampin should be continued for an additional 18 weeks. Isoniazid is also known as INH. Streptomycin, Isoniazid, Rifampin, Ethambutol is incorrect because streptomycin is a secondary antitubercular drug and not a primary antitubercular drug of choice in the treatment of MTB. Isoniazid, Ciprofloxacin, Ethambutol, and Pyrazinamide is incorrect because ciprofloxacin is a secondary antitubercular drug and not a primary antitubercular drug of choice in the treatment of MTB. Isoniazid, Capreomycin, Streptomycin, Rifampin is incorrect because both capreomycin and streptomycin are secondary antitubercular drugs and not primary antitubercular drugs of choice in the treatment of MTB.

One of the key biochemical characteristics by which Escherichia coli serogroup O157 can be screened from stool specimens is?

MacConkey agar, with sorbitol (SMAC) instead of lactose, is a screening culture media helpful in the detection of E. coli O157 from stool specimens. E. coli O157 produces heavy growth on SMAC but does not ferment sorbitol. However, SMAC should not be the only test to identify potential O157 strains. In general, all species of the genus Escherichia ferment lactose and produce a positive methyl red reaction. Most species of E. coli are motile; nonmotile species are not limited to any serogroup but do exist in some O157 isolates.

Of the following organisms, which organism is known to cause bacterial vaginosis?

Mobiluncus species is correct because this species is known to cause bacterial vaginosis and can be found growing along with Gardnerella vaginalis. Mobiluncus is rarely encountered in infections outside of the female genital tract. Actinomyces israelii is incorrect because infections by this organism can be seen in intrauterine actinomycosis infections associated with long-term implantation of intrauterine devices. Bacteroides fragilis is incorrect because this organism is a major component of the large bowel flora and is often associated with enteric abscesses, including those involving the appendix. Clostridium botulinum is incorrect because this organism is associated with "floppy infant" syndrome, due to a botulinum toxin released from ingested Clostridium botulinum organisms replicating in the bowel causing neurological effects.

MacConkey agar without crystal violet is a culture medium useful in the presumptive identification of:

Mycobacterium fortuitum is the correct answer because it has the capability of growing on modified MacConkey agar without crystal violet, in contrast to other Mycobacterium species that do not grow. This media is helpful in the identification of rapidly growing mycobacteria. Mycoplasma pneumoniae is incorrect because the culture medium for the presumptive identification of Mycoplasma pneumoniae is methylene blue-glucose biphasic medium (New York City Medium). Stuart's medium, trypticase soy broth supplemented with 0.5% bovine serum albumin, Mycotrans, and A3B broth are all additional media that can be used for the isolation of Mycoplasma pneumoniae. Legionella pneumophila is incorrect because the preferred media for the isolation of Legionella pneumophila is buffered charcoal yeast extract agar (BCYE). Vibrio parahemolyticus is incorrect because it is best isolated from the medium thiosulfate citrate bile sucrose (TCBS) agar.

A yeast identification system's biotype number for an unknown isolate did not differentiate between Candida tropicalis and Candida parapsilosis. This isolate could be identified as C. parapsilosis on a cornmeal agar preparation if it produced:

Observing the patterns of growth and sporulation on cornmeal agar aids in the identification of an unknown yeast if the results derived from profile numbers generated by automated or kit systems do not provide sufficient information. Candida parapsilosis produces pseudohyphae arranged in a 'sagebrush' appearance with large hyphae. Blastoconidia are not characteristically present. The presence of chlamydoconidia, spherical blastoconidia, and pseudohyphae is characteristic of Candida albicans. Candida albicans is the most commonly isolated yeast. It causes thrush, endocarditis, meningitis, fungemia, and other serious infections, especially in immunocompromised patients. Candida glabrata produce tightly compacted blastoconidia and no pseudohyphae or hyphae. This organism has been isolated from serious infections including endocarditis, meningitis, and disseminated infections. Candida krusei produce elongated blastoconidia arranged in clusters at the septa of branching pseudohyphae. It is important to differentiate this yeast from the others as it is typically resistant to the azole class of antifungal agents.

Throughout history, plague pandemics have killed millions. Transmission of the human plague may still occur through bites of rodent fleas or contact with infected animals. What is the causative agent of plague?

Of the four bacterial organisms listed, gram-negative, non-spore-forming, Yersinia pestis is responsible for the plague. It causes three types of disease with fever present in all: bubonic plague, with the formation of buboes in the lymphatic system; septicemic plague from organism dissemination; and pneumonic plague, an extremely infectious form transmitted through respiratory droplets. It may be deadly without prompt antibiotic treatment. In addition to Yersinia pestis, gram-negative Vibrio cholerae has also caused pandemics throughout the world. It is transmitted through contaminated water or food. Vibrio cholerae O1 causes acute and frequent diarrhea, known as "rice-water" stools, as a result of the toxin, choleragen. It is rare in the U.S. but may be deadly within hours, if untreated. Bacillus anthracis is a spore-producing, gram-positive bacillus responsible for anthrax, also known as "wool-sorter's disease." The endospores produced by this organism can survive extreme conditions, and vegetative cells are capable of producing anthrax toxins. Nearly all reported cases of anthrax are cutaneous, but transmission may also occur via inhalation or ingestion. The risk of death is reduced if prompt treatment is administered. Clostridium botulinum is a spore-producing, gram-positive, anaerobic bacilli responsible for foodborne or wound botulism. C. botulinum produces a potent neurotoxin called botulinum, which may lead to paralysis, respiratory failure, and death, without prompt antitoxin treatment. All organisms listed here are considered bioterrorism agents, according to the Centers for Disease Control and Prevention.

The 10-day-old colonies grown on SABHI agar incubated at 30° C as illustrated in the top photograph are characteristic of those recovered from skin nodules of workers living in South America, especially Brazil. Although a dimorphic fungus might be suspected from the appearance of the slow-growing, gray-white, cob-web-like colonies, the final identification can be confirmed by observing the microscopic appearance of tease mounts prepared from the surface mycelium, as observed in the bottom photograph. Based on these descriptions, what is the most likely identification?

Paracoccidioides brasiliensis exhibits slow growth with a cottony, hair-like gray white mycelium suggests one of the dimorphic fungi, the final identification is made by observing a stained microscopic mount prepared from a tease mount. Illustrated in the photomicrograph are narrow, septate hyphae from the tips of which oval, pigmented 2-3 µm in diameter delicate conidia are borne. This "lollipop" appearance is similar to those observed with Blastomyces dermatitidis. The separation can be made by observing the different presentation of the yeast form or by serological or molecular assays. Sporothrix schenckii mold colonies grow relatively rapidly, usually within 3-5 days when incubated at 30° C. Areas of dark pigmentation may be observed In mature colonies. Small smooth, oval conidia measuring 2-4 µm in diameter are also borne in profusion laterally from delicate hyphae; however, distinctive are the daisy-like clusters of conidia produced from the tips of straight, delicate conidiophores. Histoplasma capsulatum in the mold form produces non-distinct delicate cobweb-like colonies. The identification is made by the microscopic observation of singly produced 10 - 20 µm, circular, roughened, spike-like (tuberculate) macroconidia being borne latterly from the sides rather than from the tips of slender conidiophores. Coccidioides immitis colonies are not distinctive from other dimorphic molds except when areas of black pigmentation may be observed. Distinctive are the barrel-shaped arthroconidia separated by an empty space (dysjunctors) as observed microscopically in stained mounts. Conidia at the terminal ends of conidiophores are not produced.

All of the following may cause systemic mycoses EXCEPT:

Piedraia hortae does not cause systemic mycosis but instead causes a condition known as black piedra. This disease is characterized by the formation of brown or black nodules attached to the hair shaft. Scalp hair is the most frequently infected area. Black piedra does not become systemic. The remaining organisms may cause systemic mycoses Actinomycosis is caused by infection with Actinomyces israeli. Actinomyces infections are most often found in the head, neck, or jaw area but may also involve the thorax, pelvic area, and central nervous system. Nocardia brasiliensis causes mycetoma, lymphocutaneous infections, and cellulitis; in immunosuppressed individuals, Nocardia can cause invasive pulmonary infections and disseminated infections that can affect the brain, skin, and central nervous system. Disseminated nocardiosis has a poor prognosis. Coccidioidomycosis is caused by infection with Coccidioides immitis, which generally begins as a respiratory infection but may disseminate in a small percentage of patients to the bones, joins, brain or meninges.

Observed in the image is the microscopic appearance of a sporulation representative of one of the dematiaceous fungi causing chromomycosis. Note that spherical conidia are being produced laterally from the sides of a conidiophore. Which of the following choices represents the type of sporulation illustrated in this image?

Rhinocladiella sporulation is correct. Conidia are produced singly, directly, and laterally in compact rows from the sides of the hyphae. This form of sporulation is commonly observed as one of the identifying forms produced by certain strains of Fonsecaea pedrosoi. Acrotheca sporulation is incorrect. Sporulation is characterized by sympodial branches of conidiophores that are produced from the tips of septate hyphae, simulating the prongs of a coat rack. Elliptical conidia in short chains are produced from the tips of these branching conidiophores. Cladosporium sporulation is incorrect. Sporulation presents long straight chains of elliptical, dark-staining conidia each separated by a delicate scar known as a dysjunctor. Phialophora sporulation is incorrect. Sporulation is in the form of short urn-shaped phialides each with a narrow bottle-like opening from which oval-shaped, yellow-pigmented conidia are produced from within each phialide, forming loose aggregates at the terminal opening.

A 67-year-old man was seen in the emergency room complaining of cough, fever, and piercing right posterior chest pain. X-ray of the chest revealed consolidation of the right middle lobe of the lung. Sputum culture grew the bacterial species shown in the upper photograph. The lower photomicrograph illustrates a gram-stain of the sputum specimen. The most likely cause of the pneumonia is:

The clinical setting of lobar pneumonia in an elderly patient is most likely associated with Streptococcus pneumoniae. The mucoid, alpha-hemolytic colonies seen in the blood agar plate and the gram-positive diplococci seen in the sputum specimen are virtually confirmatory. Enterococcus faecalis can produce a gram stain similar to that illustrated; however, the colonies are not mucoid and lobar pneumonia by this species would be less likely. Klebsiella pneumoniae can appear as mucoid colonies on blood agar and can cause lobar pneumonia similar to that described; however, the bacterial cells are gram-negative bacilli. Staphylococcus intermedius would more likely cause necrotizing bronchopneumonia, the colonies are not mucoid and the cocci arrange in loose clusters or tetrads rather than pairs.

Which of the following organisms is both a transient colonizer of skin, as well as the most common cause of bacterial skin infections?

The correct answer is Staphylococcus aureus. Staphylococcus aureus is a transient colonizer of skin, while coagulase-negative staphylococci are permanent skin colonizers. Staphylococcus aureus is responsible for up to 45% of skin and soft tissue infections. Cutibacterium (Propionibacterium) acnes is s permanent skin colonizer and is not a common cause of skin infections. Streptococcus pyogenes is not a skin colonizer but does fall at #7 on the list of the most common bacterial causes of skin and soft tissue infections. Escherichia coli is not a skin colonizer but does fall at #4 on the list of most common bacterial causes of skin and soft tissue infections.

The image to the right is a gram stain of a positive blood culture. The bacterial genus most likely to grow out in culture is:

The correct answer is Staphylococcus. The bacteria seen in this gram stain are small gram-positive cocci that are arranged in small clusters. Staphylococcus is the only genus listed that exhibits gram-positive cocci in clusters. Streptococcus spp. would exhibit gram-positive cocci, but they would be in pairs or chains. Listeria spp. and Corynebacterium spp. would be gram-positive, but they would be rods instead of cocci.

This suspicious form was found on a blood smear:

The correct answer is artifact. Although this suspicious form may be mistaken for a parasite due to its size and shape, it does not contain recognizable interior structures. A plasmodium schizont would have visible merozoites inside. Loa loa does not lay eggs. Leishmania amastigotes are only found inside of macrophages.

The glycoproteins neuraminidase (NA) and hemagglutinin (HA) are used to subtype the following virus:

The glycoproteins neuraminidase (NA) and hemagglutinin (HA) are used to subtype the influenza A virus. These typable glycoproteins project from the envelope and allow for strain differentiation. Determining the NA and HA is useful in vaccine development disease surveillance. The Human Papilloma Virus (HPV) is divided into various genotypes based on the viral DNA sequence. They do not have an envelope from which the glycopeptides would protrude. There are two types of Herpes Simplex Virus (HSV); HSV-1 and HSV-2. HSV-1 has been linked to the oropharyngeal mucosa and HSV-2 has been linked to genital sites but this division is definitive. Hepatitis B Virus (HBV) is a DNA virus that is a significant cause of liver damage (hepatitis) related to morbidity and mortality.

A 25-year-old female patient suspects of having an urinary tract infection (UTI) and presents to her doctor for a check-up. Her urine specimen is slightly turbid and the chemical and microscopic urinalysis results are as follows: Specific Gravity 1.009 Glucose 0 pH 8 Protein 1+ WBC 15/hpf RCS 1/hpf Crystals None Casts 2 hyaline/hpf Epithelial cells 0/hpf Bacteria 2+ What should her doctor do next?

The obvious choice here is to order a culture and sensitivity in order to identify the organism causing the infection. Slight turbidity with the presence of WBCs is consistent with this case. The combination of the bacteria present from the infection, along with the cells that have been drawn to the area of infection, can create a turbid appearance in the urine. These laboratory results are not normal and they don't appear to be a result of genital contamination. A culture and sensitivity should be ordered. Clinitest tests urine for reducing substances which indicates an inborn error of carbohydrate metabolism such as galactosemia. The patient is clearly not a newborn and this test is not necessary.

Which of the following best describes the organisms seen in this illustration:

The organisms in this image are demonstrating a gram-variable phenomenon. For the most part, the organisms are staining primarily gram-negative, however, on the tips of some of the rods, there is a gram-positive staining morphology. This can be defined as gram-variable. Gram-positive organisms stain dark purple due to the crystal violet. During the gram stain procedure, crystal violet dye is added first to the slide for 30 sec. The crystal violet is washed off with water and Gram's iodine is added to the slide. The crystal violet and iodine form a complex together that does not wash out in organisms with thick cell walls containing teichoic acid. Therefore, these organisms retain the purple color and do not counterstain with safranin. Gram-negative organisms stain pink from the counterstain, safranin dye. These organisms do not have a thick cell wall for the crystal violet and iodine complex to adhere to. So during the gram stain procedure after the crystal violet and iodine complex is washed out, the slide is stained with safranin dye. This is a pink dye and all organisms including epithelial cells stain pink with this dye. Acid-fast stain is used for Mycobacteria. The carbolfuchsin stain binds to mycolic acid in the cell walls, which is red in color. After the de-colorization stage, the sample is stained with methylene blue. So, any Mycobacterium in the sample will stain red and all other organisms and cells will stain blue.

Which of the following arthropods is classified as Arachnida?

There are five classes of arthropods, Chilopoda, Pentastomida, Insecta, Arachnida, and Crustacea. Arachnida includes ticks, mites, spiders, and scorpions. Fleas and lice are both classified as Insecta. Other Insecta organisms are mosquitos and other bugs such as bedbugs and cockroaches. Crabs are classified as Crustacea. Other organisms that are also Crustacea are crayfish and copepods. The other classifications are Chilopoda, which are centipedes, and Pentastomida, which are tongue worms.

A woman presents to her physician complaining of a foul-smelling yellow vaginal discharge. The following was reported from the sample of the vaginal secretions: Wet prep: Many WBCs seen, no Clue Cells seen, microorganisms (20µm) seen with jerky motility Vaginal pH: 6.0 Amine (Whiff) Test: Positive What is the most likely cause of this vaginal infection?

Trichomonas vaginalis is the correct answer. Trichomoniasis typically produces a yellow-green vaginal discharge with a pH of >4.5, 2+-4+ WBCs, positive Whiff test, and motile trophozoites observed microscopically. Candidiasis is associated with a white, curd-like vaginal discharge, 3.8-4.5 pH, 3+-4+ WBCs, negative Whiff test, and budding yeast/pseudohyphae of C. albicans seen microscopically. . Bacterial vaginosis usually produces a thin, white/gray vaginal discharge, >4.5 pH, rare or absent WBCs, positive Whiff test, and an increased number of Clue Cells seen microscopically. Atrophic vaginitis produces a purulent discharge, >4.5 pH, 3+-4+ WBCs, negative Whiff test, and bacteria seen microscopically with an increased number of RBCs.