Some of the E. coli bacteria that were originally isolated from a contaminated salad were tetracycline resistant while the others were susceptible. However, when tetracycline-resistant and -susceptible strains were grown together, all of them became tetracycline resistant. This efficient transfer of resistance depends on which of the following?
Many antibiotic resistance genes in bacteria are carried on conjugative plasmids (c) also known as R factors. These plasmids are transferred from one bacterium to another, of the same or related species, by conjugation. Conjugation is a process by which the donor bacterium (male cell) is brought in direct contact with the recipient bacterium (female cell) through the conjugative pilus. In E. coli, the conjugative pilus is referred to as a sex pilus or F pilus. The conjugative plasmid carries genes necessary for its transfer including the pilus genes. The plasmid is transferred into the recipient cell as a strand of ssDNA, which recircularizes within the recipient cell. The complementary DNA strand is then synthesized in the recipient cell. The ssDNA in the donor cell is also replicated; thus, both the donor and the recipient now carry a copy of the plasmid. Due to the direct contact between bacteria through the pilus, conjugation is an efficient process of DNA transfer. A transposon (e) is a mobile genetic element that is capable of moving from one replicon (a chromosome or a plasmid) to another but is incapable of initiating DNA transfer. Recombinase enzymes (d) are required for the recombination process that occurs between homologous regions of DNA (as in integration of a DNA in a replicon). Competent cells (b) are part of the bacterial population that is capable of taking up DNA from a solution (naked DNA). This may occur naturally or artificially (in the laboratory). E. coli competent cells are only generated in the laboratory through treatment with certain chemicals. Cell lysis and release of donor DNA (a) may be part of a mechanism through which naturally competent bacteria such as Neisseria take up naked DNA during infection
A 3-year-old girl from a family that does not believe in immunization presents to the emergency room with a sore throat, fever, malaise, and difficulty breathing. A gray membrane covering the pharynx is observed on physical examination. Corynebacterium diphtheriae is confirmed as the etiologic agent of this infection. Which of the following is required to prove that C. diphtheriae is the etiologic agent of this child’s infection?
The vignette describes a child with diphtheria. The finding of pleomorphic grampositive bacilli on Gram stain (d) or demonstration of metachromatic granules by methylene blue stain (c) are suggestive that C. diphtheriae might be present. Culture for C. diphtheriae requires a special medium containing tellurite, such as Tinsdale agar. C. diphtheriae can be differentiated from other species of Corynebacterium by production of cysteinase (a), which causes the organism to produce colonies surrounded by a brown halo on this agar. However, isolation alone (b) is insufficient to prove that the strain caused diphtheria. Only toxigenic strains of C. diphtheriae cause diphtheria, so any isolate identified as C. diphtheriae must be tested for toxin production (e) using the Elek test. The toxin genes are carried on a lysogenic bacteriophage, and not all strains carry this phage. PCR can also be performed for the diphtheria toxin genes, but even this is not conclusive as these genes may not be expressed.
A 3-year-old girl from a day care center is brought to the local public health clinic because of a severe, intractable cough. During the previous 10 days, she had a persistent cold that had worsened. The cough developed the previous day and was so severe that vomiting frequently followed it. The child appears exhausted from the coughing episodes. A blood cell count shows a marked leukocytosis with a predominance of lymphocytes. Which of the following is most appropriate for diagnosis of this child’s illness?
The vignette describes a child with pertussis caused by Bordetella pertussis. Culture of B. pertussis requires plating of a nasopharyngeal swab on special medium containing charcoal and blood with and without cephalexin for best result. The specimen must be collected before any antibiotics are given and the closer to onset of symptoms the better the result. However, culture alone (a) is not sufficiently sensitive. DFA is no longer recommended by the Centers for Disease Prevention and Control (CDC) due problems with both false positive and false negative tests, whether used alone (c) or with culture (d). PCR (e), which appeared at first to be more sensitive than culture, has been shown over the course of time to have about the same sensitivity as culture. Therefore, the best approach is to collect two swabs and use one for culture and one for PCR (b). Be aware there is great inconsistency in textbooks and reviews regarding the best methods for diagnosis of pertussis. The CDC is the best guide in this case.
A 55-year-old male presents with severe bilateral pulmonary infiltrate, elevated temperature, leucocytosis, elevated enzymes, and elevated creatine kinase. He and six of his friends had recently visited their favorite restaurant, which had a large water fountain that was misty on the day of his visit. Which of the following would be expected on a Gram stain that was counterstained with safranin for 3 minutes (prolonged counterstain)?
Several strains of S. pneumoniae are isolated from various patients. Some demonstrate high virulence while others appear to be nonvirulent. Mixing these cultures in the laboratory causes the nonvirulent strains to become pathogenic in laboratory animal experiments. Uptake by a recipient cell of soluble DNA released from a donor cell is defined as which of the following?
A nonspore-forming, gram-positive bacillus was isolated from a throat specimen from a primary school-aged child who had not been vaccinated with the DTaP vaccine. The strain isolated carried a bacteriophage on which the gene for toxin was encoded. How did this strain become toxigenic?
An increase in antibiotic resistance has been observed in Staphylococcus aureus, Pseudomonas aeruginosa, and E. coli strains isolated from patients in medical centers. Direct transfer of a plasmid between two bacteria is defined as which of the following?
E. coli has a doubling time of 30 minutes in human urine at room temperature. A patient had 1000 bacteria per mL of urine when a urine specimen was collected. How many bacteria would be present if the specimen sat at room temperature for 4 hours before being plated for culture in the laboratory?
A 24-year-old female presented with pain during urination. Gram stain of the uncentrifuged urine revealed the presence of gram-negative rods. What virulence factor is essential for the survival of these uropathogenic bacteria in the urinary tract?
A 30-year-old hospitalized patient with an intravenous (IV) catheter developed fever and systemic infection. The source of the infection was bacteria that contaminated the catheter during its insertion. The IV catheter had to be removed because the bacteria grew within the catheter forming a biofilm. Biofilm development depends on the ability of the bacteria to produce which of the following?
A 55-year-old British teacher presents with weight loss, weakness, muscle atrophy, and declining cognitive function. Her history reveals that her favorite meal is soup made with cow brain, which she has eaten almost every week since she was 10 years old. Which of the following best describes the most likely etiologic agent of her symptoms?
Over 200 isoniazid-resistant strains of Mycobacterium tuberculosis isolated from different patients in the northwestern region of Russia are screened by a PCR-restriction fragment length polymorphism assay. This analysis reveals a 93.6% prevalence of a specific G to C mutation in the katG in strains from patients with both newly and previously diagnosed cases of tuberculosis. Which of the following best describes the type of mutation that resulted in isoniazid resistance in these strains?
Seven closely related isolates of Candida albicans exhibited progressive decreases in susceptibility to posaconazole. Sequencing of the gene involved in azole resistance in these strains revealed predicted proteins with single amino acid changes. Which of the following best describes the type of mutation that has resulted in decreased susceptibility to posaconazole in these strains?
A 25-year-old man presents to the emergency room with several red, swollen, tender bite wounds on both arms that he stated occurred yesterday when he rescued his dog from a dogfight involving three other dogs, whose owner(s) is unknown. His right wrist and left elbow are also swollen and there is axillary lymphadenopathy on the left side. Gram stain of purulent material from the worst wound shows small gram-negative pleomorphic coccobacilli. The patient reports his last tetanus vaccination was 2 years ago. In addition to antibiotics, which of the following should be included in this patient’s treatment?
A patient presents to the emergency room with vomiting, diarrhea, high fever, and delirium. Upon physical examination, you notice largebuboes, which are painful on palpation, and purpura and ecchymoses suggestive of disseminated intravascular coagulation. Gram stain onaspirate of a bubo reveals gram-negative rods with bipolar staining. Which of the following antibiotics is the drug of choice for empiric therapy?
A 3-year-old girl who has missed several scheduled immunizations presents to the emergency room with a high fever. She is irritable and has a stiff neck. Fluid from a spinal tap reveals 20,000 white blood cells per milliliter with 85% polymorphonuclear cells. Which of the following is the drug of choice for empiric therapy against organisms of childhood meningitis?
During the course of his hospital stay, a severely burned 60-year-old male develops a rapidly disseminating bacterial infection. Small gramnegative rods that are oxidase positive are cultured from green pus taken from the burn tissue. Which combination of antibiotics is the best choice for empiric therapy against this etiologic agent?
Laboratory results of a clinical specimen from a patient with hospital-acquired pneumonia reveal the presence of methicillin-resistant Staphylococcus aureus (MRSA). Which of the following drugs is the best empiric treatment?
A young, essentially healthy woman was concerned about her loss of job time due to recurrent UTIs. Her physician prescribed an antibiotic, which would control microbial nucleotide synthesis by inhibiting dihydrofolic acid reductase in bacteria up to 50,000 times more than in mammalian cells. Which of the following agents work by this mechanism?
A 52-year-old woman presents with fever of 103°F, headache, right flank pain, nausea and vomiting, and urinary frequency with hematuria and dysuria. Renal ultrasound demonstrates a right urinary stone with right hydronephrosis. Which of the following antibiotics is the most appropriate treatment option?
A 75-year-old African American male with neurogenic bladder presents to the emergency room with hypertension, fever up to 104.6°F, and nausea and vomiting. The urine from his foley catheter gives a positive culture for an antibiotic-resistant strain of Enterococcus faecalis. He had previously been given an antibiotic that binds to D-alanine-D-alanine. To which of the following antibiotics is this isolate most likely resistant?
A patient with leukemia has a chest CT finding that suggests aspergillosis. Which of the following antimicrobials affects ergosterol synthesis and would most likely be used in this patient’s treatment?
A 6-year-old girl is diagnosed with meningitis. A lumbar puncture reveals numerous neutrophils and gram-negative diplococci. She is admitted to the hospital for antibiotic treatment, which is complicated by the fact that she is known to be allergic to β-lactams. What is the mechanism of action of the alternative drug of choice to treat this infant’s meningitis?
A clinical laboratory performs real-time PCR for detection of MRSA in the area where specimens are plated. The supervisor discovers that the last 50 specimens tested were positive. However, companion cultures were positive for only 15 of the specimens. What is the most likely reason for this discrepancy?
Guests at a party consumed beef broth that was boiled earlier in the day but left at room temperature for several hours. The individuals presented with symptoms of food poisoning, including watery diarrhea and abdominal cramps, 8 to 10 hours later. The symptoms lasted 24 hours. The agent that caused the symptoms is most likely which of the following?
A 25-year-old male presented with severe urethritis. Over the past 2 days, he had developed fever and chills. Gram stain of the urethral discharge revealed the presence of gram-negative diplococci. The fever and chills are most likely due to the release of excessive amounts of which bacterial component?
A 2-year-old had been treated three times with ampicillin for acute otitis media. Fluid aspirated from his middle ear during placement of tympanostomy tubes grows Moraxella catarrhalis, which is resistant to ampicillin. Which enzyme is responsible for this resistance?
A patient admitted to surgical intensive care for dehiscence of her surgical incision was screened for MRSA by PCR for the mecA gene. Nasal, rectal, and wound swabs were submitted for testing. The nasal and wound samples were positive for MRSA by PCR; however, the wound culture grew Staphylococcus epidermidis but no MRSA. What is the most likely reason for the discrepancy in the results of testing on the wound samples?
Neisseria meningitidis, group B, is identified as the cause of a local meningitis outbreak in a military training camp. Which of the following protects this organism from complement-mediated phagocytosis by neutrophils?
A child attending classes in a preschool is noted by his teacher to have several skin lesions on his arms. The lesions are pustular in appearance and some have broken down and are covered with a yellow crust. Which of the following protects the most likely etiologic agent of this child’s infection from phagocytosis and provides serologic specificity?
A pharmacologic compound intended for injection was contaminated with gram-negative bacilli during production. The product was filtered before packaging. Quality control cultures were sterile so the product was shipped for use. However, numerous persons who received the product developed fever and several developed hypotension. Which of the following was most likely responsible for the reaction observed in these patients?
A 64-year old female with a history of COPD visited her physician because of recent poor health symptoms, including cough, fever and night sweats, weakness, and 20-pound weight loss. Chest x-ray demonstrated a nodular infiltrate in the upper lobe. Laboratory staining of sputum revealed acid-fast organisms. Which characteristic of the most likely organism is responsible for this staining result?
In 2001, a number of governmental offices received mailed envelopes that contained an unknown white powder. Several employees were contaminated and developed cutaneous and/or inhalational anthrax. Which of the following is responsible for the antiphagocytic properties of this organism?
Twelve elderly residents living in an assisted care facility suffered from sinusitis, otitis media, and mild pneumonias during midwinter. Despite the fact that they had all received the 13-valent pneumococcal conjugate vaccine recently licensed for adults, S. pneumoniae was isolated from 10 of the patients. Which of the following is the best explanation for the pneumococcal infections?
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