Week 1: Formative quiz Flashcards
Antibiotic that is most commonly implicated for the adverse effects listed: Redman Syndrome
Vancomycin
Redman Syndrome is a reaction to infusion of vancomycin. It is not an allergic reaction but rather a flushing reaction associated with rapid infusion of intravenous vancomycin. It can be resolved by slowing down the infusion rate. (Antibiotics-adverse effects)
antibiotic that is most commonly implicated for the adverse effects listed: Rash in patient with infectious mononucleosis
Ampicillin. When patients with EBV-associated infectious mononucleosis (Ebstein-Barr Virus) are prescribed an aminopenicillin, such as ampicillin or amoxicillin, many patients will develop a rash. This does not mean that the person has an allergy to the aminopenicillin. (Antibiotics-adverse effects)
Antibiotic that is most commonly implicated for the adverse effects listed: Esophageal ulcer
Doxycycline
Tetracycline derivatives (minocycline and doxycycline) are associated with drug induced esophageal ulceration. Recumbent posture after drug ingestion is a predisposing factor. The most important measure to prevent this condition is to instruct patients to take drugs in the upright position with at least 100 ml of water. (Antibiotics-adverse effects)
Choose the mechanism of action for the class of antibiotic: Clindamycin
Protein synthesis inhibitor
Protein synthesis inhibitor. Clindamycin is a lincosamide antibiotic that binds to the 50S subunit of the ribosome and is therefore a protein synthesis inhibitor (Antibiotics-mechanisms of action)
Choose the mechanism of action for the class of antibiotic: Penicillins
Cell wall synthesis inhibitor
Cell wall synthesis inhibitor . Penicillins are β-lactam antibiotics. They bind to the transpeptidase (aka PBP or penicillin binding protein) and as such are cell wall synthesis inhibitors. (Antibiotics-mechanisms of action)
A previously healthy 23-month-old girl presents with a fever, vomiting and flank pain. A urine is collected by in-and-out catheterization and the urine is sent for urinalysis (UA) and urine culture. The UA has pyuria (leukocytes in the urine) and many bacteria. She is admitted for intravenous antibiotics for presumptive pyelonephritis (kidney infection). A Gram-negative rod is isolated on the preliminary urine culture report. Which of the following is the MOST APPROPRIATE antibiotic to be prescribed empirically while waiting for culture results?
Gentamicin. Gentamicin is the only correct choice of those antibiotics that are listed because none of the others ever has activity against Gram-negative rods (Antibiotics-spectrum)
Which of the following orally administered antibiotics is not appropriate for outpatient treatment of a severe systemic infection because it has negligible oral absorption?
Vancomycin. Negligible amounts are absorbed when taken orally. The other drugs are fairly well absorbed when taken orally (Antibiotics-route)
An elderly patient in a nursing home is given a proton pump inhibitor (PPI) to prevent acid reflux. She develops shortness of breath with coughing. Her chest X-ray indicates she has developed pneumonia. What host defense barrier has been impaired that increased her susceptibility to infection?
Stomach acidity (low pH). The low pH in the stomach helps to kill potentially pathogenic organisms so interfering with this can increase risk of pneumonia. (Host-Pathogen Interaction)
A college student goes to Student Health complaining of vomiting and diarrhea over the past few hours. The physician who sees her is not concerned and tells her that a virus is going around and she will be well in a day. Within 24 hours, she is no longer vomiting and the diarrhea has ended. What host defense factor was responsible for limiting the course of this infection?
Type 1 Interferon. (Host-Pathogen Interaction)
A previously healthy 9 year-old boy presents in early December with a two day history of a sore throat. Several classmates have been sick the past 2 weeks with Strep throat so his mother brings him in to see you today to rule out Strep. In addition to the sore throat, he has had a runny nose and sneezing but no fever and no other symptoms. On examination his temperature is 37.0˚C and his other vital signs are normal. He is talkative, interactive and breathing comfortably. His neck is supple. He has shotty (minimal) anterior cervical lymphadenopathy bilaterally. His conjunctivae are normal with no erythema and no drainage. His tympanic membranes are pearly with good landmarks and good mobility. He has profuse serous rhinorrhea. His oral mucosa is moist. His tonsils are mildly enlarged with erythema but no exudates. He has no palatal petechiae. The remainder of his physical exam is normal. Which one of the following is the MOST LIKELY diagnosis?
Viral upper respiratory tract infection. It is unlikely to be Strep because he does not have fever and he DOES have rhinorrhea and sneezing. He does not have a history of seasonal allergies and it would be an unusual time of the year to have allergies. If he is fully immunized and has not traveled outside the U.S., would be very unlikely to have diphtheria and he should be more ill. Would expect fevers and significant lymphadenopathy for infectious mononucleosis. (ENT Infection - Differential Diagnosis)
An adult with chronic liver disease is treated with azithromycin and ceftriaxone for community-acquired pneumonia. He improves and is ready to be discharged on oral medications. His sputum cultures are positive for Streptococcus pneumoniae. The isolate is resistant to azithromycin. What is the most likely mechanism for this pattern of resistance to azithromycin?
A gene associated with protection of the ribosomal target. (Antibiotic Resistance)
Choose the most likely mechanism of genetic transfer between cells described below. This mechanism is the mating or quasi-sexual exchange of genetic material from one bacterium (the donor) to another bacterium (the recipient) and usually occurs between members of the same or related species.
Conjugation. Conjugation is transfer of plasmid from one bacteria to another related bacteria or transfer of genomic DNA from 1 to another bacteria. Transduction involves transfer of genetic material by way of a bacteriophage. Transformation is when there is cell lysis and naked DNA gets picked up by other bacteria. Transposons are not a mechanism of transfer but rather a genetic element that is transferred aka jumping genes. (Bacteriology, Antibiotic Resistance)
For the definition below, choose the most likely genetic element it describes. These are small genetic elements that replicate independently of the bacterial chromosome. Most are circular, double-stranded DNA molecules. Some can integrate into the host chromosome.
Plasmids. Many bacteria share their DNA with other bacteria. The transferred DNA can be integrated into the recipient cell’s genomic chromosomal DNA or stably maintained as an extrachromosomal element – i.e., a plasmid. Bacteriophages are extrachromosomal genetic elements that can survive outside of a host cell because a protein coat (it is a bacterial virus) protects the nucleic acid genome. Transposons are mobile genetic elements that can transfer DNA within a cell, from one position to another in the genome, or between different molecules of DNA (ie plasmid to plasmid or plasmid to chromosome) aka jumping genes. (Bacteriology, Antibiotic Resistance)
A 21-year-old male military recruit is seen in the Emergency Department with sudden onset severe headache, neck stiffness, and fever. He is hypotensive and has a bilateral lower extremity petechial rash. Cerebrospinal fluid is sent for culture. Growth of small transparent colonies is noted on chocolate agar. Gram stain reveals intracellular gram negative cocci in pairs. Which of the following stains is responsible for the color seen on Gram stain of this organism?
Safranin. Gram-negative organisms are decolorized by 95% alcohol and then take on a safranin counter-stain that dyes the organism red. Our patient has Neisseria meningitidis infection with meningitis and meningococcemia. (Laboratory Diagnosis of Bacterial Infection)
Which of the following components is found in BOTH human cells and bacteria.
Phospholipid bilayer. Cholesterol is not found in bacteria. Some bacteria can form endospores. Ergosterol is found in fungi. LPS is found in Gram negative organisms. (Bacteriology)
A 1-week-old infant is brought to the Emergency Department by his parents with a 2 day history of fever, vomiting, poor feeding, and irritability. Blood and cerebrospinal fluid are sent for culture. The next day, multiple bacterial colonies surrounded by a clear zone are seen on blood agar. A catalase test is performed and is negative. Which of the following organisms is the MOST LIKELY cause of this infant’s illness?
Streptococcus agalactiae. Streptococcus agalactiae / Group B Strep is β-hemolytic (clear zone on blood agar). Like all Strep, it is catalase negative. GBS is one of the top three causes of meningitis in infants younger than 3 months old. Streptococcus pneumoniae is alpha-hemolytic. Staph aureus is â-hemolytic but it is catalase positive. (Laboratory Diagnosis of Bacterial Infection)
This bacterial external structure consists of polysaccharide. It allows the bacteria to firmly adhere to various structures including skin, heart valves, and indwelling catheters. It is found in certain strains of Pseudomonas aeruginosa and Staphylococcus epidermidis. Which external specialized structure does this describe?
Glycocalyx. A (capsule) is not correct. Although most bacterial capsules consist of polysaccharide, capsules are not seen in Staph epidermidis etc. Fimbriae are also known as pili and aid in adherence. Flagella help with movement / mobility of the bacteria. Spores are dormant forms that only certain Gram positive bacteria can become when under stressful environmental conditions. (Bacteriology)
For the bacteria listed, choose the correct gram stain morphology. Staphylococcusepidermidis
Gram-positive cocci in clusters. (Laboratory Diagnosis of Bacterial Infection)
A 16-year-old female gymnast was admitted with pyelonephritis (kidney infection). Her urine grew Escherichia coli that was susceptible to all antibiotics in the antibiotic susceptibility profile. She has no known drug allergies. Her fever, flank pain and vomiting have resolved, she is well hydrated and is ready for discharge home. You would like to switch her to cephalexin, an oral antibiotic to which her organism was susceptible. When switching from parenteral to enteral therapy, which of the following pharmacologic properties is the MOST IMPORTANT one to consider in this setting?
Bioavailability of the antibiotic. Lipid solubility would be important if she had a central nervous system infection – lipophilic drugs can penetrate the blood brain barrier better. She needs an antibiotic that is excreted in the kidneys and achieves good concentration in the urine. When switching from a parenteral to an enteral (oral antibiotic) already there is another factor introduced and that is oral absorption. A drug that is not well absorbed will have lower serum concentrations that one that is absorbed very well (ie bioavailability). (Antibiotic Overview)
Which one of the following sites is sterile (i.e., no normal flora / microbiome)?
- Gingiva
- Large intestine
- Middle ear
- Nares
- Vagina
Middle ear. The middle ear is one of those sites that is always sterile in the normal host. Other sites include blood, brain, renal parenchyma, bladder, synovial fluid, peritoneal fluid, pleural fluid, etcetera. (Host-Pathogen Interaction, Bacteriology)
These culture media support the growth of most organisms that do not have fastidious growth requirements. Examples of media in this category include blood agar and chocolate agar. What type of culture media does this best describe?
- Enriched, nonselective culture media
- Selective differential culture media
- Specialized culture media
Enriched, nonselective culture media. The first media that a sample (urine, wound, spinal fluid) is inoculated onto for incubation, and is one that will support the growth of most organisms.. Specialized culture media would be for isolation of fastidious organisms. Selective differential media are designed for the recovery of specific organisms that may be present in a mixture of other organism – e.g. trying to find Staph aureus from a nasal swab since the nose is colonized with many bacteria. Some commonly used enriched, nonselective culture media include blood agar, chocolate agar, Mueller-Hinton agar, Thioglycolate broth, Sabourand dextrose agar. (Laboratory Diagnosis of Bacterial Infection)
Which of the following lab tests is used to differentiate Staphylococcus from Streptococcus?
Catalase. All Staphylococci are catalase positive while All Streptococci are catalase negative. Bacitracin is a test used to differentiate Streptococci. Group A streptococcus is sensitive to Bacitracin. Coagulase is a test used to differentiate Staph aureus from all other Staph. Oxidase and urease are tests used to differentiate Gram-negative bacilli. (Laboratory Diagnosis of Bacterial Infection)
The medical technologist was performing a Gram stain on an isolate from a blood culture when she was interrupted by a phone call. When she returned to the Gram stain, she forgot the step that uses acetone / ethanol. If the organism that she was gram staining was Escherichia coli, what color would it appear on this incorrectly performed Gram stain?
Purple. The procedure for a gram stain begins with heat fixing the organism to a slide, then Step 1: Staining it with crystal violet. Step 2: Adding Gram’s iodine. The crystal violet is precipitated by the iodine and is trapped in the thick peptidoglycan layer of Gram-positive bacteria. Step 3: Acetone or ethanol is then used. It doesn’t affect the gram positives, but it decolorizes (washes away) the blue- purple stain – i.e. the crystal violet-Gram’s iodine complex – because Gram negatives that have a thin peptidoglycan layer. Step 4: Safranin – a counter stain – is then added. The safranin will not change the “Purple” color of the gram positives. For gram negatives, they would be colorless after the ethanol step if there was not a counter stain. The safranin imparts a pinkish / red color to the Gram negatives. So if the ethanol step was left out, the E coli would remain purple. Remember what Dr. Culbreath said: P = Peptidoglycan (thick), Positive (gram), and Purple. Also – due to degradation of the peptidoglycan, Gram stain is not a dependable test for bacteria that are starved (e.g. old cultures or stationary phase cultures) or for bacteria that have been treated with antibiotics (which can affect the gram stain and morphology). Bacteria that cannot be classified by Gram stain include mycobacteria, which have a waxy outer shell, and mycoplasma – which have no peptidoglycan, chlamydiae, which don’t pick up stain well, and treponemas, which are too small to see in light microscopy. (Laboratory Diagnosis of Bacterial Infection)
A Gram positive coccus is isolated from a blood culture in a patient with a prosthetic heart valve. On Gram stain, this organism is arranged in pairs or short chains (see Gram stain below). The isolate is α-hemolytic on blood agar plate. It is catalase negative, resistant to bile, resistant to optochin, and positive for PYR (pyrrolidonyl arylamidase). Which of the following is the MOST LIKELY organism?
- Enterococcus faecium
- Leuconostoc
- Streptococcus agalactiae
- Streptococcus pneumoniae
- Streptococcus pyogenes
Enterococcus faecium. Enterococci will appear as elongated gram-positive cocci arranged in pairs and short chains. These bacteria cannot be distinguished from S. pneumoniae by Gram stain alone. Enterococci can be identified by the PYR test (positive test for the enzyme pyrolidonyl arylamidase). S. pneumoniae can be rapidly identified by exposing the cell to bile (or another detergent) which will dissolve the bacteria (“bile soluble”). the PYR test is negative. Leuconostoc is a catalase-negative gram positive coccus that is an opportunistic pathogen. It resembles streptococcus but it is inherently resistant to vancomycin, a trait that has not been seen in streptococci.Streptococcus agalactiae (Group B streptococcus) is negative for PYR, and is β-hemolytic, not α-hemolytic. Streptococcus pyogenes (Group A Streptococcus) is positive for pyrrolidonyl arylamidase, but it is β-hemolytic, not α-hemolytic.(Laboratory Diagnosis of Bacterial Infection)
Your patient returns today with another soft tissue abscess. He has had several methicillin-resistant Staphylococcus aureus skin and soft tissue infections in the past 3 months. Assuming this is a Staph aureus infection, which of the following tests is the best way to diagnose a Staph infection?
- Bacterial culture of an aspirate of the pus
- Direct fluorescent antibody for Staph on a sample of the pus
- Gram stain of the pus
- Polymerase Chain Reaction of the pus
- Serology for Staph aureus
- Currently there is no PCR for Staph aureus that is commercially available. Serology is not useful either, because everyone has been exposed to Staph aureus (since it is normal flora) and would have antibodies so it would not differentiate past exposure from current infection. A Gram stain could be suggestive of Staph, but coagulase-negative Staph is universally present on the skin, so Gram positive cocci in clusters would not be diagnostic in this setting. There is no DFA for Staph either. DFA is used for viral infections like HSV & VZV. (Laboratory Diagnosis of Bacterial Infection)
Bacterial culture of an aspirate of the pus. Culture is the best method for diagnosing a Staph aureus infection. It also permits susceptibility testing so you can determine if it is MRSA or MSSA (methicillin-susceptible)
A 3 day old term neonate presents with sepsis syndrome. Her CSF (cerebrospinal fluid) is culture negative but her blood is growing a β-hemolytic Gram positive coccus. Streptococcus agalactiae (Group B Streptococcus) can be tentatively identified and differentiated from Streptococcus pyogenes (Group A Streptococcus) by which of the following tests?
- Bile solubility
- CAMP test
- Catalase
- Coagulase
- Optochin
CAMP test. Streptococcus agalactiae (Group B Strep) can be tentatively identified and differentiated from Group A strep by the following tests: negative catalase test, positive CAMP test and hydrolysis of hippurate. Group A strep are catalase negative too so that doesn’t help differentiate them. Coagulase is a test used to differentiate species of Staph. Optochin and bile are used to diagnose Strep pneumoniae. (pneumococcus). Group B streptococci are identified definitively by the demonstration of the group-specific carbohydrate or the use of commercially prepared molecular probes. Group B streptococci produce a diffusible, heat-stable protein (CAMP factor) that enhances β hemolysis of Staphylococcus aureus. S. aureus (streaked from the top to the bottom of the agar plate) produces sphingomyelinase C, which can bind to erythrocyte membranes. When exposed to the group B CAMP factor, the cells undergo hemolysis (compare the two positive reactions of enhanced hemolysis to the immediate left of the vertical line) (Laboratory Diagnosis of Bacterial Infection)
A 21-year-old sexually-active college student living in the dorms was seen by student health with a complaint of two days of a sore throat. Examination of the pharynx revealed patchy erythema and exudates on the tonsillar pillars. A throat swab was sent to the lab. Gram stain of the smear revealed gram-positive cocci in chains and gram-negative diplococci. What is the significance of Gram stain of the smear in his case?
- It is not useful since diagnosis cannot be made by Gram stain alone.
- It provides a rapid means of diagnosing the infection.
- It is more specific than Streptococcal antigen testing.
- It suggests pharyngitis caused by Neisseria gonorrheae.
- It is evidence of infection with Group A Streptococcus and Neisseria.
It is not useful since diagnosis cannot be made by Gram stain alone. This is an important concept. Gram stains are most useful when performed on a specimen from a normally sterile site (i.e. cerebrospinal fluid, pleural fluid, peritoneal fluid, synovial fluid, middle ear fluid) OR from an abscess.Gram stains of body sites that are not sterile (e.g. throat, nose, stool, skin etc) do not give you much information – it confirms that there are bacteria present but doesn’t tell you if it is a pathogen or a colonizer because the gram stain gives gram morphology only, not the culture results, and many bacteria may appear similar on Gram stain. Most gram stains offer only preliminary information not diagnostic information (i.e. a Gram-positive cocci in clusters seen on a Gram stain of synovial fluid is suggestive of Staph aureus). Most of the time most physicians will not / should not narrow their empiric therapy based on a gram stain but rather should wait for the culture results. However, sometimes results of a gram stain of a sterile site will lead someone to prescribe an additional therapeutic agent while waiting culture confirmation.One setting where a gram stain can be diagnostic is in detecting gonococcal infection in men with purulent urethritis. It is very sensitive (>90%) and specific (98%). However its sensitivity in diagnosing infection in asymptomatic men is < 60%. It is also relatively insensitive in detecting gonococcal cervicitis in both symptomatic and asymptomatic women. (Laboratory Diagnosis of Bacterial Infection, Tutorial 1)
