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A 28-year-old female with folliculitis is not responsive to a 10-day treatment course with penicillin. An enzyme produced by the etiologic agent is most likely responsible for this treatment failure. At which site on the molecule shown does this enzyme act to destroy penicillin?
The structural integrity of the β-lactam ring in penicillins is essential for their antimicrobial activity. Many resistant strains of staphylococci produce a β-lactamase (penicillinase) that cleaves the β-lactam ring of penicillin at the carbon-nitrogen bond (4 in diagram). Other organisms, including coliform bacteria and Pseudomonas, produce cephalosporinases, metallo-β- lactamases, and carbapenamases that cleave the β-lactam rings of cephalosporins and/or carbapenems, as well as penicillins, at the same site. Penicillin amidase inactivates penicillin by disrupting the bond between the radical and nitrogen in the free amino group (1 in diagram). Bonds 2, 3, and 5 are not commonly broken by bacterial enzymes.
A 35-year-old male was treated with an aminoglycoside for an infection with a gram-negative rod. Which of the following bacterial processes is involved in transport of this antibiotic into the cell?
Aminoglycosides passively diffuse across the outer membrane of gram-negative bacteria through porins. The passage of aminoglycosides into the cytoplasm is an active process dependent on oxidative phosphorylation (d). Facilitated diffusion (a) involves specific protein carriers. Group translocation (c) occurs in the absence of oxygen. Fermentation (b) is a mechanism for production of energy. Transpeptidation (e) is a reaction involving the transfer of one or more amino acids from one peptide chain to another, or in the addition of amino acids to the growing peptide chain during translation.
A 36-year-old male developed a painful purulent urethral discharge following a 2-week vacation to Thailand. A Gram stain reveals gramnegative diplococci. Iron, which it obtains through surface receptors, is essential for the expression of the microorganism’s virulence factors. From which of the following molecules does it obtain iron?
Unlike E. coli, Salmonella, Pseudomonas, and other gram-negative bacilli, Neisseria gonorrhoeae does not produce siderophores (e). Instead the pathogenic Neisseria obtain iron through surface receptors that interact with lactoferrin (b) and transferrin, host molecules that sequester iron. Ferric oxide (a) is produced by some environmental bacteria. LPS (c) does not play a role in the iron uptake or metabolism of the bacteria. Pyocyanin (d) is a blue redox pigment produced by P. aeruginosa that can generate reactive oxygen intermediates and affect the electron transport chain and vesicular transport of eukaryotic cells.
A female medical student is admitted to the emergency department with symptoms of severe urinary tract infection (UTI). The Escherichia coli strain isolated from urine produced extended-spectrum β-lactamase. In which anatomic area of the cell would this enzyme be located?
The periplasm is the space between the outer membrane and the cytoplasmic membrane of gramnegative bacteria. The periplasmic space (c) in E. coli has been shown to contain enzymes involved in transport, degradation, and synthesis, including β-lactamase. The other structures are labeled in the diagram above (capsule—A; outer membrane—B; cytoplasmic membrane—D; cytoplasm). The chromosome and other structures found within or on the cell surface, such as the flagellum and pilus present in the diagram, were not indicated among the choices.
A 52-year-old male develops abscesses following surgery to repair an abdominal gunshot wound. Gram stain of the foul-smelling exudate from his abscess reveals numerous polymorphonuclear leukocytes (PMNs or neutrophils) and several gram-negative rods that did not grow on blood plates in the presence of O2. Metabolism of O2 results in toxic reactive oxygen species. Which of the following enzymes inactivates superoxide free radicals
Toxic oxygen radicals, superoxide and hydrogen peroxide, are generated during aerobic metabolism. Superoxide dismutase (e) catalyzes the reaction that detoxifies superoxide . Both catalase (b) and peroxidase (d) detoxify hydrogen peroxide, but peroxidases also act on lipid peroxides. The reaction catalyzed by catalase is . ATPase (a) converts ATP into ADP plus a free phosphate ion. Permeases (d) are membrane transport proteins that facilitate the passage of specific molecules across into or out of the cell.
A 52-year-old male develops abscesses following surgery to repair an abdominal gunshot wound. Gram stain of the foul-smelling exudate from his abscess reveals numerous polymorphonuclear leukocytes (PMNs or neutrophils) and several gram-negative rods that did not grow on blood plates in the presence of O2. Metabolism of O2 results in toxic reactive oxygen species. Which of the following antibiotics would be the best choice for treating this patient’s infection?
The etiologic agent of this man’s infection is an anaerobe, most likely Bacteroides fragilis. Ampicillin (a) is unlikely to be successful as most isolates of B. fragilis produce β-lactamase. Chloramphenicol (b) is rarely used in the United States because of its toxicity, although it is used for treatment of serious rickettsial infection such as Rocky Mountain spotted fever and as an alternative for treatment of bacterial meningitis in patients who have hypersensitivity to penicillins and cephalosporins. Ciprofloxacin (c), a fluoroquinolone antibiotic, has no spectrum of activity against anaerobes. Oxygen is necessary for transport of aminoglycosides like gentamicin (d) into the bacterial cell; thus, they have no spectrum of activity against anaerobes. Of the drugs listed, metronidazole (e) is the best choice for infections with anaerobic gram-negative bacilli. Additional antibiotics with activity against anaerobic gram-negative bacilli are carbapenems and β-lactam/β-lactamase inhibitor combinations. Bacteroides has become increasingly resistant to clindamycin, which used to be a good alternative, with up to 25% of isolates now resistant.
A teenaged boy suffered a foot laceration while swimming in a polluted water area in a river. He did not seek medical treatment, and the wound developed a foul-smelling exudate. One of the bacteria isolated from the abscess exudate was missing superoxide dismutase, catalase, and a peroxidase. Which of the following statements best describes this microorganism?
Superoxide dismutase is an enzyme found in both prokaryotic and eukaryotic cells that can survive in an environment of O 2. Lack of this enzyme, as well as peroxidase and catalase, ensures that a bacterium will not grow in the presence of O2, making it an anaerobe (d). Capnophiles (a) prefer an atmosphere with increased CO2 and ambient O2; Haemophilus ducreyi is capnophilic. Microaerophiles (c) such as Campylobacter jejuni prefer increased CO2 and reduced O2. Facultative anaerobes (b) can grow in the presence or absence of O 2. Obligate aerobes (e), such as Mycobacterium tuberculosis, require O2 for growth unless they can utilize an alternative terminal electron acceptor.
A 20-year-old pregnant female patient presents to the emergency room with a 4-day history of fever, chills, and myalgia. Two days prior to this, she had noted painful genital lesions that had begun as vesicles. Pelvic examination revealed extensive vesicular and ulcerative lesions on the left labia minora and majora with marked edema, and inguinal lymphadenopathy. Which of the microorganisms causing genital ulcers can be demonstrated by dark field microscopy?
The microbes that cause genital ulcers are Chlamydia trachomatis LGV serovars 1 to 3, Haemophilus ducreyi, herpes simplex virus types 1 and 2, Klebsiella granulomatis, and Treponema pallidum. The various methods for diagnosis are outlined in the below table. Treponema pallidum (e) can be demonstrated by darkfield microscopy in primary and secondary syphilitic lesions.
A 20-year-old pregnant female patient presents to the emergency room with a 4-day history of fever, chills, and myalgia. Two days prior to this, she had noted painful genital lesions that had begun as vesicles. Pelvic examination revealed extensive vesicular and ulcerative lesions on the left labia minora and majora with marked edema, and inguinal lymphadenopathy. Which of the following tests would prove that the patient is infectious for herpes simplex virus (HSV) type 2?
See the table above. Culture (a) is the best method to prove infectivity. However, viral culture decreases in sensitivity as the lesions begin to heal, so nucleic acid amplification testing or PCR (d) is increasingly used. PCR detects viral DNA, which may be present in non-infective particles. Because herpes simplex is shed intermittently upon reactivation, negative culture or PCR does not rule out latent infection. Viral antigens, detected by direct immunofluorescent assay (c) (or enzyme immunoassay), may be present after infectious particles have been neutralized or disrupted by antivirals. Cytologic detection by Tzanck smear (e) of genital lesions or cervical Pap smear is insensitive and nonspecific and should not be relied upon. Serologic detection (b) using tests that are type-specific may be used in cases or recurrent genital symptoms or atypical symptoms with negative culture; clinical diagnosis of genital herpes without laboratory confirmation, or a partner with genital herpes. Screening of the general population should not be done.
Gram-negative diplococci were demonstrated in a Gram stain of urethral drainage from an 18-year-old male who presented with symptoms of urethritis. Continuous passage of this strain on laboratory medium resulted in the reversion of a fimbriated to a nonfimbriated strain. Which of the following is the most likely implication of this phenomenon?
The vignette and figure detail a typical case of Neisseria gonorrhoeae. Bacteria may shift rapidly between the fimbriated (fim +) and the nonfimbriated (fim–) states. Fimbriae function as adhesions to specific surfaces and, consequently, play a major role in pathogenesis. Lack of fimbriae prevents colonization of the mucosal surface by the bacterium (c). Pili (fimbriae) are hairlike appendages that extend several millimeters from the gonococcal surface. Fim changes would have no effect on capsule presence (a) or loss of serologic specificity (d). Such changes would not cause the death of the organism (b), and Neisseria organisms never revert to a gram- positive staining result (e) due to the major differences found in the structures of gram-positive and gram-negative cell walls.
Twenty-eight hours after eating undercooked chicken, a 50-year-old farmer presents to the emergency room with abdominal pain, cramping, bloody diarrhea, and nausea. An isolate from the stool is biochemically identified as Salmonella enterica; however, the isolate cannot be serotyped. Which of the following has this organism lost?
LPS, a component of the outer membrane of gram-negative microorganisms consists of three regions: lipid A, which forms the outer leaflet of the lipid bilayer outer membrane; the core polysaccharide; and the O-antigen polysaccharide (e) side chain that confers serospecificity or serotype for the Enterbacteriaceae. Strains that have lost the O antigen cannot be serotpyed. Flagella (b) confer the H antigens, which are also used in the typing of Salmonella strains after the O typing; strains that have lost flagella cannot be H-typed. The capsule (a) confers the K antigen; pili (d) can be conjugative or be involved in motility or adhesion (usually called fimbriae). Neither the K antigens nor the pilus- nor fimbria-related antigens are utilized in the serotyping of Salmonella for epidemiologic purposes. The mannose receptor (c) is found on macrophages and used by them to characterize pathogenic microbes in the host innate response.
A 2-year-old infant is diagnosed with meningitis. A lumbar puncture reveals numerous neutrophils and gram-positive cocci in pairs that appear encapsulated. She is admitted to the hospital and started on intravenous (IV) β-lactams. Which of the following targets would most likely play a role in the development of resistance to this antibiotic in the most likely etiologic agent of this child’s meningitis?
Transpeptidases, or penicillin-binding proteins (PBPs) (c), are inactivated when bound to penicillin. Altered PBPs no longer bind the β-lactam antibiotic. The acquisition of a new PBP or modification of the existing one is the mechanism by which S. pneumoniae, the most likely etiologic of this child’s meningitis, become resistant to β-lactams. Altered PBPs are also seen in N. gonorrhoeae, S. aureus, and other bacteria. Bactoprenol (a) is not known to be involved in antibiotic resistance. Fluoroquinolones target the DNA gyrase (b), while rifamycins target the bacterial RNA polymerase (e). Bacteria do not utilize reverse transcriptase (d).
A cattle farmer develops necrotic lesions on his arms and face following a traumatic encounter with a bull. Selective inhibition of synthesis of dipicolinic acid by the etiologic agent of the infection would most likely inhibit the formation of the infective structure. Which of the following would be inhibited?
The vignette presents a case of cutaneous anthrax. Anthrax is acquired by cutaneous introduction, ingestion, or inhalation of endospores from contaminated animals such as cattle, sheep, and goats. Inhibition of dipicolinic acid synthesis would prevent formation of bacterial spores (a), but would not affect vegetative cells (b, e). Mycobacteria do not form spores. Dipicolinic acid is not found in fungal arthrospores (c) or microconidia (d).
A 30-year-old male presents to the emergency room with high fever and malaise, which he reports that it began 4 days ago and got progressively worse each day. He appears underweight and very ill. Physical examination reveals needle marks in both antecubital fossae. Upon listening for heart sounds, you hear a distinctive systolic heart murmur. You order blood cultures and make a presumptive diagnosis of acute bacterial endocarditis. Following is the growth curve of the organism growing in a nutrient medium at 35°C with both O2 and added CO2 present. In which of the following growth phases would the organism most likely be resistant to β-lactam antibiotics?
The bacterial growth cycle is characterized by four phases as shown in the figure for this question. In the lag phase (a), the bacteria are not dividing but exhibit dynamic metabolic activity as they adjust to the new environment. Depending on the species and the temperature, pH, and nutrients available, the bacteria begin dividing within a few hours and enter the log phase (b) where they grow exponentially for 12 to 18 hours. As nutrients become depleted and/or toxic waste products accumulate in the culture, growth slows so that the number of new cells and cells that die are balanced; this is the stationary phase (c). When nutrients are exhausted or the culture medium becomes too toxic, the number of viable cells decreases dramatically; this is the death phase (d). Similar phases occur when the bacteria are introduced into the host, although the timing may vary due to the specific host environment and the host defenses encountered. Antibiotics that affect the cell wall, such as β-lactams, are not effective in the lag phase (a).
A 30-year-old male presents to the emergency room with high fever and malaise, which he reports that it began 4 days ago and got progressively worse each day. He appears underweight and very ill. Physical examination reveals needle marks in both antecubital fossae. Upon listening for heart sounds, you hear a distinctive systolic heart murmur. You order blood cultures and make a presumptive diagnosis of acute bacterial endocarditis. Following is the growth curve of the organism growing in a nutrient medium at 35°C with both O2 and added CO2 present. On which of the following growth phases would treatment with gentamicin have a maximal effect?
Aminoglycosides exert three effects on protein synthesis: (1) block initiation of translation, (2) cause incorporation of incorrect amino acids into the growing peptide chain, and (3) elicit premature termination of translation. Protein synthesis occurs throughout the growth cycle, but optimum effectiveness of aminoglycosides is seen in the log phase (b).
An outbreak of diarrhea is thought to be related to a group of vendors who were selling hot dogs at the county fair. Stool cultures are positive for Shigella in almost all individuals who ate the hot dogs. Growth of the isolated colonies in nutrient liquid medium without the transfer to fresh medium will eventually induce the death phase of the organism. Which of the following is a limiting factor in microbial growth under laboratory conditions?
Differentiation of the gram-negative bacilli that cause gastroenteritis begins with aerobic (Enterobacteriaceae and Vibrio) versus microaerophilic (b) (Campylobacter) growth. Morphology on a lactose-containing selective agar such as MacConkey agar separates Salmonella and Shigella, which do not ferment lactose (a), whereas E. coli and Vibrio do. Shigella can be differentiated from Salmonella on agar containing iron; Shigella do not produce H2S (c) while Salmonella do. Vibrio species are oxidase positive (d); the Enterobacteriaceae are not. Urease production (e) distinguishes Proteus spp. from Salmonella.
A 62-year-old woman with diagnosed type 2 diabetes lived alone and did essentially nothing to manage her illness, including disregarding her physician’s instructions. She was taken to her local emergency room (ER) with severe, multiple infected foot lesions, which yielded a variety of opportunistic microbes with a mixture of antibiotic susceptibilities. The physician decided to treat with systemic and topical antimicrobials. Which of the following antimicrobial agents must only be used topically?
Penicillin (d) is well-tolerated as is itraconazole (c). Vancomycin (e) is irritating and can result in phlebitis at the site of injection; nephrotoxicity of aminoglycosides is exacerbated by concomitant use of vancomycin, but can be controlled by careful dosage. Gentamicin (b) and other aminoglycosides are ototoxic and nephrotoxic, but these adverse effects can usually be controlled by monitoring of serum concentration and careful dosage. Bacitracin (a) is highly nephrotoxic, so much so that it cannot be administered systemically. Because bacitracin is poorly absorbed, it can be used topically, providing local antibacterial activity but no systemic toxicity.
A 42-year-old alcoholic man presents with fever, chills, cough, and chest x-ray suggestive of pneumonia. The Gram-stained smear of sputum shows many PMNs and gram-positive cocci in pairs and chains. Which of the following is the correct order of the procedural steps when performing the Gram stain?
First described in 1884 by a Danish physician, Hans Christian Gram, the Gram stain has proved to be one of the most useful diagnostic laboratory procedures in microbiology and medicine. The Gram stain procedure is characterized by the following steps: (1) fixation of the bacteria to the slide, (2) crystal violet (acridine dye) treatment, (3) iodine treatment, (4) decolorization using alcohol/acetone wash, and (4) counterstaining using safranin. Gram-positive bacteria have thick outer walls with no lipids, whereas gram-negative bacteria have a thin wall and an outer membrane. The difference between gram-positive and gram-negative organisms is in the cell-wall permeability to these complexes on treatment with mixtures of acetone and alcohol solvents. Thus, gram-positive bacteria retain purple iodinedye complexes, whereas gram-negative bacteria do not retain these complexes when decolorized using an alcohol/acetone wash.
A 78-year-old man presents to the local emergency department with a severe headache and stiff neck. The cerebrospinal fluid (CSF) specimen is cloudy. Analysis reveals 400 white blood cells per cubic millimeter (95% PMNs), a protein concentration of 75 mg/dL, and a glucose concentration of 20 mg/dL. While in the ER, a resident does a Gram stain of the CSF but mistakenly forgets the iodine treatment step. If the meningitis is caused by Streptococcus pneumoniae, how will the bacteria seen on the resident’s slide appear?
If the iodine treatment step is omitted during the Gram stain process, the purple iodinedye complexes will not form. The crystal violet will wash away during the alcohol/acetone decolorization washing step and all cells will appear red (d). Gram staining of pus or fluids along with clinical findings can guide the management of an infection before culture results are available in the clinical setting.
A 28-year-old female just returned from a 1-week cruise with stops along the coast of Mexico. Forty-eight hours after her return she is reported to have headache, fever, abdominal cramps, and constipation. Over the next 5 days, her fever increases with continued complaints of myalgias, malaise, and anorexia. A blood culture is positive for S. enterica ser. Typhi. Her condition improves with a treatment course of a ciprofloxacin. Which of the following is the function of porins that would prevent the effective use of this antimicrobial?
Porins are protein trimers that function in outer-membrane (OM) permeability. Porins permit the transfer of molecules across the OM (e) including the passage of many antimicrobial drugs such as the fluoroquinolones. Porins participate in multidrug-resistance efflux pumps that export drugs like ciprofloxacin from the cell. Porins play no role in the metabolism required for hydrolysis of antimicrobials (a), metabolism of metabolic intermediates (b), serologic stabilization of the O antigen (c), or inactivation of hydrophobic antimicrobials (d).
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