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40 Cards in this Set
- Front
- Back
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What two preparatory procedures are performed on a slide prior to Gram staining?
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Air drying and fixation with methanol or gentle heat.
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What two media are commonly used to inhibit the growth of Gram-negative bacilli while allowing Gram-positive bacteria to grow?
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Colistin-nalidixic acid (CNA) and phenylethyl agar (PEA) are two common media that select for Gram-positives.
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What chemical is used as a catalyst to achieve anaerobic conditions for culturing bacteria?
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This is palladium.
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Review the key biochemical features of the genus Staphylococcus.
Catalase: Bacitracin: Growth in NaCl: |
Staphylococci are catalase-positive, bacitracin-resistant, coagulase-positive
(S. aureus) or -negative (S. epidermidis and S. saprophyticus), and grow in NaCI. |
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True/False. The 'Medusa head' colony morphology in a Gram-positive, spore-forming rod that is catalase-positive is diagnostic of Bacillus anthracis.
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False. This pattern can also be seen in the other major pathogen in this genus, B. cereus.
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Both Nocardia spp. and Actinomyces have a similar Gram stain morphology.
Describe this morphology and state which stain is used to differentiate between the two. |
Both are Gram-positive, long, beaded, branching bacilli. Nocardia spp. are partially acid-fast, while Actinomyces spp. are not.
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Which Nocardia species are most likely in these patients?
22-year-old Venezuelan male with leg ulcer: 60-year-old US female with multiple centra l nervous system (CNS) abscesses: |
The leg ulcer is due to N. brasiliensis. The systemic illness with CNS abscesses is due to N. asteroides.
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List three selective and differential media that would be useful in the workup of a Salmonella outbreak.
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Three good media are xylose-lysine deoxycholate (XLD), Hektoen enteric (HE), and bismuth sulfite (BS). Salmonella colonies will have black centers, in contrast to Shigella.
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All members of the Enterobacteriaceae family are oxidase ???? and nitrate ????
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Enterobacteriaceae are all oxidase-negative and nitrate-positive (reducers).
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How are Escherichia coli, Klebsiella spp. and many of the Enterobacter spp. differentiated from other members of the Enterobacteriaceae family on MacConkey agar (MAC)?
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All are lactose fermenters that will manifest as pink colonies on MAC.
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Clinical Consultation: A 36-year-old oyster fisherman was hospitalized for gastroenteritis and became septic over a 24-hour period. Your differential d iagnosis includes members of the Vibrio genus, including V. vulnificus because he is septicemic, V cholerae, and V parahaemolyticus. Isolates from blood cultures are motile, curved, Gram-negative rods (GNRs). How will the following tests support a general diagnosis of Vibrio spp.?
Nitrate: Oxidase: Growthat42°C: Growth in 3% NaCl: Which Vibrio species is supported by positive salicin and cellobiose tests? |
Vibrios are nitrate--, oxidase-- and NaCl-positive. They do not grow at 42°C, but C. jejuni does, and should be ruled out in a case of gastroenteritis. V vulnificus is positive for salicin and cellobiose.
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Two genera of GNR that are implicated in gastroenteritis are Aeromonas and Plesiomonas. A common biochemical test that suggests these two genera, rather than one of the Enterobacteriaceae, is ???
which is positive in Aeromonas and Plesiomonas. |
Aeromonas and Plesiomonas are both oxidase-positive, but Enterobacteriaceae are negative.
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Campylobacter jejuni is the most common cause of bacterial enteritis in the USA. Review its characteristic features.
Growth temperatures___ and selective for C. jejuni, particularly____ Oxygen concentration for growth:____ CO2:_____ Fecal leukocytes:____ Motility:____ Gram morphology:___ Oxidase:___ |
Campylobacter species grow at 37°C and C. jejuni grows at 37°C as well as 42°C, requiring reduced oxygen (5-10%) and increased C02 (10%). Fecal leukocytes are seen in only 25% of cases, so this is not helpful in the diagnosis. It is motile, is an S-shaped small GNR, and is oxidase positive.
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Clinical Consultation: An 8-year-old girl with cystic fibrosis is hospitalized with pneumonia. The differential diagnosis includes Pseudomonas aeruginosa and Burkholderia cepacia. Are there media that will select for B. cepacia?
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As B. cepacia portends a worse prognosis for cystic fibrosis patients, distinguishing it from P. aeruginosa is important. Three media, Pseudomonas cepacia (PC), oxidative fermentative base, polymyxin B, bacitracin lactose (OFBL), and B. cepacia-selective agar (BCSA) are all helpful, as are the following biochemical characteristics:
Lysine decarboxylase is negative in P. aeruginosa and positive in B. cepacia. Arginine dihydrolase is positive in P. aeruginosa and negative in B. cepacia . Maltose oxidation is negative in P. aeruginosa and positive in B. cepacia. |
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The HACEK organisms are well-known agents of endocarditis. They may be distinguished by their biochemical characteristics. Except for Kingella hingae, they each have a partirular test, which is positive in just one of the organisms and negative in all of the others, including Kingella. Review which HACEK organism is uniquely positive for the following tests.
Lactose: Indole: Lysine decarboxylase: Catalase: |
Lactose- Haemophilus aphrophilus.
Indole - Cardiobacterium hominis. Lysine decarboxylase - Eikenella corrodens. Catalase - Actinobacillus actinomycetemcomitans. |
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What culture medium is used to isolate Legionella spp.?
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.Buffered charcoal yeast extract (BCYE) is used to grow Legionella spp.
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What is the principal virulence factor of Neisseria meningitidis?
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This is the lipopolysaccharide-endotoxin complex, which activates the clotting cascade.
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What are the incubation parameters for Neisseria spp?
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2-8% C02 at 35°C.
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Because Moraxella spp. wi ll occasionally yield positive carbohydrate reactions, what two biochemical tests distinguish them from Neisseria spp?
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DNase and nitrate reduction are both positive in Moraxella spp. but negative in Neisseria spp.
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The tetravalent polysaccharide vaccine for N. rneningitidis protects against what four serogroups?
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The vaccine protects against serogroups A, C, Y, and W135. It does not protect against serogroup B.
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Review the differential characteristics of the Haemophilus species by identifying which bacteria require which supplemental factors.
Requires X and V: Requires X only: Requires V only: Requires neither: |
X and V are required by H. influenzae and H. hemolyticus.
X only is required by H. ducreyi. Chancroid, the disease caused by H. ducreyi, is sexually transmitted. One may think of the 'X-rated' disease agent as requiring only the X factor. V only is required by H. parainfluenzae and H. paraphrophilus. The two 'paras' go together. H. aphrophilus requires neither. |
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Haemophilus aphrophilus is the H in the HACEK organisms. The characteristic test in differentiating Haemophilus among the HACEKs.
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Lactose fermentation.
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What is the action of the pertussis toxin of Bordetella pertussis?
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The pertussis toxin inhibits intracellular signal transduction factors.
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B. pertussis is a slow-grower but, after 4 days, one may look for this characteristic colony morphology.
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B. pertussis colonies are described as 'a drop of mercury' .They are small and shiny.
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What are the typical biochemical reactions of Brucella spp.?
Catalase: Oxidase: Urease: |
Brucella is positive for catalase, oxidase, and urease.
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How well do Pasteurella spp. grow on the following media?
BAP: EMB: MAC: |
Pasteurella grows well on BAP but poorly on EMB and MAC. This growth pattern is a key aid to diagnosis of P. multocida.
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What nutrient is necessary for the growth of Francisella?
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Francisella requires cystine or cysteine for growth.
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What is the etiologic agent of Haverhill disease?
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Streptobacillus moniliformis is the agent of Haverhill disease. In this section of the text, this disease is also called rat bite fever. However, in Chapter 58, the spirochete Spirillum minus is identified as the agent of rat bite fever.
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Which toxin is predominantly responsible for the enterotoxicity of Clostridium difficile?
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Toxin A. There is also a toxin B, which plays a minor role in human disease.
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A double zone of hemolysis on BAP and boxcar-shaped, large, Gram-positive rods are presumptively diagnostic for what anaerobe in a case of massive tissue necrosis?
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Clostridium perfringens. It is importa nt to identify patients with gas gangrene as opposed to those with an anaerobic cellulitis, as the former disease may require amputation while the latter generally does not.
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Briefly describe the traditional antimicrobial resistance testing procedure for the penicillinase-resistant penicillins, using disk diffusion.
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A 1 microgram oxacillin disk is placed on Mueller-Hinton agar and incubated 24 hours at 35°C.
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Clinical Consultation: A 25-year-old male has a necrotizing fasciitis. The blood agar plate (BAP) shows small white colonies with beta-hemolysis. To confirm a suspicion of group A Streptococcus, which three biochemical tests would you perform,
beginning with one to rule out Staphylococcus? |
Start with catalase (negative). Then perform PYR hydrolysis (positive). Finally, test for bacitracin (S).
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What two tests offer presumptive identification of group B Streptococcus, if positive?
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Hippurate hydrolysis and cAMP test are both positive in group B Streptococcus.
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What is the first test to perform to presumptively identify S. pneumoniae in a mucoid alpha-hemolytic isolate?
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Optochin (P) is the first test to do. It is an old standard, and S. pneumoniae is positive.
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Clinical Consultation: A blood culture on a 67-year-old female has grown out Gram-positive cocci in chains that are gamma-hemolytic. Describe how L-pyrrolidonyi-P naphthylamide (PYR) hydrolysis, growth in 6.5% NaCl and esculin hydrolysis will help you distinguish between Enterococcus and viridans.
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Enterococci are PYR-positive, grow in 6.5% NaCl, and are esculi n-positive.
Viridans is negative for all three, except that about 10% of viridans will test esculin-positive |
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Does S. pneumoniae require penicillin sensitivity testing?
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Yes. Use the 1 microgramg oxacillin disk.
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Name the immunodiffusion test that discriminates between oral and skin diphtheroids or corynebacteria, and the more pathogenic Clostridium diphtheriae.
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The Elek test is an immunodiffusion test, rarely performed in routine laboratories now that diphtheria is so uncommon. A paper impregnated with a strip of diphtheria antitoxin is incubated with a streak of culture laid out at a 90° angle to the strip. A precipitin line at 45° between the two lines indicates toxin production and thus C. diphtheriae.
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Clinical Consultation: A 3-day-old female has meningitis. Small colonies with a narrow zone of beta-hemolysis are isolated . What three tests, including growth patterns and motility characteristics, will distinguish this as Listeria monocytogenes and rule out group B Streptococcus?
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L. monocytogenes is catalase-positive, grows at 4°C and displays tumbling motility at room temperature. In this age group, group B Streptococcus must be considered.
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Clinical Consultation: A 31-year-old fisherman develops an elevated cutaneous eruption with a violaceous zone around the site of inoculum, which was a cut he sustained on the back of his hand. After 5 days, a BAP reveals some growth. What will be the result of these tests if your suspicion of Erysipelothrix rhusiopathiae is correct?
Oxidase: Catalase: Motility: H2S in TSI: |
E. rhusiopathiae is oxidase-, catalase- and motility-negative. It is H2S-positive in TSI.
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What are the three components of the complex anthrax toxin?
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The three toxins are edema factor, contributing to pulmonary edema; protective antigen; and lethal factor
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