ID lab final
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Types of enriched, non-selective media
- Mueller Hinton
- Thioglycolate broth
- Sabouraud dextrose
What is blood agar?
An enriched, nonselective culture media containing a basal medium and blood
What is chocolate agar?
An enriched, nonselective culture media that is a modified blood agar medium. It consists of blood or hemaglobin that has been added to heated basal media. It supports the growth of most bacteria, plus Haemophilus and pathogenic Neisseria strains (which won't grow on blood agar)
What is Mueller-Hinton agar?
Enriched, nonselective culture media. It is recommended media for susceptibility testing in bacteria
What is Thioglycolate broth?
An enrichment broth used to recover low numbers of aerobic and anaerobic bacteria. Supplement with hemin and vitamin K to enhance recovery of anaerobic bacteria.
What is Sabouraud dextrose agar?
An enriched medium used to isolate fungus. You can make it selective by adding antibiotics and reducing the pH.
How can you differentiate Staph from Strep?
- Staph is catalase +
- Strep is catalase -
How can you differentiate GAS from GBS?
- GAS: Bacitracin sensitive
- GBS: Bacitracin resistant
How can you differentiate Staph aureus from other types of staph?
- Staph aureus: Coagulase +
- Other staph (aka coagulase negative staph or CoNS): Coagulase -
Which bacteria are indole +?
Which bacteria are oxidase +?
Pseudomonas aeruginosa, Shigella, Salmonella, and H. influenza
Which are urease +?
Klebsiella, proteus, staph saprophyticus
What test differentiates between the coagulase negative staph (Staph epidermidis vs. staph saprophyticus?)
- Staph epidermidis: Novobiocin sensitive
- Staph saprophytius: novobiocin resistant (also urease +)
How can you ID strep pyogenese?
- GAS: G+ coccus growing in chains, catalase negative, beta hemolytic, bacitracin sensitive
- Also PYR positive
What two organisms are PYR +?
Streptococcus pyogenes (the only strep positive for PYR) and Enterococcus
What G+ bacteria are soluble in bile (aka sensitive to bile)?
- Strep pneumonia, Enterococcus, nonenterococcus species (strep bovis)
- Big difference between Enterococcus and nonenterococcus: Enterococcus can grow in 6.5% NaCl
What test differentiates between Strep pneumo and viridans strep (S. mutans)?
- Strep pneumo: Optochin sensitive
- Viridans: Optochin resistant
How do you differentiate between Enterococcus and non-enterococcus species?
- Enterococcus: Bile insoluble with 6.5% NaCl
- Non-enterococcus: Bile insoluble WITHOUT 6.5% NaCl
What is the best way to diagnose a staph infection in a soft tissue abscess?
Bacterial culture of the pus. It is also useful because it allows for susceptibility testing.
- Bonus: Could you use PCR, DFA, or Serology?
- No, because there is no PCR or DFA available, and because everyone has been exposed to Staph, a serology would be positive.
- Double bonus: Could you gram stain the pus?
- Yes, but it is not diagnostic because CoNS is found on the skin and would show up on the stain.
What organisms are CAMP positive?
GBS and Listeria monocytogenes
Aside from bacitracin sensitivities, how can you differentiate GAS from GBS?
- GAS: CAMP test negative
- GBS: CAMP test positive
When are gram stains most useful as a diagnostic procedure?
When they are performed on a specimen from a normally sterile site (CSF, synovial fluid, middle ear fluid, etc) or from an abscess
What is one setting in which a gram stain can be diagnostic?
Detecting gonococcal infection in men with purulent urethritis (doesn't work in asx men, or asx/sx women)
What organisms are catalase +?
Staph, serratia, pseudomonoas, nocardia, and aspergillus
How long should blood cultures be incubated before being finalized as "no growth"?
For five days
- Bonus: Is it routine to Gram stain blood samples before incubation?
- No because the frequency of true positive cultures is slow, and because in order to gram stain, you must have a high number of organisms in the bloodstream. Once the bottles have been incubated, then a gram stain can be done
How do you diagnose Enteroviral meningitis?
Enteroviral PCR on CSF
- Bonus: Does a DFA exist for enteroviruses?
How do you diagnose Influenza pneumonia?
DFA of nasopharyngeal swab
How do you diagnose cutaenous fungal infections like tinea capitis (seen as circular pruritic scaly area with possible hair loss at the center of the lesion)?
- KOH prep: Take hair or skin/nail sample and look for fungal hyphae by direct microscopy
- ALSO: Calcofluor white (fluorescent stain) is used for direct microscoy of fungal infections
How do you diagnose Cryptococcus species?
India Ink: modified KOH in which ink is added to contrast material. The polysaccharide capsule of Cryptococcus excludes the ink, creating a halo around the yeast cell
What is a Tzank test? What do you see in it? Is is still used a lot?
- Tzank test: Direct examination of cells from the base of a vesicle to ID HSV and VZV (can't differentiate)
- See: multinucleate giant cells and Cowdry type A bodies (intranuclear inclusions)
- Not used much anymore because there are DFA for HSV1, HSV2, and VZV
What method do you want to use in a person who has the clinical scenario for endocarditis?
Blood culture: must take several from different sites at different times over 24-48 hr period before starting antibiotics
For what organism is Thayer-Martin agar used?
- Neisseria gonorrheae: require cystine and energy source for growth. Will not grow on blood agar, but will on chocolate
- TM agar: has VPN media (vancomysin for GP, polymyxin for GN, and Nystatin for fungus)
How do you diagnose acute Varicella infection?
VZV DFA of a vesicle
- Could you do serology and look for IgG?
- No, in acute infection IgG titers won't have developed
When would you use a Shell vial fluorescent antibody culture of a vesicle?
What is the preferred method for diagnosing enteroviral meningitis?
- Bonus: Does serology work in diagnosing enteroviral infections?
- No, because it is an indirect measure, and because there are so many enteroviruses it isn't practical
What is the best method for diagnosing HSV infection?
- Infection with a rash: DFA and HSV culture of the base of the lesion
- HSV encephaliis: HSV PCR on CSF
What is the best way to diagnose a respiratory virus?
Nasopharyngeal swab for DFA (TOP!!!) and culture
What is the best way to diagnose EBV?
Specific serologic tests for Ab to EBV viral antigens (more dependable than monospot)
Monospot still used because it is less expensive and turnaround in faster; however, it can be FN in early infection, unreliable in young children, and FP in other situations
Viral culture is not practical to diagnose EBV infection
What is the likely source of infection in neonatal meningitis caused by strep agalactiae (GBS)?
Mom's vagina -> use INTRPARTUM antibiotics
Why is Mycobacteria no easily seen on Gram stain?
Their cell wall has waxy lipids (mycolic acids) that don't stain well
What is the best way to diagnose Mycobacterium tuberculosis?
Sputum culture with positive nucleic acid test for TB
What is the significance of seeing acid-fast organisms on a stain of pus taken from a post-surgical lesion or other infection?
It indicates "rapid growers" of nontuberculosis mycobacteria. These can grow is 7-10 days
What is the best way to diagnose a patient with a lung abscess due to mixed anaerobic flora?
What is the best way to diagnose Influenza pneumonia?
PCR or DFA of nasopharygneal swab
What organism grows on aerobic culture with buffered charcoal yeast extract? What do you see?
- Legionella: facultative intracellular aerobic bacteria that is slender, pleomorphic, nonfermentative, GNR
- Nutritionally fastidious and requires L-cysteine and iron salts to grow
What two organisms are branching filamentous G+? How do you distinguish them?
- Actinomyces: Anaerobe, not acid fast
- Nocardia: Aerobe, partially acid fast
How do you diagnose mycoplasma pneumonia?
Is there a DFA for TB?
What two media grow Bordetella pertussis?
- Bordet-Gengou media
- Regan-Lowe charcoal media supplemented
What are the most common causes of actue viral encephalitis? How do you diagnose them?
- Etiologies: HSV, enteroviruses, and arboviruses (West Nile)
- Dx: HSV PCR, enteroviral PCR, and West Nile PCR on CSF
What is the best way to determine etiology of brain abscesses?
Bacterial cultures of aerobic and anaerobic bacteria of fluid aspirated from the abscess
What organisms have a positive quelling reaction?
Those with capsules: Klebsiella, E. coli (K antigen), Strep pneumo, HIB
How do you diagnose Trachomonas vaginalis?
How are vibrio infections diagnosed?
- Can you do serology (IgG titers)?
What organisms can you grow on stool culture for bloody diarrhea?
Yersinia enterolytica, Salmonella, Shigella, Campy, EHEC
On what organisms can you do an EIA?
- C. difficile: for the toxin, confirm with culture.
On what organisms can you do a stool O&P?
- Entamoeba histolytic
What is the most appropriate diagnostic test for malaria?
- Microscopic exams of thick and thin peripheral blood smear preps
- Thick smear: used to find trophozoites
- Thin smear: used to determine species
How is acute Chagas's disease diagnosed in the US? Chronic in the US? What about outside the US?
- Acute in US: Peripheral blood smear
- Chronic in US: Serology
- Outside US: Xenodiagnosis
What is the most appropriate test to diagnose schistosomiasis?
Microscopic exam of stool for eggs
What is the best way to diagnose Taenia solium infection?
Microscopic exam of stool for eggs
What is the best test to evaluate for Strongyloides?
Microscopic exam of stool for larvae
How do you diagnose syphilis?
- If primary: Biopsy of chancre with darkfield microscopy
- If secondary: VRDL/RPR with FTA-ABS follow-up
- If tertiary: FTA-ABS
If RPR and FTA-ABS are both positive, you want to treat the patient, even if he or she lacks sx!
On what type of media should a stool sample be incoulated to find enteric pathogens?
What are examples of selective differential media?
- MacConkey agar
- Mannitol salt agar
- Xylose-lysine deoxycholate agar
- Lowenstein-Jensen medium
What is MacConkey agar?
A selective differential medium that selects for GN bacteria and differentiates lactose-fermenting from non-fermenting
What is Mannitol salt agar?
A selective medium used to isolate staph: Staph can grow in high salt, and Staph aureus can ferment mannitol to produce yellow-colored colonies
What is Xylose-lysine doxycolate (XLD) agar?
Selective agar used to detect Salmonella and Shigella
What is Lowenstein-Jensen medium?
Selective differential media used to isolate mycobacteria; malachite green is added to inhibit GP growth
What is specialized culture media and what are some examples?
- Media used to detect specific organisms with fastidious growths
- Examples: buffered charcoal yeast extract for Legionella, Regan Lowe for Bordetella, and MacConkey sorbitol for EHEC
What is the best way to diagnose a person with signs and sx of acute HIV infection?
HIV antibody and HIV RNA (viral load), because the HIV Ab test may be negative if it is too early in the infection
How can you diagnose infant botulism?
Culture stool for C. botulinum and/or C. botulinum toxin assay on stool
How do you diagnose Yersinia pestis?
DFA and culture on aspirate of bubo
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