Lab Diagnosis

Question 1

Lab Diagnosis of Enterobacteriaceae

Answer 1

Grows readily on media

Blood agar for normally sterile material

Selective media for specimens with lots of normal flora like feces sputum, urine (MacConkey agar, Eosin Methylene Blue agar)

Biochemical ID (many commercial systems available to ID genus and species except salmonella)
api 20E

Enterotube

Question 2

MacConkey Agar

Answer 2

Contains bile salts crystal violet
Lactose
Peptone
pH indicator

Question 3

Eosin Methylene Blue Agar

Answer 3

Has eosin
Methylene Blue
Lactose peptone

Question 4

Lab Diagnosis of Vibrio Cholerae

Answer 4

Survives poorly in an acidic or dry environment so has to be collected and cultured fast

Grows on variety of simple media (MacConkey Agar or TCBS-Thiosulfate Citrate Bile Salts Sucrose)

V. cholerae grows without salt (most virbos are halophilic)
Gram -

Question 5

Campylobacter Lab Diagnosis

Answer 5

Feces refrigerated and processed in a few hours (avoid oxygen exposure) Selective filtration (can pass through 0.45 filter)

Microscopically darting motility in fresh stools and cultured organisms appear small, curved rods with gull wing shape

Culture growth at microaerophilic atmosphere supplemented with CO2 at an elevated temp 42C and grown on selective media (supplement with blood/charcoal to remove oxygen radicals and antibiotics “vancomycin” to inhibit contaminating microbes)

Question 6

Helicobacter pylori Lab Diagnosis

Answer 6

Direct microscopy of gastric biopsy

Antigen detection (h. pylori in stool)

Culture on media with blood, hemin or charcoal (protects for oxygen free radicals)

Serology (detects presence of anti H. pylori antibodies in serum)

Urease test

Question 7

Neisseria Meningitidis Lab Diagnosis

Answer 7

Bacteria abundant in CSF and blood

Spinal tap/blood smear and gram stain

Culture on selective (Modified Thayer-Martin-contains Vancomycin, Colistin, Nystatin, and Trimethoprim) and non-selective media (chocolate agar)

Oxidase test

Commercial serological test to detect capsular antigen (can give false pos)

Question 8

Neisseria Gonorrhoeae Lab Diagnosis

Answer 8

Gram stain of discharge (Small gram - diplococci in presence of PMN)

Culture

Susceptible to drying and cooling (use pre-warmed selective media or nonselective media)

Some strains inhibited by vancomycin, fatty acids, and trace metals in protein hydrolysates and agar found in nonselective media

Question 9

Positive ID of Gonorrhoeae

Answer 9

Presumptive test
Oxidase +
Catalase +
Gram - diplococci

Commensals found in oropharynx can be confused with N. gonorrhoeae

Question 10

Norcardia Lab Diagnosis

Answer 10

Microscopy is sensitive and specific (branching partially acid-fast organisms)

Culture on selective media such as buffered charcoal yeast extract (BCYE) takes up to a week

Genus levels ID can be made microscopically

Species level requires genomic analysis

Question 11

M. Tuberculosis Lab Diagnosis

Answer 11

Radiographic evidence

Tuberculin skin test detects exposure to bacteria (PPD inoculated intradermal layer, read 48h later, exposed person mounts immune response, rxn read by measuring induration)

Microscopically detects acid fast bacteria in samples

Culture (Lowenstein-Jensen or Middlebrook agar) takes 3+ weeks

Question 12

Tuberculoid Leprosy

Answer 12

Few erythematous hypigmented plaques with flat centers and raised demarcated borders

Sensory loss and nerve enlargement

Few or no acid fast bailli observed

Low infectivity

Reactive to lepromin

Question 13

Lepromatous Leprosy

Answer 13

Many erythematous macules, papules, or nodules

Extensive tissue destruction

Foam macrophages with few lymphocytes

High infectivity

Non-reactive to lepromin