Gram Negative Rod/UTI Lecture and Small Group

Question 1

1. Discuss the methods used by the clinical microbiology lab to process urine samples to assess for bacteriuria.

Answer 1

examination of Gram stain smear of fresh uncetrifuged urine, smear with at least one organism per oil immersion field correlates with significant colony count, indicative of cocci or rod shaped org

E. coli, Proteus: Gram neg rods
Enteroccoci and Stpahylococci- Gram positive cocci
Pseudomonas and Serratia are also possible (gram negative rods)

Question 2

2. Interpret a urine colony count and determine when it is clinically significant.

Answer 2

patients with more than 100,000 (10^5) colony forming units per milliliter or urine usually have UTI, contamination is indicated by 1,000 or few bacteria per mL, although some patients with true bacteriuria have counts much lower than 100,000 cfu/mL

Question 3

3. Discuss the reasons for artificially low and artificially high urine colony counts.

Answer 3

artificially low: frequent urination, consuming large volumes of fluid, hydration IV

artificially high: contamination due to collection method, or not plated within 1 hr of collection (or not refrigeration)

Question 4

4. Explain the principles of facultative/ aerobic Gram-neg rod identification.

Answer 4

GNR can be classified as a lactose fermenter or non lactose fermenter by the way the colony looks on the EMB plate (dark v. colorless, E.coli metallic green sheen)

further inoculation of plates with selective media can differentiate

carbohydrate fermentation tests
nitrogen utilization test (H2S black color)

API strip used to identify Gram neg rods

Question 5

5. Interpret the results of various antimicrobial susceptibility tests and explain how these aid in choosing optimal antimicrobial treatment.

Answer 5

MIC : x 4 antimicrobial level goal at infection site
broth tube dilution (most accurate)- comparing turbidity
microdilution method- commercially available

MBC- lowest concentration that will kill all or 99.9% of bacterium: use for severe infection where bactericidal activity is necessary; MBC is always equal or higher than MIC

kirby bauer test: disk diffusion test (overnight incubation with impregnated disks)

E test: combo of dilution and diffusion test- quantifies the antimicrobial susceptibility (continuous and exponential gradient of antibiotic concentration is created directly underneath the strip) zone edge intersects at MIC

Question 6

1. Name the most common Gram-negative rod causing uncomplicated urinary tract infections (UTI)

Answer 6

E. coli

Question 7

2. Name the most common Gram positive coccus causing community acquired UTIs in sexually active females.

Answer 7

Staph. saprophyticus

Question 8

1. List the 3 classic symptoms of cystitis.

Answer 8

frequent urination, burning on urination, suprapelvic pain

Question 9

2. Describe the 2 most common routes by which UTIs progress, from colonization to clinical infection of the bladder/kidney.

Answer 9

ascending from the urethra: bacteria from feces, skin and vgaina enter the urethral meatus
descending from blood seeding: particularly associated with S. aureus and candida

Question 10

3. Describe the laboratory tests used to diagnosis UTIs and evaluate their place in statistical v. lab-certified medicine.

Answer 10

clarity of urine (turbidity)

urine reagent strips (dip sticks) can detect leukocyte esterase and bacterial nitrate (85% sensitive if patients are not neutropenic and bacteria present produce nitrate reductase

place measured amount of urine on agar and counting of bacterial colonies that grow- streak plate method deliver specific quantity

colony is multiplied by the dilution used, report will include the predominant org is more than one is present

selective media include EMB, MacConkey agar plate

Question 11

4. Define the 2 types of recurrent UTIs why they occur (disruption of host defenses as well as patient factors and how to prevent them.

Answer 11

relapse: same antibiotic susceptibility pattern
reinfection: org with different antibiogram or different organism altogether, reinfecion with same organism

Question 12

5. Discuss when it is appropriate to treat an aysmptomatic bacteriuria if detected.

Answer 12

pregnant women and whenever instrumentation is being introduced

Question 13

What are causes treatment failure?

Answer 13

compliance

appropriate collection

microbe susceptibility (TMP-sulfa, longer course, what is local resistance)

Question 14

What are possible drug treatments of UTI?

Answer 14

TMP/sulfa, nitrofuratoin, ciprofloxacin and ampicilin

Question 15

What are host defense and conditions that cause increased UTI?

Answer 15

urine flow/flushing, urine pH, osmolality and urea, as well as prostate protective secretions are defenses

tumor, stone, stricture, prostate hyperplasia and pregnancy can lead to mechanical disruption

neurogenic: spinal cord injury, multiple sclerosis

functional - reflux seen in children

diabetes increase glucose in urine

Question 16

Describe ways to reduce UTI.

Answer 16

stop spermicide use
complete antibiotic course
consider genetic predisposition (bacterial receptors, or lack of protective coating)
void after intercourse (30min)
scrupulous hygienic practices
cranberry juice
topical estriol cream (post menopausal)
probiotics
avirulent colonizers, 
avoid catheters or use coated catheters
vaccines