ID Lecture 5: Clinical Microbiology Lab Data | Quizlet

Colonization

occurs when organisms inhabit a specific site but do NOT cause signs and symptoms of infection

Contamination

occurs when bacteria are accidentally introduced into a specimen during collection, transport, or processing

Infection

occurs when an organism invades a specific site and evokes a host immune reponse

What are some reasons to run cultures?

Identify infection

Source control

Confirm coverage of antimicrobials

Refine therapy based on susceptibility

What are some reasons NOT to run cultures?

Illnes is predictive of infectious source and the pathogen is readily predictable

Getting a good culture is painful/difficult

What are some limitations of microbiological lab tests?

May not identify pathogens

Organisms identified may not be causing the illness

What are the various microbiology lab tests?

Gram stain

Culture

Susceptibility testing

What does a gram stain tell us about a bacteria?

Morphology

What is the D-test?

detects if S. aureus is resistant to Clindamycin if it is alredy resistant to Macrolides

Minimum inhibitory concentration (MIC)

the smallest concentration (highest dilution) of drug that visibly inhibits growth

What are the broth dilution methods used to determine an MIC?

Macro and Microdilution

What is Macrodilution?

broth dilution where serial twofold dilutions of an antibiotic are placed in a liquid growth media

What are the pros and cons of a macrodilution MIC?

Pros - provides an EXACT MIC

Cons - resource and labor-intensive

What is Microdilution?

like macrodilution, but in a smaller volume of broth on smaller trays

What are the pros and cons of microdilution?

Pros

- can test susceptibility to multiple antibiotics

- easy to use

- quicker results

- decreased labor and costs

Cons

- limited number of antibiotics and concentrations in trays

- abx concentrations are limited

- MIC a range instead of being exact

Breakpoint

concentration of drug that is readily achievable in SERUM using standard doses of antibiotic

used with MIC to determine susceptibility

MIC < BP

Susceptible

MIC = BP

Intermediate

MIC > BP

Resistant

Antibiogram

cumulative report of the most commonly isolated pathogens in an institution along with their susceptibilities

What are the limitations to antibiograms?

Duplicate isolates reported

Number of isolates reported

Collection bias (inpt vs outpt)

Whole hospital antibiogram may not reflect unit specific patterns