Unit #4: Bacterial Culture & Sensitivety

What is a bacterial culture?

Sterile sample from area of interest grown in a media

What are some reasons to perform a bacteria culture?

• Determine bacteria type(s)

• Determine which bacteria may be pathogenic

• Determine Tx

What does a bacterial sensitivity test do?

• determine which antibiotics the bacteria is sensitive and resistant to

What are some pros and cons of in clinic bacterial culture & sensitivey?

Pros

• Low cost

• Quick results

• Less chance for bacteria growth during shipment

Cons

• Not as thorough

• False neg

What is an in clinic aerobic bacterial culture?

But medium in incubator (35-37 degrees)

Name 4 factors that abnormal proliferations of normal flora can be due to

• Poor ventilation of ear canal

• External ear conformation (folded floppy ears vs. straight upright ears)

• • Hair

• •Obstructions

• •Foreign material presence

• atop

• Adverse food reactions

Ture or False: It is ok if we culture a small amount of bacteria from a blood culture since it may have introduced while we were obtaining our sample.

False

These two Gram positive cocci are considered normal flora

• Staphylococcus aureus

• Staphylococcus ppseudintermedius

What are non-fastidious organsisms?

Non fussy bacteria that require minimum nutrients for growth

What are fastidious organisms?

Require supplemental components for growth

What is a basic media?

• Basic nutrients (sugars, proteins, phosphate, NaCl, water)

• Not used often

• Only grow non-fastidious organisms

What is an enriched media?

Supplements added to basic media to enhance growth

(Eg: blood agar plates)

What is selective media?

Inhibit growth of some organisms and enhance growth of others

How long should you wait to see results on an in clinic culture?

3 days

What are 6 sterile sites of the body?

• blood

• CSF

• Lower resp tract

• Inner/middle ear

• Upper urinary tract (Bladder, kidneys, ureters)

• Synovial fluid (joints)

*Bacteria should NEVER be seen here

When interpreting culture results from regions of the body that contain normal flora we should always ask ourselves…

What is a pathogen and what is the normal flora here

6 steps for interpreting culture results

1. # of days for growth

2. # of quadrants growth is present

3. Macroscopic morphology (color)

4. Changes to media surrounding organism

5. Gram stain (-) (+)

6. Microscopic exam of bacteria (shape)

7. Biochemical tests

What are we describing during macroscopic/colony morphology

Appearance of medium (moist, color)

Isolated colonies are described based on the following 7 things

1. Size

2. Colour

3. Optical properties

4. Texture

5. Shape

6. Elevation

7. Hemolysis

What is colony size measured in

MM’s

*if <3mm then termed pinpoint

Color of colony can be 1 of 2 things…

• pigmented

• Unpigmented

3 optical properties of culture

• opaque

• Translucent

• Transparent

Shape of colony can be described as 4 things

• cirular

• Spindle

• Irregular

• Filamentous

Edge of colony can be described as 5 things

• entire

• Lobster

• Spreading

• Filamentous

• Curled

Elevation of culture can be described as 5 things

• convex

• Domed

• Flat

• Crateform

• Umbonate

When are classical hemolytic reactions determined after?

24hrs in incubation on sheep blood agar

What is a catalase test? What will it tell us?

• allows to tell difference between Staphylococcus spp. and Streptococcus spp. (since both gram + cocci)

• Place sample of colony on glass slide & place drip of 3% hydrogen peroxide

Bubbles= staph

No bubbles= strep

What does an oxidase test tell us?

• differentiates gram - bacteria

• Testing if bacteria produces cytochrome oxidase

(+) Test: Blue color in 1 min

(-) Test: No color in 1 min

What should a swab be placed in when shipping to lab

Transport medium

When shipping bacterial sample to the lab, how many layers of protections must be between sample and outside of package?

3

When a culture is performed in the lab, when is the preliminary report done? When is the final culture report done?

Preliminary: After 24 hrs of culture

Final Report: After 3 days of culture

T or F: can begin animal on antibiotics after only the perliminary report on the culture results- dont have to wait for final culture report

True

What are 2 methods to performing an Antimicrobial Susceptibility Test?

• Disk diffusion method (Kirby-Bauer method)

• Broth micro dilution method

What does the Disk Diffusion Method do?

• measures area that is growth free (zone of inhibition) of bacteria to determine MIC

• Determines if bacteria is sensitive or resistant to an antibiotic

T or F: we can compare MIC from on antibiotic to another

F

What is the MIC (Minimum Inhibitory Concentration)

Lowest concentration of an antibiotic that would inhibit bacterial growth

What must the zone of inhibition be compared to in order to assign “sensitivity” “resistance” or “intermediate” on a sensitivity?

Compared to the specific criteria assigned by the CLSI (clinical and laboratory standards institute)

How do you determine the MIC in the broth

Incubate overnight @ 37C

What does sensitive indicate on a send out C & S culture

Best choice for antibiotics

What are some things that anti microbial choice is based off of?

• where is the infection

• Side effects of antibiotic

• Pt hx

• Cost

• Administration

6 common areas that are cultured

• wounds/abcesses

• Urine

• Feces

• Ear

• Skin

• Fluids (centesis)

Collection method for wounds and abcesses for culture

• collect from area where there’s active infection (pus)

• Syringe collect pus

• May need to clip hair around wound

• Aspirate of abscess may be required if anaerobic culture required

How to obtain urine sample for culture

• Cysto

• Gram stain

• Microanalysis

• Culture

• Lab culture- transport media

Characteristics of a fecal culture

• send off test

• Microscopic cytology

• Gram stain often not helpful

• Often anaerobic culture may be required

A dog has been swimming and now presents with an ear infection (otitis externa). What gram-negative rod is often the culprit.

Pseudomonas aeruginosa (naturally found in water)

When should we always perform an ear cytology

Otitis

*yeast overgrowth wont grow on bacteria culture media

Which bacteria is normal flora in low numbers (ears) (3)

• staphylococcus spp

• Streptococcus spp

• Mallassezia

When would we perform a skin culture

Skin infection that aren’t related to wounds or abcesses

What are some areas that are predisposed to skin infections

Folds

Interdigital regions

Groin

4 different fluids we could culture

• abnormal fluids from body cavities (eg: peritoneal fluid in pleural space of abdomen)

• Blood culture

• Synovial fluid (joint culture)

• Milk culture (mastitis in cattle)

How to perform California mastitis test

• disinfect teat and squirt some milk out prior to collection

• Collect milk from 4 quarters into paddle

• Reagent is added in 1:1 ratio

• Swirl paddle around watching for gel to form

True or False: It is acceptable to take a sterile swab of necrotic tissue since the area will still contain pathogenic bacteria

False

What does a + and - test look like on a California mastitis test

Negative- milk remains liquid

Positive+ milk forms thick gel

What is endogenous bacteria

Patients own microorganisms

What is exogenous bacteria

Direct or indirect contact w another animal w an infection