Infectious Disease Intro

What does an infection imply?

The presence of an organism within tissues with invasiveness that often results in a response of the host's immune defenses

What is colonization?

Presence of an organism at a body site without production of disease in a host

When do organisms become pathogenic?

When introduced into other body sites or fluids through trauma or breakdown of host defenses

Staphylococcus epidermis is a part of which normal flora? Where is it considered a pathogen?

In normal flora of the skin. Pathogen in cerebrospinal fluid and line infections (catheters)

What are "non-specific" ID lab tests?

Tests that are usually abnormal in infected patients but can also be abnormal in patients with noninfectious disorders. Interpret results in a broader clinical context. Example: WBC count

What is a general normal WBC count range? Keep in mind that this range varies depending on the facility

4.8-10.8 x 10^3 cells/mm^3

Which elevated WBC indicates a potential bacterial infection?

Neutrophils and bands

When might leukocytosis be present?

- Presence of systemic infection

- In elderly patients

Will sepsis present with high or low WBC count?

Could be either

Overall, WBC count is non-specific because...

it can be elevated by any stress response in the body

What are 2 acute phase reactions?

1. Erythrocyte sedimentation rate (ESR)

2. C-reactive protein (CRP)

When are Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) elevated?

In the presence of an inflammatory process. Large elevations are associated with infections, but they can also be elevated in non-infectious conditions

What is the difference between ESR and CRP?

- ESR helps track improvement of chronic infections and effectiveness of therapy

- CRP has a short half-life and is more useful in assessing response to therapy for acute infections

How do procalcitonin (PCT) levels change during a bacterial infection?

It is produced through alternative pathways in response to inflammatory cytokines and can be a more specific marker for bacterial infections than CRP or ESR

PCT levels can be valuable when...

determining when to initiate (>0.5) and discontinue (<0.25) antibacterial therapy, particularly in respiratory tract infections

What does a "left shift" mean when assessing a WBC count?

There is an increased number of immature white blood cells, especially neutrophil precursors (bands)

A gram stain can be used to determine/examine...

- material from normally sterile body fluids

- shape and staining characteristics

- cell wall

- cell membrane permeability

- gram (+) or (-)

Bacteria are identified what what 4 things?

1. Gram stain

2. Growth characteristics

3. Shape

4. Color

Why is there controversy with sputum being used for a gram stain?

Because the sputum is often contaminated or mixed with normal microbiota

What characteristics of a sputum sample may improve its gram stain diagnostic utility?

1. Predominance of 1 organism in high numbers

2. Presence of many polymorphonuclear leukocytes (PMN)

3. Few squamous epithelial cells

What is a MIC?

Minimal inhibitory concentration, the lowest antibiotic concentration that inhibits visible growth of the bacteria

What is a MBC?

Minimum bactericidal concentration, the lowest antibiotic concentration that kills 99.9% of the bacteria

How are MIC's determined? 3 ways

1. Automated using a plate with different concentrations. Vitek, Microscan, Phoenix. Report can be S, I, R

2. Disk diffusion (Kirby-Bauer). Clear zone around paper disk of antibiotic determines susceptibility (larger = better)

3. E test strips. Most time intensive and most expensive. MIC is at the tip of the "egg", or where bacteria grows into the strip

Can you compare MIC for different antibiotics?

NO

Which is higher in clinical trials, bacteriological cure vs clinical outcome?

Bacteriological cure

Why are susceptibility tests limited in predicting the success of therapy?

- Only considers organism and drug

- Does not consider the body

- Ideally the drug will eliminate the organism and not cause toxicity to the body

What are the benefits of using a combination therapy?

- Expand the spectrum of activity

- Minimize the development of resistant strains

- Produce synergistic effects

What type of bacteria is most commonly found in the sinuses?

Gram (+) (strep. pneumo), upper gram (-) (H.flu)

What type of bacteria is most commonly found in the lungs?

Gram (+) (strep. pneumo), upper gram (-) (H.flu), atypicals (legionella, mycoplasma)

What type of bacteria is most commonly found in the gut/UTI?

Gram (+) (Enterococcus), lower gram (-) (E.coli), anaerobes (bacteroides B fragile)

What type of bacteria is most commonly found on the skin?

Gram (+) (Staph and Strep)