Bloodstream and Antibiotics

What is a bloodstream infection (BSI)?

Presence of microorganisms in the blood.

What is bacteremia?

Presence of bacteria in the blood.

What is fungemia?

Presence of fungi in the blood.

What is parasitemia?

Presence of parasites in the blood.

What is viremia?

Presence of viruses in the blood.

Why are bloodstream infections clinically important?

"They can lead to sepsis, significant morbidity, and mortality."

What is sepsis?

A life-threatening dysregulated inflammatory response to infection.

Common symptoms of sepsis?

"Fever, tachycardia, tachypnea, altered mental status, and difficulty breathing."

Organisms associated with occult neoplasms in BSI?

Clostridium septicum and Streptococcus bovis group.

Risk factors for BSI?

"Surgery, invasive procedures, abscesses, pneumonia, UTI, cellulitis, immunosuppression, and central lines."

What is transient bacteremia?

"Brief bacteremia after manipulation of nonsterile sites (e.g., dental work, tooth brushing)."

What is intermittent bacteremia?

Periodic release of organisms from an extravascular infection source.

What is continuous bacteremia?

"Constant release of bacteria into blood, classically seen in endocarditis."

Most important variable for blood culture recovery?

Volume of blood collected.

Recommended adult blood volume per culture set?

20-30 mL.

What constitutes a blood culture set?

One aerobic and one anaerobic bottle from a single draw.

How many blood culture sets should be collected for a septic episode?

Typically 2-3 sets.

When should blood cultures be collected relative to antibiotics?

Before antibiotics are given.

Should blood culture bottles be refrigerated?

No.

Purpose of SPS in blood culture bottles?

Anticoagulant that inhibits clotting and improves organism recovery.

How do automated blood culture systems detect growth?

By monitoring CO2 production or pressure changes.

Standard incubation time for routine blood cultures?

5 days at 35C.

Standard incubation time for fungal blood cultures?

About 14 days.

What is the first step after a bottle flags positive?

Perform a Gram stain.

Why are positive blood culture Gram stains critical values?

Rapid communication can significantly affect patient outcomes.

Common causes of no organisms seen on Gram stain despite positivity?

Poorly staining organisms or rare false positives.

Why is culture still required after rapid molecular ID?

"To confirm ID, detect off-panel organisms, and perform AST."

Common significant blood culture isolate: Staphylococcus aureus. Significance?

Always considered clinically significant.

Common contaminants in blood cultures?

"Coagulase-negative staphylococci, Corynebacterium spp., Cutibacterium acnes, Bacillus spp. (non-anthracis)."

Target blood culture contamination rate?

"<3%, ideally ~1%."

How is catheter-associated BSI suggested by time-to-positivity?

Line culture becomes positive >2 hours before peripheral culture.

Major diagnostic criterion for infective endocarditis?

Multiple positive blood cultures with typical organisms.

Most common causes of infective endocarditis?

Staphylococci and streptococci.

What does HACEK stand for?

"Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella."

Common cause of culture-negative endocarditis?

Prior antibiotic therapy.

Most common fungus isolated from blood cultures?

Candida species.

Sensitivity of blood cultures for candidemia?

Approximately 50%.

Rapid-growing mycobacteria can grow in what?

Standard blood culture bottles.

Difference between susceptibility and sensitivity?

People are sensitive; bacteria are susceptible.

Two major AST approaches?

Genotypic and phenotypic methods.

Examples of phenotypic AST methods?

"Broth microdilution, Etest, Kirby-Bauer, agar dilution."

Gold standard AST methods?

Broth dilution and agar dilution.

What does MIC stand for?

Minimum inhibitory concentration.

Definition of MIC?

Lowest antimicrobial concentration that inhibits visible growth.

What does Kirby-Bauer measure?

Zone of inhibition size.

Does Kirby-Bauer provide an MIC?

"No, only S/I/R interpretation."

Advantage of direct Kirby-Bauer from blood cultures?

Provides rapid preliminary AST results.

Why must AST methods be standardized?

To ensure accuracy and reproducibility.

Who publishes AST standards used in the U.S.?

CLSI (Clinical and Laboratory Standards Institute).

What are breakpoints?

"MIC or zone thresholds that define S, I, or R categories."

What data inform breakpoint decisions?

"MIC distributions, PK/PD data, and clinical outcomes."

What is cascade reporting?

Selective release of AST results to promote stewardship.

Example of intrinsic resistance reporting rule?

Do not report nitrofurantoin except for urine isolates.

mecA in Staphylococcus aureus indicates what?

MRSA.

Protein encoded by mecA?

PBP2a.

Methods to detect MRSA?

"Cefoxitin/oxacillin AST, mecA PCR, or PBP2a assay."

Mechanism of beta-lactam antibiotics?

Bind PBPs and inhibit peptidoglycan cross-linking.

Examples of carbapenems?

"Meropenem, ertapenem, imipenem."

Examples of beta-lactam/beta-lactamase inhibitor combinations?

"Amoxicillin-clavulanate, piperacillin-tazobactam."

Major beta-lactam resistance mechanisms?

"Altered PBPs, beta-lactamases, efflux pumps, and decreased permeability."

Most common ESBL gene?

CTX-M.

Typical ESBL phenotype?

"Ceftriaxone resistant, carbapenem susceptible."

What does AmpC de-repression cause?

Resistance to third-generation cephalosporins and cephamycins.

HECK-YEs organisms?

"Hafnia, Enterobacter, Citrobacter freundii, Klebsiella aerogenes, Yersinia enterocolitica."

Major carbapenemase genes?

"KPC, NDM, IMP, VIM, OXA-48."

How does vancomycin work?

Binds D-Ala-D-Ala and blocks cell wall synthesis.

Why doesn't vancomycin work against Gram-negatives?

It is too large to cross the outer membrane.

Genes associated with VRE?

vanA and vanB.

Mechanism of fluoroquinolones?

Inhibit DNA gyrase and topoisomerase IV.

Examples of fluoroquinolones?

"Ciprofloxacin, levofloxacin, moxifloxacin."

Common fluoroquinolone resistance mechanisms?

"Target mutations, efflux pumps, and permeability changes."