Microbiology and Meningitis Quiz

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Questions and Answers

What is the recommended blood culture volume for adults?

5 mL

1 mL CSF (correct)

1000 mL

10 mL

Which media are most commonly used for bacterial cultures?

MAC and EMB

Chocolate agar and Manitol salt agar

BAP and CHOC (correct)

Nutrient agar and Blood agar

In CSF findings, which cell type is typically associated with bacterial infections?

Eosinophils

Neutrophils (correct)

Lymphocytes

Monocytes

What is the sensitivity range of Gram stain smears for detecting organisms?

75-90% Signup and view all the answers

Which organism is most commonly associated with viral encephalitis?

Viruses Signup and view all the answers

What is a key feature of fungal meningitis in CSF findings?

Occasional lymphocytes Signup and view all the answers

What should antigen detection tests be used for in the context of meningitis?

To supplement smears and cultures Signup and view all the answers

Which condition is rarely isolated in cases of anaerobic bacteria-related brain abscesses?

Fungal infections Signup and view all the answers

What term describes bacteria present in the bloodstream without any clinical symptoms?

Bacteremia Signup and view all the answers

Which type of bacteremia is characterized by bacteria periodically released into the bloodstream?

Intermittent bacteremia Signup and view all the answers

What is a possible cause of pseudobacteremia?

Contamination from blood culture materials Signup and view all the answers

What is a significant consequence of septic shock?

Mortality rate greater than 50% Signup and view all the answers

Endocarditis is associated with which type of bacteremia?

Intravascular bacteremia Signup and view all the answers

Which of the following is NOT a recognized pattern of bacteremia?

Chronic bacteremia Signup and view all the answers

Which of the following sites is NOT typically associated with sources of extravascular bacteremia?

Catheters Signup and view all the answers

Which organism is NOT typically associated with meningitis in cases of extravascular bacteremia?

Staphylococcus aureus Signup and view all the answers

What is the recommended method for blood collection in suspected bacteremia cases?

Venipuncture using a needle and syringe Signup and view all the answers

Which of the following anticoagulants is recommended during blood specimen collection?

Sodium polyanethol sulfonate (SPS) Signup and view all the answers

What is crucial to do after blood is collected in a syringe?

Immediately inoculate into blood culture bottles Signup and view all the answers

Which organisms are among the most common causes of nosocomial bacteremia?

Candida albicans Signup and view all the answers

Why is the indwelling intravascular catheter not recommended for blood collection?

It may lead to contamination. Signup and view all the answers

What is the primary purpose of using sodium polyanethol sulfonate (SPS) in blood cultures?

To prevent clotting and neutralize antimicrobial agents Signup and view all the answers

During site preparation for blood collection, what is recommended to clean the skin?

Application of alcohol followed by iodine Signup and view all the answers

What is the role of anticoagulants like sodium polyanethol sulfonate (SPS) in blood specimen collection?

To prevent clotting and inhibit phagocytosis. Signup and view all the answers

Study Notes

Bacteremia and Sepsis

Bacteremia is bacteria in the bloodstream
Septicemia is bacteremia with clinical signs and symptoms like fever, chills, hypothermia, hyperventilation, and septic shock.
Exotoxins and/or endotoxins can evoke a massive immune response
Mortality rate is greater than 50%

Types of Bacteremia

Primary bacteremia: No other known infected site
Secondary bacteremia: Associated with an infected body site
Occult bacteremia: No known cause, often in children
Pseudobacteremia: False bacteremia, contaminated materials are the source of organisms in blood culture media

Bacteremia Patterns

Transient: Bacteria are in the bloodstream for minutes to hours, often from a traumatized body site (e.g., mucous membrane, skin, minor surgery) or early stages of some diseases (e.g., meningitis, osteomyelitis)
Intermittent: Bacteria are periodically released into the bloodstream (e.g., caused by pneumonia, meningitis, and abscess)
Continuous: Bacteria are present constantly in the bloodstream; often in individuals with infected intravascular sites

Bacteremia Sources

Intravascular: Associated with the vascular system (e.g., infected heart valves, catheters, veins)
Extravascular: Outside the vascular system; lymphatic vessels carry organisms to the bloodstream (e.g., urinary tract infections, respiratory tract infections, abscesses)

Intravascular Bacteremia Causes

Bacteria infection of the endocardium (e.g., viridans streptococci - normal oral flora, Enterococci, coagulase-negative staphylococci (e.g. S. epidermidis), S. aureus, Corynebacterium - normal skin flora)

Extravascular Bacteremia Causes

Bacteria from a localized site of infection (e.g., Meningitis caused by N. meningitidis, H. influenzae type B, and S. pneumoniae. Other sites are enterics, anaerobes, NFGR, and other GNR.)

Bacteremia Detection

Mortality rates are high, so prompt detection is crucial
Obtain blood via an aseptic venipuncture
Isolate, identify, and test for the organism's susceptibility to different antibiotics/antimicrobials
Common causes of nosocomial bacteremia: coagulase-negative staphylococci, enterococcus, and Candida albicans; Enterobacteriaceae (E. coli., K. pneumonia, Enterobacter spp.), Serratia spp. Nonfermenting GNR (e.g., P. aeruginosa, A. baumannii)

Bacteremia Collection Sites and Methods

Venipuncture: Peripheral aseptic venipuncture is preferred; indwelling intravascular catheters aren't recommended due to potential contamination; only use if no other option.
Methods: Needle and syringe (preferred method), butterfly, tube method via lysis centrifugation system

Bacteremia Collection Procedure

Specimen container preparation (disinfect tops of culture bottles & tubes with 70% alcohol or iodine)
Site preparation (clean skin with alcohol to remove debris and oil, swab with iodine in concentric circles, let iodine dry for at least 1 minute)
Specimen collection
Site care (remove iodine with alcohol)

Bacteremia Collection Difficult Cases

Difficult to recover organisms in clotted specimens
Recommended anticoagulant is SPS

Sodium Polyanethol Sulfonate (SPS)

Prevents clotting
Inactivates complement
Inhibits phagocytosis
Neutralizes some antimicrobial agents
Inhibits N. gonorrhoeae and N. meningitidis
0.025% concentration in blood culture media
SPS blood collection tubes available but not always recommended.

Blood Volume

Larger Volume = More positive results
Recommended amount varies with patient age; children have high levels of bacteremia and require 100-1,000 organisms/mL; infants and children need 1-5 mL; and adults need 10 mL minimum

Blood-to-Broth Ratio

1:5 to 1:10
Prevention of clots, and dilution of inhibitory factors

Timing and Number of Cultures

Usually 2-3 sets of aerobic and anaerobic blood culture sets
Getting 1 set is 80% detection rate
2 sets = 90% detection
3 sets = 99% detection
Take cultures from different sites
Collect before antimicrobial therapy

Newer Recommendation

Three blood cultures collected simultaneously or one right after the other

Transport

High priority transportation (STAT)
Store at 37°C if transport is delayed

Culture Media - Broth

Includes BHI, Brucella, Columbia, Thioglycollate, Trypticase Soy, and special formulations
Key components: SPS, nutrients, headspace with CO2, antibiotic removal device, and osmotic stabilizers

Culture Media - Agar

Lysis-centrifugation method uses agar media
Uses biphasic culture bottles (agar and broth)

Incubation Conditions

35-37°C
Time varies according to the organism; some require longer incubation

Manual Culture Methods – Conventional Broth

Broth bottles examined daily for growth, turbidity, gas production, discoloration
Blind subcultures

Manual Culture Methods – Biphasic

Castaneda bottle: tipped to cover agar and allow inoculation; colonies appear after incubation
Newer versions use blood added and use a paddle device attached to aerobic bottle with media containing CHOC or MAC; paddle bathed, and paddle/broth is examined daily for growth

Manual Culture Methods – Lysis-Centrifugation

Collect blood in a special isolator tube; anticoagulate and lyse the blood; centrifuge the tube
Inoculate onto agar plates; incubate and check daily for appearance of colonies

Instrumentation

BACTEC systems: Measure CO2 production by metabolizing organisms
Semiautomated system: BACTEC 460
Continuous-Monitoring Systems: BacT/Alert, BACTEC 9000, ESP

BACTEC 460

First automated system
Uses radioactive carbon; microorganisms metabolize carbon and produce radioactive CO2; amount of radioactive CO2 in headspace is determined

Continuous Monitoring Systems (BacT/Alert, BACTEC 9000, ESP)

Uses special bottles, incubator, agitator, and detection system monitoring for growth
Computer monitors the bottles

BacT/Alert

Colorimetric method
Monitors CO2 production
Bottles with gas-permeable sensors; sensor changes from green to yellow as gas is generated

BACTEC 9000

Uses fluorescent sensors in the base of the bottles
Fluorescence increases as CO2 is generated

ESP/VersaTREK

Manometric system
Bottles attached to a pressure transducer; measures gas pressure inside the bottles

Positive Blood Cultures

Critical values detected by appearance of turbidity, hemolysis, gas production; colonies on subculture plates
Instrumentation-assisted identification and antimicrobial susceptibility testing

Gram-Stained Smears

First step in work-up
Determine morphology and gram stain reaction

Subculture

Positive cultures subcultured onto agar media to isolate colonies
Media selection depends on gram stain reaction; aerobic: BAP, CHOC, or both in CO2; MAC, EMB added for GNR; mixed cultures using PEA or CNA; anaerobic: anaBAP

Reporting Results

Notify physician immediately upon detection of critical values
Document report with time, date, performed tests, and number of positive cultures; including gram-stained smear results
Preliminary report after 48 hours; final report if no growth after 7 days of incubation

Contaminant

Determine clinical significance of isolate
Probable contaminant: Skin flora (e.g., Corynebacterium spp., Propionbacterium acnes, S. epidermidis, Micrococcus); growth of multiple organisms from one culture; Growth of the same organism in repeated cultures. Growth of same organism in blood and from a sterile body site

Special Cultures

Brucella spp.: Prolonged incubation (30 days); Nutritionally deficient streptococci; Suspected GPC if no growth on subcultured plates; overlaid culture with a streak of S. aureus
Campylobacter: Will grow in blood culture, but subculture must be in microaerophilic conditions (e.g., CAMPY agar)

Antimicrobial Therapy

Empiric initially
Narrow spectrum after ID and susceptibility testing is completed

Central Nervous System

Introduction: CNS includes the brain, spinal cord, and meninges.
Term: CSF is clear and colorless liquid bathing the brain and spinal cord. It's tested for culture, cell count, glucose, and protein levels. Meningitis is inflammation of the meninges, and purulent meningitis is also known as pyogenic meningitis (pus in the meninges), often caused by bacteria. Aseptic meningitis is typically caused by a virus. Encephalitis is inflammation of the brain, and meningoencephalitis is inflammation of the brain and meninges.

Routes of Infection

Hematogenous: Most common route where organisms from a site of infection enter the bloodstream and travel to the CNS. N. meningitidis is an example.
Contiguous spread: Organisms spread from an infected adjacent site (like sinusitis) to the meninges.
Trauma: Breach of CNS protective barrier (e.g., skull fracture) allows microbial contamination.
Surgery: Microbial contamination can occur during surgical procedures.
Shunts: Placed to drain fluid from the CNS, these can serve as a portal of entry for microbes

Acute Bacterial Meningitis Symptoms

Flu-like symptoms
Headache
Fever
Nausea
Vomiting
Nuchal rigidity (stiff neck)
Photophobia (sensitivity to light)
Mental status changes

Causative Agents

Neonates: E. coli; other GNRs (Klebsiella, Enterobacter); S. agalactiae; Listeria monocytogenes
Young children (6 months-5 years): H. influenzae type b; N. meningitidis; S. pneumoniae
Older Children/Adults: Meningococci; Pneumococci; GNRs; L. monocytogenes (immunocompromised individuals)
Elderly: Same as above but with possible shunt

Specimen Collection

Lumbar puncture: Spinal needle inserted between the 3rd and 4th lumbar vertebrae to collect CSF
Specimen transported at room temperature (RT) within 15 minutes of collection; STAT processing; and stored in incubator or at RT for pathogens that are not sensitive to refrigeration (e.g., meningococci, pneumococci, H. influenzae)
3-4 tubes are collected; Tube #1 (protein and glucose); Tube #2 (gram stain and culture); Tube #3 (cell count and differential).

Microbiology Processing

STAT processing
1 mL CSF → centrifuge and collect sediment for smear and culture
Gram stain sensitivity: 75-90%
Thick smears

Culture Media

BAP, CHOC media are standard/most common
MAC or EMB if GNR is suspected from gram stained smears
anaBAP requested for anaerobic bacteria
Anaerobes rarely isolated
Incubate at 35-37°C with CO2

Antigen Detection Tests

Available for group B streptococci, Hib, meningococci, pneumococci
Should supplement but not replace smear and culture tests

Other Types of Meningitis

Spirochetes: T. pallidum, B. burgdorferi
Viral: Aseptic meningitis
Mycobacterial: M. tuberculosis
Fungal: Rare, chronic meningitis in immunocompromised individuals
Paraistic: Rare

CSF Findings

Table describing organisms, cells (e.g., neutrophils, lymphocytes, or eosinophils), and diagnostic tests for various types of meningitis

Brain Abscesses

Caused by N.F. organisms (anaerobic bacteria, staphylococci, viridans, other streptococci)
Aspiration and biopsy material transported in anaerobic conditions
Examined microscopically; cultured aerobically and anaerobically

Encephalitis/Meningoencephalitis

Most common: Viruses
Bacteria: L. monocytogenes, Rickettsia, and Mycoplasma
Parasites: Naegleria and Acanthamoeba

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Description

Test your knowledge on microbiology, specifically focusing on blood cultures, bacterial infections, and meningitis. This quiz covers key concepts such as culture media, cell types associated with infections, and diagnostic methods. Perfect for medical students and professionals looking to refresh their understanding of these critical topics.