40 Questions
What type of medium is used to grow larger quantities of a culture of bacteria?
Liquid medium
What is the purpose of enriched media?
To support the growth of fastidious organisms
What type of organisms require chocolate agar for growth?
Haemophilus influenzae and Neisseria gonorrhoeae
What is the main difference between blood agar and chocolate agar?
Blood agar contains sheep blood, while chocolate agar contains red blood cells that have been lysed
What is the primary use of selective media?
To isolate specific bacterial species from specimens with large numbers of bacteria
What type of medium is MacConkey agar?
Selective medium
What is the primary purpose of using a manual system for rapid identification of Enterobacteriaceae and other gram-negative bacteria?
To identify the metabolic profile of the organism
What can be inferred from the failure to culture an organism?
The culture medium is inadequate, or the incubation conditions do not support bacterial growth
What is suggested by the growth of an organism on blood agar and chocolate agar, but not MacConkey agar?
The organism is a gram-positive isolate or a fastidious gram-negative species
What is the role of the photometer in the Vitek System?
To measure the color changes in the reagent cards
What is the advantage of using automated systems for bacterial identification?
They reduce the time required for identification
What is the principle behind immunologic detection of microorganisms?
The specificity of antigen-antibody binding
What is the purpose of the data bank in the manual system for bacterial identification?
To compare the test results with known bacteria
What is the purpose of inoculating the reagent cards in the Vitek System?
To initiate the biochemical reactions
What is the advantage of using a manual system for bacterial identification?
It is more cost-effective and easy to use
What is the purpose of the microtubes in the manual system?
To provide a substrate for biochemical reactions
What happens if the complement has not been depleted by initial antigen-antibody complexes?
The complement will bind to the antibody–red blood cell complexes, causing the cells to lyse
What is the term for the test panel used to evaluate patients with fever of unknown origin?
Febrile agglutinins
What is the purpose of the direct hemagglutination test?
To detect the presence of antibodies against red blood cells in a patient's serum
What is the term for the test that detects IgM autoantibodies that agglutinate human red blood cells at 4°C but not at 37°C?
Cold agglutinins
What is the purpose of the latex agglutination test?
To detect the presence of a specific antigen or antibody in a patient's serum
What is the result of the complement binding to the antibody–red blood cell complexes?
The cells will lyse
What is the purpose of the direct agglutination test?
To evaluate patients suspected of being infected by Brucella abortus or Francisella tularensis
What is the result of the addition of antigen to antibody-coated latex beads in the latex agglutination test?
The latex beads will agglutinate
What is the purpose of alkaline denaturation of the double-stranded DNA?
To produce single-stranded DNA for hybridization
What is a major advantage of nucleic acid amplification methods over direct detection with nucleic acid probes?
They allow specific DNA or RNA target sequences of the pathogen to be amplified millions of times
What is the purpose of washing the filter after hybridization?
To remove unbound probe
What is a major use of nucleic acid amplification techniques?
To detect organisms that cannot be grown in vitro
What is the purpose of binding the DNA to a solid support such as a nitrocellulose membrane?
To make the DNA available for hybridization
What is a characteristic of nucleic acid amplification methods?
They are sensitive and specific for the target organism
What is a step involved in the detection of amplified sequences?
Blotting the product onto a membrane such as a nitrocellulose filter
What is the purpose of the microorganism-specific, labeled probe?
To hybridize with the target DNA
What characteristic do all clinically important bacteria have in common?
They are heterotrophs
What is the purpose of growth factors in bacteria?
To provide essential organic compounds for growth
What type of organisms require a large number of growth factors?
Fastidious organisms
What is the primary characteristic of aerobic bacteria?
They grow in the presence of oxygen
Which type of organism prefers to use oxygen as the terminal electron acceptor when present?
Facultative organisms
What is the purpose of enriched media in isolating pathogenic bacteria?
To promote the nonselective growth of any bacteria
Which type of organism grows only in the absence of oxygen and may be killed by small amounts of oxygen?
Obligate anaerobes
What is the energy production mechanism used by anaerobic bacteria?
Fermentation
Study Notes
Growth Requirements of Bacteria
- Clinically important bacteria are heterotrophs, requiring organic carbon for growth
- Heterotrophs may have complex or simple requirements for organic molecules
- Most bacteria require varying numbers of growth factors, which are organic compounds required for cell growth but cannot be synthesized by the organism itself
- Organisms requiring many growth factors or specific ones are called fastidious
Oxygen Requirements
- Bacteria can be categorized based on their growth responses to oxygen presence and absence
- Aerobic bacteria (aerobes) grow in the presence of oxygen and use oxygen as the terminal electron acceptor in energy production
- Obligate aerobes have an absolute requirement for oxygen
- Facultative organisms grow in the presence or absence of oxygen, using oxygen preferentially as the terminal electron acceptor when present
- Microaerophilic organisms require or tolerate oxygen but at a lower partial pressure than atmospheric oxygen
- Anaerobic bacteria grow in the absence of oxygen, producing energy by fermentation
- Obligate anaerobes grow only in the absence of oxygen and may be killed by small amounts of oxygen
Media
- Enriched media are used to promote the non-selective growth of any bacteria that may be present in a clinical sample
- Enriched media are fortified with blood, yeast extracts, or brain or heart infusions to support the growth of fastidious organisms
- Selective media allow growth of specific bacterial species from specimens that normally contain large numbers of bacteria
- Isolation of bacteria is usually performed on solid medium, while liquid medium is used to grow larger quantities of a culture
Immunologic Detection of Microorganisms
- Immunologic methods take advantage of the specificity of antigen-antibody binding to identify microorganisms
- Direct agglutination measures the ability of a patient's serum antibody to directly agglutinate specific killed microorganisms
- Direct hemagglutination measures the presence of antibodies directed against red blood cells
- Latex agglutination test uses latex particles coated with either antibody or antigen to detect serum antigens or antibodies
- Other tests include complement fixation and hybridization assays
Nucleic Acid Detection
- Nucleic acid detection methods include hybridization assays and nucleic acid amplification techniques such as PCR
- Hybridization assays use a microorganism-specific probe to detect the target DNA or RNA sequence
- Nucleic acid amplification techniques amplify specific DNA or RNA target sequences of the pathogen, allowing for detection without culturing the microorganism
- Amplified sequences are detected by various methods, including agarose gel electrophoresis and blotting onto a nitrocellulose filter followed by probe hybridization
Learn about the organic carbon requirements for bacterial growth, including heterotrophs and autotrophs, and the role of growth factors.
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