Detection of Microbes

1. What are the major laboratory tests used to identify microbes?

CBC, microscopy and staining, culture, biochemical tests, antigen detection, serology (antibody detection), and molecular techniques such as PCR and gene probes.

2. What is the most common method of identifying a causative pathogen, and when are alternative methods needed?

• Culture: The most common method; the clinical specimen is grown on an appropriate culture medium, usually agar.

• Alternative methods: Antigen detection, serology, and molecular testing are useful when an organism cannot be cultured or is difficult to grow.

3. What is the purpose of a CBC when evaluating a possible infection?

A CBC helps evaluate the patient's blood cells. For example, an elevated white blood cell count can suggest an infection.

4. Why are specimens stained before viewing them under a microscope?

Staining provides contrast, making microorganisms easier to see. Specimens are also fixed, often using heat, to prevent them from washing away during staining.

5. What does the Gram stain identify, and what are its results?

The Gram stain differentiates bacteria based on their cell wall characteristics:

• Gram-positive: Purple due to crystal violet.

• Gram-negative: Pink/red due to safranin counterstain.

It is the most commonly used bacteriological stain.

6. What does a Gram stain reveal about bacterial morphology and arrangement?

It can reveal:

• Shape: Cocci (round) or bacilli (rods).

• Arrangement: Clusters, chains, or random groupings.

• Example: Staphylococcus = Gram-positive cocci in grape-like clusters.

• Example: E. coli = Gram-negative rods.

7. Why can some bacteria, such as Mycoplasma, not be classified as Gram-positive or Gram-negative?

Gram staining depends on bacterial cell wall characteristics. Mycoplasma lacks a cell wall, so it cannot be classified as Gram-positive or Gram-negative.

8. What does the Auramine-Rhodamine stain identify, and what are its important characteristics?

• Identifies Mycobacteria, especially Mycobacterium tuberculosis, and Cryptosporidium.

• Organisms appear fluorescent red/maroon.

• Potassium permanganate is used as the counterstain.

• It allows rapid scanning of specimens.

• It is faster and more sensitive than the older Ziehl-Neelsen stain and has replaced it in routine clinical use.

• A positive result indicates acid-fast organisms but does not identify the exact species.

9. What is the Ziehl-Neelsen stain, and how should its results be interpreted?

• It is the older acid-fast staining method.

• Acid-fast organisms appear pink/red.

• Acid-fast-negative organisms appear blue.

• It is not the same as a Gram stain: blue does not mean Gram-positive in this stain.

• It is less commonly used clinically today, but remains relevant for board exams.

10. What does Wright-Giemsa stain identify, and which organisms should you know?

Used especially on peripheral blood smears to identify:

• Parasites: Plasmodium (malaria), Babesia, Trichomonas, and Toxoplasma.

• Bacteria: Chlamydia and Rickettsia.

11. What does India ink stain identify, and what is its clinical significance?

• Identifies Cryptococcus neoformans, an encapsulated fungus.

• Commonly performed on cerebrospinal fluid (CSF).

• Useful in diagnosing cryptococcal meningitis.

• The capsule appears as a clear halo around the fungal cell.

12. What does a capsule stain identify, and why is the capsule important?

• Identifies encapsulated bacteria and fungi.

• Examples include Streptococcus and Cryptococcus.

• The capsule can increase virulence and pathogenicity by helping the organism evade host defenses.

• Not all bacteria or even all strains of the same species have capsules.

13. What does a flagellar stain identify, and what additional information can it provide?

• Identifies bacterial flagella, which allow motility.

• Can reveal the number and arrangement of flagella.

• Example: Proteus is highly motile.

• Flagellar arrangement can help with bacterial identification.

14. What does an endospore stain identify, and what colors are seen?

• Identifies endospore-forming bacteria, especially Bacillus and Clostridium.

• Malachite green stains the spores green.

• Vegetative cells appear red.

• Endospores initially form inside the vegetative cell, called the mother cell, and are eventually released.

15. Which two bacterial genera form endospores, and what are their characteristics?

• Bacillus: Gram-positive rods; generally aerobic.

• Clostridium: Gram-positive rods; anaerobic.

• Both form endospores.

16. What are the most common sources of clinical specimens for culture?

Blood, urine, stool, and cerebrospinal fluid (CSF).

17. What are other possible sources of clinical culture specimens?

Sputum, skin lesions, throat swabs, pus, abscess aspirates, synovial fluid, and biopsied tissue.

18. What are the main forms of culture media?

• Broth: Liquid medium used for enrichment and microbial growth.

• Agar slants: Solid media in a slanted tube.

• Agar plates: Solid media commonly used to grow and identify bacteria and fungi.

19. What is the difference between selective and differential culture media?

• Selective: Favors the growth of certain organisms while inhibiting others.

• Differential: Distinguishes organisms based on biochemical characteristics, such as whether they ferment lactose.

Some media are both selective and differential.

20. What is blood agar used for, and what does non-fastidious mean?

• Blood agar is a general-purpose, non-selective medium that grows many bacteria.

• It is especially useful for non-fastidious bacteria.

• Non-fastidious: Organisms that do not require many special nutrients to grow.

• It can also be used to observe hemolysis.

21. What is chocolate agar, how is it prepared, and which organisms require it?

• Made by heating blood agar, which lyses red blood cells and releases their contents.

• Its brown color resembles chocolate; it contains no actual chocolate.

• Supports fastidious organisms that require additional nutrients.

• Important examples:

  • Haemophilus influenzae

  • Neisseria gonorrhoeae

• Some organisms, such as N. gonorrhoeae, may also require reduced oxygen levels.

22. What is Sabouraud dextrose agar used for?

• Used to grow most fungi.

• Provides nutrients, including carbohydrates, that support fungal growth.

• Commonly used for fungal culture.

23. What are MacConkey agar and EMB agar used for, and how do they work?

Both are selective and differential media.

• Selective: Grow Gram-negative enteric rods and inhibit Gram-positive bacteria.

• Differential: Distinguish lactose fermenters from non-fermenters.

• Bile salts help inhibit Gram-positive organisms.

• Lactose fermentation produces acid, changing the pH indicator and the appearance of the medium.

24. Which organisms are lactose fermenters versus non-fermenters on MacConkey agar?

• Lactose fermenters: E. coli and Klebsiella.

• Lactose non-fermenters: Usually Salmonella and Shigella.

• Fermenters produce acid and appear pink/red on MacConkey agar.

• Non-fermenters generally produce pale or colorless colonies.

25. What is Lowenstein-Jensen agar used for, and why is it important?

• Used to grow Mycobacteria, especially Mycobacterium tuberculosis.

• A sputum specimen may be cultured on this medium when TB is suspected.

• Culture on Lowenstein-Jensen is the gold standard for confirming TB according to the lecture.

26. What is buffered charcoal yeast extract (BCYE) agar used for?

Used to grow Legionella pneumophila, which causes Legionnaires' disease, a type of pneumonia.

27. What is Bordet-Gengou agar used for?

Used to grow Bordetella pertussis, the bacterium responsible for pertussis (whooping cough).

28. How are viruses cultured in the laboratory, and why?

Viruses are grown using tissue cell lines because they are acellular and cannot reproduce independently like bacteria.

29. What are the major oxygen conditions used for bacterial culture?

• Aerobic: Requires oxygen.

• Anaerobic: Grows without oxygen.

• Microaerophilic: Requires low oxygen levels, generally less than 5%.

• Facultative anaerobic: Can grow with or without oxygen.

30. Why do bacteria have different oxygen requirements? Give examples.

Their oxygen requirements depend on whether they possess enzymes that break down toxic oxygen radicals.

• Anaerobes lack sufficient protective enzymes and cannot tolerate oxygen.

• Microaerophiles tolerate low oxygen but not normal atmospheric levels.

• Facultative anaerobes can grow with or without oxygen.

• Clostridium is anaerobic.

• Neisseria may require reduced oxygen levels.

• Bacillus is generally aerobic.

31. What temperatures are important for growing microorganisms in culture?

• 35–37°C: Ideal for most pathogenic bacteria; close to human body temperature.

• 25°C/room temperature: Preferred by many fungi.

• 40–42°C: Only a few organisms can grow well at these temperatures.

• Fungi often prefer cooler temperatures, helping explain why many cause superficial infections of skin and other cooler body areas.

32. What is the catalase test, how does it work, and what organisms does it differentiate?

• Detects the catalase enzyme.

• Catalase breaks down hydrogen peroxide into water and oxygen.

• Adding hydrogen peroxide:

  • Positive: Bubbling occurs due to oxygen production.

  • Negative: No bubbling.

• Differentiates:

  • Staphylococcus = catalase-positive.

  • Streptococcus and Enterococcus = catalase-negative.

33. What is the coagulase test, how does it work, and what does it differentiate?

• Detects the coagulase enzyme.

• Uses plasma from blood.

• Positive: Plasma clots.

• Negative: Plasma remains liquid.

• Differentiates:

  • Staphylococcus aureus = coagulase-positive.

  • Staphylococcus epidermidis and Staphylococcus saprophyticus = coagulase-negative.

34. What is hemolysis, and what causes it?

Hemolysis is the breakdown of red blood cells on blood agar.

It is caused by bacterial hemolysins and is classified as:

• Beta: Complete hemolysis.

• Alpha: Partial hemolysis.

• Gamma: No hemolysis.

35. What is beta hemolysis, and which organisms are examples?

• Complete breakdown of red blood cells.

• Produces a clear zone around bacterial colonies on blood agar.

• Examples:

  • Group A Streptococcus.

  • Group B Streptococcus.

  • Most S. aureus strains.

  • Some Clostridium strains.

  • Approximately half of E. coli strains.

36. What is alpha hemolysis, and which organisms are examples?

• Partial breakdown of red blood cells.

• Produces a greenish or green-brown discoloration on blood agar.

• Examples:

  • Streptococcus pneumoniae.

  • Viridans streptococci.

37. What is gamma hemolysis, and which organisms are examples?

• No breakdown of red blood cells.

• No change in the blood agar.

• Examples:

  • Enterococcus faecalis.

  • Staphylococcus epidermidis.

  • Staphylococcus saprophyticus.

38. What is the optochin (P-disc) sensitivity test, and what does it differentiate?

Differentiates Streptococcus pneumoniae from other alpha-hemolytic streptococci.

• S. pneumoniae = optochin-sensitive; a clear inhibition zone forms around the disc.

• Viridans streptococci = optochin-resistant; growth reaches the disc.

39. What is the bacitracin (A-disc) sensitivity test, and what does it differentiate?

Differentiates Group A Streptococcus from other beta-hemolytic streptococci.

• Group A Streptococcus (S. pyogenes) = bacitracin-sensitive; inhibition zone forms.

• Other beta-hemolytic streptococci = generally resistant.

40. What is sugar fermentation testing, and how is a positive result identified?

• Tests whether bacteria can ferment sugars such as lactose, glucose, sucrose, or maltose.

• Fermentation produces acid.

• Acid lowers the pH.

• A pH indicator, such as phenol red, changes color, typically to yellow in the fermentation test.

• On MacConkey agar, lactose fermenters form pink/red colonies.

41. How does lactose fermentation help differentiate Gram-negative enteric rods?

• E. coli and Klebsiella = lactose fermenters.

• Salmonella and Shigella = usually lactose non-fermenters.

• MacConkey and EMB agar help distinguish these organisms.

42. What is sorbitol fermentation testing used for, and what is the important organism?

• Used to identify E. coli O157, an enterohemorrhagic, Shiga toxin-producing strain.

• E. coli O157 generally does not ferment sorbitol.

• Most other E. coli strains ferment sorbitol.

• This strain is associated with contaminated or undercooked beef and can cause serious illness, including kidney failure, particularly in young children.

43. What is the oxidase test, and which bacteria are oxidase-positive?

• Detects the oxidase enzyme involved in the electron transport chain.

• A positive result produces a purple color after the reagent is added.

• Examples:

  • Pseudomonas

  • Campylobacter

  • Helicobacter

  • Vibrio

  • Some Neisseria

  • Pasteurella

44. What is antigen detection, and what are its advantages?

• Detects microbial antigens using antibodies.

• Usually rapid, sometimes taking only minutes.

• Often has built-in controls.

• Can be performed in a clinic without the specialized laboratory procedures needed for culture and some other tests.

45. What is the latex agglutination test, how does it work, and what is it used for?

• Antibodies are attached to latex beads.

• If the matching antigen is present, it binds to antibodies and cross-links the beads.

• This produces visible clumping, called agglutination.

• Examples:

  • Rapid strep testing for streptococcal pharyngitis.

  • Cryptococcal antigen testing, including in suspected meningitis.

46. What is ELISA, how does it work, and what does a positive result look like?

ELISA = enzyme-linked immunosorbent assay.

• Uses antibodies to detect a specific antigen.

• An enzyme-linked antibody helps produce a detectable signal.

• A substrate is added, producing a color change.

• The amount of color is proportional to the amount of antigen present in the assay.

• It can detect not only presence but also estimate concentration.

47. What are important clinical examples of ELISA testing?

Examples discussed in lecture include:

• Autoantibodies: anti-double-stranded DNA and antinuclear antibodies (ANA), associated with lupus testing.

• Hepatitis A, B, and C testing.

• HIV testing.

• PSA testing.

• LH and FSH testing.

• hCG testing, including pregnancy testing.

48. What is serology, and what does it detect?

• Serology traditionally detects antibodies in serum.

• Serum is the liquid portion of blood remaining after clotting.

• It indirectly helps identify exposure to a pathogen or an immune response.

• It can also be used to assess immunity after vaccination.

49. What are the major serological techniques used to detect antibodies?

• ELISA.

• Agglutination.

• Direct immunofluorescence.

• Complement fixation.

• Western blot.

These methods detect antibodies or antibody-associated reactions in patient samples.

50. What are important clinical uses of serology?

• Detecting fungal infections, including:

  • Coccidioidomycosis.

  • Histoplasmosis.

  • Blastomycosis.

  • Sporotrichosis.

• Checking antibody titers after vaccination, such as measles vaccination.

• Blood typing through antigen-antibody agglutination reactions.

51. What are molecular diagnostic techniques, and what do they detect?

Detect small biological molecules, including:

• DNA.

• RNA.

• Proteins.

They are rapid, highly sensitive, and highly specific, and can sometimes detect extremely small amounts of genetic material.

52. When are molecular techniques especially useful?

• When a microorganism cannot be cultured.

• When the organism is difficult to grow.

• When rapid identification is needed.

• When a highly sensitive and specific result is important.

53. What is FISH, and how does it identify microorganisms?

FISH = fluorescent in situ hybridization.

• Uses a DNA probe designed to bind a specific genetic sequence.

• The probe is attached to a fluorescent tag.

• If the matching genetic material is present, the probe hybridizes and fluoresces.

• Can help identify organisms such as Staphylococcus aureus, Enterococci, and Candida.

54. What are nucleic acid amplification tests (NAATs), and what are examples?

• Detect and amplify microbial nucleic acids, such as DNA or RNA.

• PCR is one type of NAAT.

• Examples:

  • COVID-19 testing.

  • Influenza testing.

  • Clostridioides difficile testing, which can use non-PCR amplification methods.

55. What is the difference between PCR-based and non-PCR-based molecular tests?

• PCR-based: Use polymerase chain reaction to amplify nucleic acids.

• Non-PCR-based: Use other amplification techniques.

• Both can detect microbial genetic material.

• Examples include PCR for COVID-19 and influenza, and non-PCR amplification methods for C. difficile.

56. A patient has fever, painful urination, flank pain, and a urine culture growing pink colonies on MacConkey agar. What does this suggest?

• MacConkey agar selects for Gram-negative enteric rods.

• Pink colonies indicate lactose fermentation.

• E. coli is a common lactose-fermenting Gram-negative rod and a common cause of urinary tract infections.

• The findings are consistent with an E. coli UTI.

57. A patient has suspected tuberculosis. Which specimen, culture medium, and confirmatory method are important?

• Specimen: Sputum.

• Culture medium: Lowenstein-Jensen agar.

• Organism: Mycobacterium tuberculosis.

• Confirmation: Culture remains the gold standard for confirming TB according to the lecture, even when imaging or molecular tests suggest the diagnosis.

58. A patient has suspected cryptococcal meningitis. Which stain and antigen test can help identify the organism?

• Stain: India ink on CSF, showing an encapsulated fungus with a clear halo.

• Antigen test: Cryptococcal antigen detection, such as latex agglutination.

• Organism: Cryptococcus neoformans.

59. How do microscopy, culture, biochemical testing, antigen detection, serology, and molecular testing work together in microbial identification?

• Microscopy: Provides initial information about shape, arrangement, and staining characteristics.

• Culture: Grows the organism and helps isolate it.

• Biochemical tests: Help confirm the organism's identity.

• Antigen detection: Rapidly detects microbial antigens.

• Serology: Detects antibodies, providing indirect evidence of infection or immunity.

• Molecular tests: Detect genetic material and can identify organisms that are difficult to culture.

60. What is the overall diagnostic approach emphasized in this lecture?

Start with clinical findings and appropriate specimens, then use microscopy and staining to narrow the possibilities. Culture is commonly used to grow and identify organisms, with biochemical testing to help confirm identity. Antigen detection, serology, and molecular techniques provide additional or alternative approaches when rapid testing is needed or culture is difficult or impossible.

Final high-yield comparison charts

Stains

Stain	Main organism or purpose
Gram	Gram-positive purple; Gram-negative pink
Auramine-Rhodamine	Mycobacteria, Cryptosporidium
Ziehl-Neelsen	Older acid-fast stain
Wright-Giemsa	Blood parasites, some bacteria
India ink	Cryptococcus neoformans
Capsule stain	Encapsulated organisms
Flagellar stain	Bacterial motility
Endospore stain	Bacillus, Clostridium

Culture media

Medium	Organism
Blood agar	Most non-fastidious bacteria
Chocolate agar	H. influenzae, N. gonorrhoeae
Sabouraud dextrose	Fungi
MacConkey/EMB	Gram-negative enteric rods
Lowenstein-Jensen	Mycobacteria, especially TB
BCYE	Legionella pneumophila
Bordet-Gengou	Bordetella pertussis
Tissue cell lines	Viruses

Biochemical tests

Test	Key result
Catalase	Staph +; Strep/Enterococcus −
Coagulase	S. aureus +
Beta hemolysis	Complete; clear zone
Alpha hemolysis	Partial; greenish zone
Gamma hemolysis	No hemolysis
Optochin	S. pneumoniae sensitive
Bacitracin	Group A Strep sensitive
Lactose	E. coli, Klebsiella ferment
Sorbitol	E. coli O157 generally does not ferment
Oxidase	Pseudomonas and several others positive