ASCP Technologist in Microbiology 2 — Questions and Answers
Question 1: Which staining technique is most commonly used to differentiate bacteria into two major groups?
- Acid-fast stain
- Gram stain (Correct answer)
- Wright stain
- India ink
Correct answer: Gram stain
The Gram stain classifies bacteria as gram-positive (purple/blue) or gram-negative (pink/red) based on cell wall structure.
The four-step procedure: crystal violet (primary stain), iodine (mordant), alcohol/acetone (decolorizer removes stain from gram-negative), safranin (counterstain for gram-negative). Gram-positive bacteria have thick peptidoglycan retaining crystal violet; gram-negative have thin peptidoglycan and an outer membrane. This classification guides initial antibiotic selection.
Question 2: What is the purpose of blood agar in microbiology?
- To selectively grow only gram-negative bacteria
- To detect hemolysis patterns that help identify bacterial species (Correct answer)
- To inhibit fungal growth
- To test antibiotic susceptibility
Correct answer: To detect hemolysis patterns that help identify bacterial species
Blood agar detects hemolysis patterns (alpha, beta, gamma) produced by bacteria, aiding identification.
Blood agar contains 5% sheep RBCs. Alpha hemolysis is partial lysis producing a greenish zone (S. pneumoniae). Beta hemolysis is complete lysis producing a clear zone (Group A Strep, S. aureus). Gamma hemolysis means no lysis (Enterococcus). These patterns are essential for preliminary identification of streptococci.
Question 3: Which organism is the most common cause of urinary tract infections?
- Staphylococcus aureus
- Escherichia coli (Correct answer)
- Pseudomonas aeruginosa
- Streptococcus pneumoniae
Correct answer: Escherichia coli
E. coli causes approximately 80% of uncomplicated UTIs due to virulence factors that allow adherence to uroepithelial cells.
Uropathogenic E. coli strains possess P fimbriae and type 1 fimbriae for attachment. UTIs are more common in women due to shorter urethra. Colony counts of 100,000 CFU/mL or more from clean-catch are significant. E. coli appears as pink lactose-fermenting colonies on MacConkey agar.
Question 4: What is the purpose of an antibiotic susceptibility test?
- To identify the species of bacteria
- To determine which antibiotics will effectively inhibit or kill the pathogen (Correct answer)
- To count bacteria in a specimen
- To detect viral infections
Correct answer: To determine which antibiotics will effectively inhibit or kill the pathogen
AST determines the minimum inhibitory concentration (MIC) of antibiotics against a bacterial isolate, guiding treatment selection.
Methods include broth microdilution (gold standard MIC), Kirby-Bauer disk diffusion, Etest (gradient strips), and automated systems (Vitek 2, MicroScan). Results are interpreted using CLSI or EUCAST breakpoints as susceptible, intermediate, or resistant. AST is critical for combating antibiotic resistance.
Question 5: Which culture medium is selective for gram-negative bacteria?
- Blood agar
- Chocolate agar
- MacConkey agar (Correct answer)
- Thioglycollate broth
Correct answer: MacConkey agar
MacConkey agar inhibits gram-positive bacteria (using bile salts and crystal violet) and differentiates gram-negatives by lactose fermentation.
MacConkey agar is selective (bile salts and crystal violet inhibit gram-positives) and differential (lactose fermenters produce pink colonies via neutral red pH indicator; non-fermenters appear colorless). This distinguishes normal flora from enteric pathogens like Salmonella and Shigella which are non-lactose fermenters.
Question 6: What does MRSA stand for, and why is it clinically significant?
- Multi-resistant Streptococcus aureus; causes severe pneumonia
- Methicillin-resistant Staphylococcus aureus; resistant to beta-lactam antibiotics (Correct answer)
- Macrolide-resistant Staphylococcus aureus; resistant to erythromycin only
- Multi-resistant Salmonella; causes food poisoning
Correct answer: Methicillin-resistant Staphylococcus aureus; resistant to beta-lactam antibiotics
MRSA carries the mecA gene encoding altered PBP2a that confers resistance to virtually all beta-lactam antibiotics.
MRSA possesses the mecA gene on the SCCmec element encoding PBP2a with low affinity for beta-lactams, conferring resistance to penicillins, cephalosporins, and carbapenems. Treatment options include vancomycin, daptomycin, and linezolid. Detection uses cefoxitin disk screening, PBP2a latex agglutination, or mecA PCR.
ASCP Technologist in Microbiology 2
Which staining technique is most commonly used to differentiate bacteria into two major groups?