Microbiology: Gram Stain & Pathogens flashcards

The deck

1. 1Why do gram-positive bacteria appear purple after Gram staining?Tap to reveal

   AnswerTheir thick peptidoglycan layer dehydrates and traps the crystal violet–iodine complex during the alcohol decolorizing step, so the purple primary stain stays in the cell and the safranin counterstain does not show.
2. 2Why do gram-negative bacteria appear pink or red after Gram staining?Tap to reveal

   AnswerTheir thin peptidoglycan layer sits beneath an outer membrane that the alcohol dissolves, releasing the crystal violet–iodine complex. The decolorized cells then take up the pink safranin counterstain.
3. 3What are the four reagents of the Gram stain, in order?Tap to reveal

   AnswerCrystal violet (the primary stain), iodine (the mordant, which fixes the dye as a complex), alcohol or acetone (the decolorizer), and safranin (the counterstain).
4. 4Which important pathogens cannot be classified by the Gram stain, and why?Tap to reveal

   AnswerMycobacterium species, because their waxy mycolic acid wall requires an acid-fast stain; Mycoplasma, because it has no cell wall; and spirochetes such as Treponema pallidum, which are too thin to resolve by ordinary light microscopy.
5. 5Staphylococcus aureus — Gram reaction, morphology, and diseases?Tap to reveal

   AnswerGram-positive cocci in grape-like clusters, and coagulase-positive. It causes skin and wound infections, abscesses, osteomyelitis, endocarditis, toxic shock syndrome, and preformed-toxin food poisoning. MRSA strains are methicillin-resistant.
6. 6Streptococcus pyogenes (Group A strep) — Gram reaction, morphology, and diseases?Tap to reveal

   AnswerGram-positive cocci in chains, beta-hemolytic. It causes strep throat, impetigo, scarlet fever, and necrotizing fasciitis, and can be followed by the immune sequelae rheumatic fever and post-streptococcal glomerulonephritis.
7. 7Streptococcus pneumoniae — Gram reaction, morphology, and diseases?Tap to reveal

   AnswerGram-positive lancet-shaped diplococci, alpha-hemolytic and encapsulated. It is the leading cause of community-acquired pneumonia and a major cause of otitis media, sinusitis, and bacterial meningitis in adults.
8. 8Enterococcus faecalis — Gram reaction, morphology, and diseases?Tap to reveal

   AnswerGram-positive cocci in pairs and short chains, part of the normal gut flora. It causes urinary tract infections, intra-abdominal infections, and endocarditis. Vancomycin-resistant enterococci (VRE) are an important hospital problem, though VRE is more often E. faecium than E. faecalis.
9. 9Clostridioides (Clostridium) difficile — Gram reaction, morphology, and disease?Tap to reveal

   AnswerA gram-positive, spore-forming, anaerobic rod. It causes antibiotic-associated diarrhea and pseudomembranous colitis after normal gut flora are disrupted. Its spores resist alcohol gel, so handwashing with soap and water is required.
10. 10Clostridium tetani — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-positive, spore-forming, anaerobic rod found in soil. It causes tetanus: the toxin tetanospasmin blocks inhibitory neurotransmitters, producing spastic paralysis with lockjaw and muscle rigidity.
11. 11Clostridium botulinum — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-positive, spore-forming, anaerobic rod. It causes botulism: its toxin blocks acetylcholine release at the neuromuscular junction, producing descending flaccid paralysis. Infant botulism is classically linked to honey.
12. 12Bacillus anthracis — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-positive, spore-forming, aerobic rod that grows in chains. It causes anthrax in cutaneous, inhalational, and gastrointestinal forms; the cutaneous form produces a painless black eschar.
13. 13Listeria monocytogenes — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-positive rod that can grow at refrigeration temperatures. It causes listeriosis from contaminated deli meats, unpasteurized dairy, and soft cheeses, and is especially dangerous in pregnancy, in newborns, and in the immunocompromised.
14. 14Escherichia coli — Gram reaction, morphology, and diseases?Tap to reveal

    AnswerA gram-negative rod that is part of the normal gut flora. It is the most common cause of urinary tract infections, and also causes traveler's diarrhea and neonatal meningitis. The O157:H7 strain causes bloody diarrhea and can lead to hemolytic uremic syndrome.
15. 15Salmonella Typhi — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-negative rod. It causes typhoid fever, a systemic illness with sustained fever spread through food and water contaminated by human waste. Non-typhoidal Salmonella instead causes gastroenteritis, classically from poultry and eggs.
16. 16Shigella — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-negative rod. It causes bacillary dysentery — bloody, mucus-containing diarrhea with abdominal cramps. It has a very low infectious dose and spreads readily person to person by the fecal-oral route.
17. 17Neisseria gonorrhoeae — Gram reaction, morphology, and disease?Tap to reveal

    AnswerGram-negative diplococci, typically seen inside neutrophils on a smear. It causes gonorrhea, and in newborns delivered through an infected birth canal it causes ophthalmia neonatorum.
18. 18Neisseria meningitidis — Gram reaction, morphology, and disease?Tap to reveal

    AnswerGram-negative encapsulated diplococci. It causes meningococcal meningitis and meningococcemia, spreads by respiratory droplets, and is known for outbreaks in crowded settings such as dormitories.
19. 19Pseudomonas aeruginosa — Gram reaction, morphology, and diseases?Tap to reveal

    AnswerA gram-negative aerobic rod that produces a blue-green pigment. It is a major cause of hospital-acquired infections: ventilator-associated pneumonia, burn wound infections, otitis externa, and chronic lung infection in cystic fibrosis.
20. 20Klebsiella pneumoniae — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-negative encapsulated rod. It causes pneumonia — classically with thick, blood-tinged 'currant jelly' sputum — as well as urinary tract infections. Carbapenem-resistant strains are a serious hospital threat.
21. 21Haemophilus influenzae — Gram reaction, morphology, and diseases?Tap to reveal

    AnswerA gram-negative coccobacillus. The encapsulated type b strain caused meningitis and epiglottitis in young children before routine Hib vaccination; nontypeable strains still cause otitis media and sinusitis.
22. 22Helicobacter pylori — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-negative curved or spiral rod. It causes chronic gastritis and peptic ulcer disease and raises the risk of gastric cancer. It is urease-positive, which neutralizes stomach acid and is the basis of the urea breath test.
23. 23Vibrio cholerae — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-negative comma-shaped rod. It causes cholera: its enterotoxin drives massive secretory diarrhea described as rice-water stools, and death results from dehydration, so rehydration is the mainstay of treatment.
24. 24Bordetella pertussis — Gram reaction, morphology, and disease?Tap to reveal

    AnswerA gram-negative coccobacillus. It causes pertussis (whooping cough), with a catarrhal stage followed by paroxysmal coughing fits and an inspiratory whoop. It is prevented by the DTaP and Tdap vaccines.

Microbiology: Gram Stain & Pathogens: FAQ

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