Why do gram-positive bacteria appear purple after Gram staining?

Their 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.

Why do gram-negative bacteria appear pink or red after Gram staining?

Their 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.

What are the four reagents of the Gram stain, in order?

Crystal violet (the primary stain), iodine (the mordant, which fixes the dye as a complex), alcohol or acetone (the decolorizer), and safranin (the counterstain).

Which important pathogens cannot be classified by the Gram stain, and why?

Mycobacterium 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.

Staphylococcus aureus — Gram reaction, morphology, and diseases?

Gram-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.

Streptococcus pyogenes (Group A strep) — Gram reaction, morphology, and diseases?

Gram-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.

Streptococcus pneumoniae — Gram reaction, morphology, and diseases?

Gram-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.

Enterococcus faecalis — Gram reaction, morphology, and diseases?

Gram-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.

Clostridioides (Clostridium) difficile — Gram reaction, morphology, and disease?

A 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.

Clostridium tetani — Gram reaction, morphology, and disease?

A 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.

Clostridium botulinum — Gram reaction, morphology, and disease?

A 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.

Bacillus anthracis — Gram reaction, morphology, and disease?

A 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.

Listeria monocytogenes — Gram reaction, morphology, and disease?

A 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.

Escherichia coli — Gram reaction, morphology, and diseases?

A 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.

Salmonella Typhi — Gram reaction, morphology, and disease?

A 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.

Shigella — Gram reaction, morphology, and disease?

A 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.

Neisseria gonorrhoeae — Gram reaction, morphology, and disease?

Gram-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.

Neisseria meningitidis — Gram reaction, morphology, and disease?

Gram-negative encapsulated diplococci. It causes meningococcal meningitis and meningococcemia, spreads by respiratory droplets, and is known for outbreaks in crowded settings such as dormitories.

Pseudomonas aeruginosa — Gram reaction, morphology, and diseases?

A 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.

Klebsiella pneumoniae — Gram reaction, morphology, and disease?

A 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.

Haemophilus influenzae — Gram reaction, morphology, and diseases?

A 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.

Helicobacter pylori — Gram reaction, morphology, and disease?

A 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.

Vibrio cholerae — Gram reaction, morphology, and disease?

A 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.

Bordetella pertussis — Gram reaction, morphology, and disease?

A 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.

What is the difference between gram-positive and gram-negative bacteria?

Gram-positive bacteria have a thick peptidoglycan cell wall that retains the crystal violet dye, so they appear purple. Gram-negative bacteria have a thin peptidoglycan layer plus an outer membrane containing lipopolysaccharide; the decolorizer washes the dye out and they take up the pink safranin counterstain.

What are the four steps of the Gram stain?

Crystal violet as the primary stain, iodine as the mordant to trap it, alcohol or acetone as the decolorizer, and safranin as the counterstain. The decolorizing step is the one that determines the result, and leaving it on too long can make gram-positive cells look gram-negative.

Which bacteria do not Gram stain?

Mycobacterium species have a waxy mycolic acid layer and require an acid-fast stain instead. Mycoplasma has no cell wall at all. Spirochetes such as Treponema pallidum are too thin to be seen by ordinary light microscopy after Gram staining, and are detected by darkfield microscopy or serology.

24 free flashcards

Microbiology: Gram Stain & Pathogens flashcards

This deck covers the Gram stain from mechanism to application: the four reagents of the procedure, why gram-positive cells hold the crystal violet while gram-negative cells take up the safranin counterstain, and which clinically important organisms resist the stain entirely. The remaining cards pair twenty major pathogens with their Gram reaction, their morphology, and the disease each one causes.

The Gram stain is still the first test run on most clinical specimens, and its result narrows the differential immediately — a gram-positive coccus in clusters from a wound is a very different problem from a gram-negative rod in sputum. Microbiology courses, nursing programs, and health-professions entrance exams all test the same core skill: classify the organism by stain and shape, then name what it causes. That pairing is what this deck drills.

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How to study this deck

Learn each organism as a three-part unit — Gram reaction, shape and arrangement, disease — and recite all three every time. Exam questions supply any one of the three and ask you for the others.
Fix the mechanism before the lists: gram-positive cells have a thick peptidoglycan layer that traps the crystal violet–iodine complex when alcohol is applied, while gram-negative cells have a thin layer plus an outer membrane the alcohol dissolves. Every colour result follows from that one difference.
Keep a short exception list of organisms the Gram stain cannot classify — mycobacteria (acid-fast, waxy mycolic acid wall), mycoplasma (no cell wall at all), and the spirochetes (too thin to resolve) — because those are exactly the ones test writers use to check whether you understand the stain rather than memorized it.

Microbiology: Gram Stain & Pathogens: FAQ

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