Bacteria Classification

Question 2

What is the purpose of the Gram stain?

Answer 2

The Gram stain differentiates bacteria into Gram-positive (purple) and Gram-negative (pink) based on their cell wall structure.

Question 3

What are the steps of the Gram stain procedure?

Answer 3

1) Apply crystal violet (purple dye), 2) Apply iodine (mordant), 3) Alcohol wash (decolorization), 4) Apply safranin (counterstain).

Question 4

What is the key difference between Gram-positive and Gram-negative bacteria?

Answer 4

Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane.

Question 5

What organisms do not stain with the Gram stain?

Answer 5

Organisms without a cell wall (e.g., Mycoplasma, Ureaplasma, Chlamydia), acid-fast bacteria, and viruses.

Question 6

What is the clinical significance of direct examination of a specimen?

Answer 6

It allows for presumptive identification of organisms, guides empiric therapy, and provides evidence of infection even if cultures are negative.

Question 7

What are the common biochemical tests used to identify bacteria?

Answer 7

Catalase, coagulase, PYR test, oxidase, and indole tests.

Question 8

What is the catalase test used for?

Answer 8

The catalase test differentiates Staphylococcus (catalase-positive) from Streptococcus (catalase-negative).

Question 9

What is the coagulase test used for?

Answer 9

The coagulase test differentiates Staphylococcus aureus (coagulase-positive) from other staphylococci (coagulase-negative).

Question 10

What are the three types of hemolysis seen on blood agar plates?

Answer 10

1) Alpha-hemolysis (partial hemolysis), 2) Beta-hemolysis (complete hemolysis), 3) Gamma-hemolysis (no hemolysis).

Question 11

What is the natural habitat of Staphylococcus aureus?

Answer 11

Skin, mucous membranes, respiratory tract, and the anterior nares of patients and medical staff.

Question 12

What are the common infections caused by Staphylococcus aureus?

Answer 12

Skin and soft tissue infections (e.g., boils, abscesses), osteomyelitis, sepsis, endocarditis, and toxin-mediated diseases (e.g., toxic shock syndrome).

Question 13

What is MRSA, and how does it differ from MSSA?

Answer 13

MRSA (Methicillin-resistant Staphylococcus aureus) is resistant to methicillin and other beta-lactam antibiotics, while MSSA (Methicillin-susceptible Staphylococcus aureus) is susceptible.

Question 14

What are the virulence factors of Staphylococcus aureus?

Answer 14

Catalase, coagulase, hyaluronidase, hemolysins, Panton-Valentine leukocidin, exfoliative toxins, TSST-1, and enterotoxins.

Question 15

What is scalded skin syndrome, and what causes it?

Answer 15

Scalded skin syndrome is a condition caused by exfoliative toxins from Staphylococcus aureus, leading to blistering and loss of the superficial skin layer, mostly in infants.

Question 16

What are coagulase-negative staphylococci (CNS), and what infections do they cause?

Answer 16

CNS (e.g., Staphylococcus epidermidis) are less virulent than S. aureus and typically cause prosthetic material infections, IV catheter infections, and urinary tract infections.

Question 17

What is the difference between Staphylococcus aureus and Staphylococcus epidermidis in terms of colony appearance?

Answer 17

S. aureus forms gold colonies, while S. epidermidis forms white colonies.

Question 18

What is the PYR test used for?

Answer 18

The PYR test is used to identify Streptococcus pyogenes (Group A Streptococcus) and Enterococcus species, which are PYR-positive.

Question 19

What are the common infections caused by Streptococcus pyogenes?

Answer 19

Pharyngitis, skin infections (e.g., impetigo, cellulitis), necrotizing fasciitis, scarlet fever, and immune-mediated diseases (e.g., rheumatic fever).

Question 20

What is necrotizing fasciitis, and what organism commonly causes it?

Answer 20

Necrotizing fasciitis is a severe infection of deep tissues caused by Streptococcus pyogenes, leading to destruction of muscle fascia and subcutaneous fat.

Question 21

What is scarlet fever, and what causes it?

Answer 21

Scarlet fever is a toxin-mediated disease caused by Streptococcus pyogenes, characterized by a red rash, sandpaper texture, and ‘strawberry tongue.’

Question 22

What is the natural habitat of Streptococcus agalactiae (Group B Streptococcus)?

Answer 22

Vagina, cervix, and gastrointestinal tract.

Question 23

What infections are caused by Streptococcus agalactiae?

Answer 23

Neonatal sepsis, pneumonia, meningitis, and postpartum sepsis.

Question 24

What is the natural habitat of Streptococcus pneumoniae?

Answer 24

Upper respiratory tract, with asymptomatic carriage common in young children.

Question 25

What infections are caused by Streptococcus pneumoniae?

Answer 25

Pneumonia, otitis media, sinusitis, sepsis, and meningitis.

Question 26

What are the two vaccines available for Streptococcus pneumoniae?

Answer 26

23-valent polysaccharide vaccine and 13-valent conjugate vaccine.

Question 27

What are viridans group streptococci, and what infections do they cause?

Answer 27

Viridans group streptococci are alpha-hemolytic and cause dental caries, abscesses, sepsis, and endocarditis.

Question 28

What are the main species of Enterococcus, and what infections do they cause?

Answer 28

Enterococcus faecalis and Enterococcus faecium cause post-surgical wound infections, intra-abdominal infections, sepsis, and endocarditis.

Question 29

What is VRE, and why is it significant?

Answer 29

VRE (Vancomycin-resistant Enterococcus) is significant because it is resistant to vancomycin, a last-resort antibiotic, making infections difficult to treat.

Question 30

What is the natural habitat of Listeria monocytogenes?

Answer 30

Gastrointestinal tract and environment, with the ability to grow at 4°C.

Question 31

What populations are most at risk for Listeria monocytogenes infections?

Answer 31

Pregnant women, infants, the elderly, and immunocompromised individuals.

Question 32

What infections are caused by Listeria monocytogenes?

Answer 32

Gastroenteritis, sepsis, meningitis, and spontaneous abortion or stillbirth in pregnant women.

Question 33

What is the appearance of Listeria monocytogenes on blood agar?

Answer 33

Small zones of beta-hemolysis.

Question 34

What is the natural habitat of Corynebacterium diphtheriae?

Answer 34

Nasopharynx, skin, and environment.

Question 35

What is the primary disease caused by Corynebacterium diphtheriae?

Answer 35

Respiratory diphtheria, characterized by pseudomembrane formation and systemic toxicity.

Question 36

What is the appearance of Corynebacterium diphtheriae on Gram stain?

Answer 36

Gram-positive bacilli with a club-like appearance.

Question 37

What is the natural habitat of Bacillus anthracis?

Answer 37

Infected animals, spores in soil, water, and vegetation, and it can be used in bioterrorism.

Question 38

What are the clinical forms of anthrax?

Answer 38

Cutaneous anthrax, inhalation anthrax, gastrointestinal anthrax, and injection anthrax.

Question 39

What is the most common form of anthrax, and what are its symptoms?

Answer 39

Cutaneous anthrax (95% of cases) presents as a painless ulcer with a black eschar.

Question 40

What is inhalation anthrax, and why is it dangerous?

Answer 40

Inhalation anthrax occurs when spores are inhaled, causing fever, respiratory failure, pleural effusions, and mediastinal widening, with a high mortality rate.

Question 41

Why is Bacillus anthracis considered a potential bioterrorism agent?

Answer 41

Its spores are easily found, can be produced in labs, and can survive in the environment for long periods, making them ideal for silent release.

Question 42

What is the appearance of Bacillus anthracis on Gram stain?

Answer 42

Gram-positive bacilli with central spores, often forming chains.

Question 43

What is the role of the capsule in Streptococcus pneumoniae?

Answer 43

The capsule is a virulence factor that helps the bacteria evade phagocytosis by the host immune system.

Question 44

What is the significance of the M protein in Streptococcus pyogenes?

Answer 44

The M protein is a virulence factor that helps the bacteria evade phagocytosis and is the basis for the different serotypes of Group A Streptococcus.

Question 45

What is the difference between alpha-hemolytic and beta-hemolytic streptococci?

Answer 45

Alpha-hemolytic streptococci cause partial hemolysis (greenish discoloration), while beta-hemolytic streptococci cause complete hemolysis (clear zones).

Question 46

What is the significance of the bile esculin test for Enterococcus?

Answer 46

Enterococcus species hydrolyze esculin in the presence of bile, which helps differentiate them from other streptococci.

Question 47

What is the significance of the oxidase test?

Answer 47

The oxidase test is used to identify bacteria that produce cytochrome c oxidase, such as Pseudomonas aeruginosa.

Question 48

What is the significance of the indole test?

Answer 48

The indole test is used to identify bacteria that can break down tryptophan into indole, such as Escherichia coli.

Question 49

What is the significance of the PYR test for Enterococcus?

Answer 49

Enterococcus species are PYR-positive, which helps differentiate them from other streptococci.

Question 50

What is the significance of the catalase test for Staphylococcus?

Answer 50

Staphylococcus species are catalase-positive, which helps differentiate them from Streptococcus (catalase-negative).

Question 51

What is the significance of the coagulase test for Staphylococcus aureus?

Answer 51

Staphylococcus aureus is coagulase-positive, which helps differentiate it from other staphylococci (e.g., Staphylococcus epidermidis).