Bacteria Classification Flashcards
What is the purpose of the Gram stain?
The Gram stain differentiates bacteria into Gram-positive (purple) and Gram-negative (pink) based on their cell wall structure.
What are the steps of the Gram stain procedure?
1) Apply crystal violet (purple dye), 2) Apply iodine (mordant), 3) Alcohol wash (decolorization), 4) Apply safranin (counterstain).
What is the key difference between Gram-positive and Gram-negative bacteria?
Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane.
What organisms do not stain with the Gram stain?
Organisms without a cell wall (e.g., Mycoplasma, Ureaplasma, Chlamydia), acid-fast bacteria, and viruses.
What is the clinical significance of direct examination of a specimen?
It allows for presumptive identification of organisms, guides empiric therapy, and provides evidence of infection even if cultures are negative.
What are the common biochemical tests used to identify bacteria?
Catalase, coagulase, PYR test, oxidase, and indole tests.
What is the catalase test used for?
The catalase test differentiates Staphylococcus (catalase-positive) from Streptococcus (catalase-negative).
What is the coagulase test used for?
The coagulase test differentiates Staphylococcus aureus (coagulase-positive) from other staphylococci (coagulase-negative).
What are the three types of hemolysis seen on blood agar plates?
1) Alpha-hemolysis (partial hemolysis), 2) Beta-hemolysis (complete hemolysis), 3) Gamma-hemolysis (no hemolysis).
What is the natural habitat of Staphylococcus aureus?
Skin, mucous membranes, respiratory tract, and the anterior nares of patients and medical staff.
What are the common infections caused by Staphylococcus aureus?
Skin and soft tissue infections (e.g., boils, abscesses), osteomyelitis, sepsis, endocarditis, and toxin-mediated diseases (e.g., toxic shock syndrome).
What is MRSA, and how does it differ from MSSA?
MRSA (Methicillin-resistant Staphylococcus aureus) is resistant to methicillin and other beta-lactam antibiotics, while MSSA (Methicillin-susceptible Staphylococcus aureus) is susceptible.
What are the virulence factors of Staphylococcus aureus?
Catalase, coagulase, hyaluronidase, hemolysins, Panton-Valentine leukocidin, exfoliative toxins, TSST-1, and enterotoxins.
What is scalded skin syndrome, and what causes it?
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.
What are coagulase-negative staphylococci (CNS), and what infections do they cause?
CNS (e.g., Staphylococcus epidermidis) are less virulent than S. aureus and typically cause prosthetic material infections, IV catheter infections, and urinary tract infections.
What is the difference between Staphylococcus aureus and Staphylococcus epidermidis in terms of colony appearance?
S. aureus forms gold colonies, while S. epidermidis forms white colonies.
What is the PYR test used for?
The PYR test is used to identify Streptococcus pyogenes (Group A Streptococcus) and Enterococcus species, which are PYR-positive.
What are the common infections caused by Streptococcus pyogenes?
Pharyngitis, skin infections (e.g., impetigo, cellulitis), necrotizing fasciitis, scarlet fever, and immune-mediated diseases (e.g., rheumatic fever).
What is necrotizing fasciitis, and what organism commonly causes it?
Necrotizing fasciitis is a severe infection of deep tissues caused by Streptococcus pyogenes, leading to destruction of muscle fascia and subcutaneous fat.
What is scarlet fever, and what causes it?
Scarlet fever is a toxin-mediated disease caused by Streptococcus pyogenes, characterized by a red rash, sandpaper texture, and ‘strawberry tongue.’
What is the natural habitat of Streptococcus agalactiae (Group B Streptococcus)?
Vagina, cervix, and gastrointestinal tract.
What infections are caused by Streptococcus agalactiae?
Neonatal sepsis, pneumonia, meningitis, and postpartum sepsis.
What is the natural habitat of Streptococcus pneumoniae?
Upper respiratory tract, with asymptomatic carriage common in young children.
What infections are caused by Streptococcus pneumoniae?
Pneumonia, otitis media, sinusitis, sepsis, and meningitis.
What are the two vaccines available for Streptococcus pneumoniae?
23-valent polysaccharide vaccine and 13-valent conjugate vaccine.
What are viridans group streptococci, and what infections do they cause?
Viridans group streptococci are alpha-hemolytic and cause dental caries, abscesses, sepsis, and endocarditis.
What are the main species of Enterococcus, and what infections do they cause?
Enterococcus faecalis and Enterococcus faecium cause post-surgical wound infections, intra-abdominal infections, sepsis, and endocarditis.
What is VRE, and why is it significant?
VRE (Vancomycin-resistant Enterococcus) is significant because it is resistant to vancomycin, a last-resort antibiotic, making infections difficult to treat.
What is the natural habitat of Listeria monocytogenes?
Gastrointestinal tract and environment, with the ability to grow at 4°C.
What populations are most at risk for Listeria monocytogenes infections?
Pregnant women, infants, the elderly, and immunocompromised individuals.
What infections are caused by Listeria monocytogenes?
Gastroenteritis, sepsis, meningitis, and spontaneous abortion or stillbirth in pregnant women.
What is the appearance of Listeria monocytogenes on blood agar?
Small zones of beta-hemolysis.
What is the natural habitat of Corynebacterium diphtheriae?
Nasopharynx, skin, and environment.
What is the primary disease caused by Corynebacterium diphtheriae?
Respiratory diphtheria, characterized by pseudomembrane formation and systemic toxicity.
What is the appearance of Corynebacterium diphtheriae on Gram stain?
Gram-positive bacilli with a club-like appearance.
What is the natural habitat of Bacillus anthracis?
Infected animals, spores in soil, water, and vegetation, and it can be used in bioterrorism.
What are the clinical forms of anthrax?
Cutaneous anthrax, inhalation anthrax, gastrointestinal anthrax, and injection anthrax.
What is the most common form of anthrax, and what are its symptoms?
Cutaneous anthrax (95% of cases) presents as a painless ulcer with a black eschar.
What is inhalation anthrax, and why is it dangerous?
Inhalation anthrax occurs when spores are inhaled, causing fever, respiratory failure, pleural effusions, and mediastinal widening, with a high mortality rate.
Why is Bacillus anthracis considered a potential bioterrorism agent?
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.
What is the appearance of Bacillus anthracis on Gram stain?
Gram-positive bacilli with central spores, often forming chains.
What is the role of the capsule in Streptococcus pneumoniae?
The capsule is a virulence factor that helps the bacteria evade phagocytosis by the host immune system.
What is the significance of the M protein in Streptococcus pyogenes?
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.
What is the difference between alpha-hemolytic and beta-hemolytic streptococci?
Alpha-hemolytic streptococci cause partial hemolysis (greenish discoloration), while beta-hemolytic streptococci cause complete hemolysis (clear zones).
What is the significance of the bile esculin test for Enterococcus?
Enterococcus species hydrolyze esculin in the presence of bile, which helps differentiate them from other streptococci.
What is the significance of the oxidase test?
The oxidase test is used to identify bacteria that produce cytochrome c oxidase, such as Pseudomonas aeruginosa.
What is the significance of the indole test?
The indole test is used to identify bacteria that can break down tryptophan into indole, such as Escherichia coli.
What is the significance of the PYR test for Enterococcus?
Enterococcus species are PYR-positive, which helps differentiate them from other streptococci.
What is the significance of the catalase test for Staphylococcus?
Staphylococcus species are catalase-positive, which helps differentiate them from Streptococcus (catalase-negative).
What is the significance of the coagulase test for Staphylococcus aureus?
Staphylococcus aureus is coagulase-positive, which helps differentiate it from other staphylococci (e.g., Staphylococcus epidermidis).
