MICROBIOLOGY Flashcards
Microbiology specimens could be?
a. For bacteria: biopsied tissue, exudates or pus (swab or aspiration).
b. For Fungi: Skin scraping; nail scraping, hair, biopsied tissue.
c. Specimens should be processed as soon as possible:
- to ensure that the pathogens does not die
- to control contaminating organism
Culture media depends on ?
type of specimens and the suspected etiological agent
Culture media for bacteria?
For bacteria, an enriched medium like blood agar and differential medium like MacConkey are encouraged since it will provide the growth of almost all pathogens, except the anaerobic pathogens.
Culture media for fungi?
a non-selective medium like Sabouraud dextrose agar (SDA) should be routinely use because it will support the growth of almost all the medically important fungi. For Dermatophytes, add cycloheximide to inhibit saprophyte fungi.
best temperature for fungi?
could be at room temperature (250 – 270C) or at 370C
incubation temperature of bacteria?
should be at 370C, could be in aerobic, microaerophilic, or anaerobic condition.
describe staphylococcus in general!
Staphylococci are catalase-positive, gram-positive organisms, grow in clusters (gram stain), common inhabitants of the skin and mucous membranes of humans
Characteristic of Staphylococcus aureus
- Hemolyze blood, coagulate plasma –> coagulase positive Staphylococci
- Virulent factors: extracellular enzymes, toxins
- Gram stain: clusters
- Blood agar : beta hemolytic
- DNAse test +, should be done if the coagulase test is negative
Characteristic of coagulase negative staphylococci
- less virulent
- more often associated with opportunistic infections (S epidermidis) or specific syndromes, such as S. saprophyticus and urinary tract infections
Coagulase test + in Staphylococcus aureus means
Coagulase test : + (produce coagulase enzyme)
• Coagulase –> deposit fibrin on the surface of staphylococci –>altering their ingestion by phagocytic cells or their destruction within such cells.
• Coagulase production is considered synonymous with invasive pathogenic potential.
• Negative findings on slide coagulation test should be followed by tube coagulation test.
Characteristic of Streptococcus pyogenes
- Group A beta hemolytic Streptococci
- Most streptococci that contain the group A antigen are S. pyogenes.
- Toxins and enzymes –> DSHPH
Toxins and enzymes in Streptococcus pyogenes
- Streptokinase (Fibrinolysin) & Deoxyribonuclease (DNAse) : Facilitate the spread of streptococci in tissue by liquefying pus
- Hyaluronidase : splits hyaluronic acid spreading of infection
- Pyrogenic exotoxins : Scarlet fever & streptococcal toxic shock syndrome
- Hemolysins : Streptolysin O –> Anti-streptolysin O serum titer ; if abnormally high (160-200 units) –> present infection or hypersensitivity which associated with post-streptococcal syndromes (e.g rheumatic fever, glomerulonephritis, arthritis)
Pseudomonas aeruginosa characteristic
- P. aeruginosa is widely distributed in nature and is commonly present in moist environments in hospitals
- The bacteria are pathogenic only when introduced into areas devoid of normal defenses: such as when mucous membranes and skin are disrupted by direct tissue damage as in the case of burn wounds
- Importance : multiple drugs resistance
- Culture: green pigment (pyocyanin)
- Kligler Agar : Glucose fermentation (-) ; Lactose fermentation (-)
- Citrate utilization test : +
explain citrate agar test for pseudomonas aeruginosa
• Using citrate agar (contain citrate, ammonuium salts, and bromothymol blue) –> Bacteria utilize citrate citrate metabolite break down ammonium salts to ammonia -> pH alkaline –> medium change from green to blue
Characteristic of Clostridium sp. ?
- Anaerobic, Gram-positive, spore forming bacteria
- Many different toxin-producing clostridia (C. perfringens and related clostridia) can produce invasive infection (including myonecrosis and gas gangrene) if introduced into damaged tissue
- The most common in invasive disease is C. perfringens (90%).
