Gram Positive and Negative Bacteria Flashcards
1
Q
Describe general properties of staphylococcus
A
- Structure: Gram positive (purple), cocci 0.5-1.5 um diameter, singly and in pairs, tetrad, short chains and irregular grape like clusters, nonmotile, non-spore forming, catalase positive, enterotoxin A and B
- Physical Requirements: Facultative anaerobes, grow in the presence or absence of air, prefer air (except S. saccharolyticus, S. aureus subsp. anaerobius)
- Species: 35 species and 17 subspecies
- Clinical Significance: Commensal on human skin, also occurs in nose frequently and throat less commonly, sometimes in gut
- Emergence of multiple antibiotic resistant strains (MRSA to superbugs)
- Methicillin: Form of penicillin to treat staph, organisms are MRSA (resistance to front line drugs)
- Infections: Bacteraemia, pneumonia, food poisoning, acute myocarditis, pericarditis, abaecess of muscle, urogenital tract and CNS
- Categorisation: Weak or striking lysis of RBC, due to release of exotoxins which target RBC and WBC, in attempt to obtain nutrients (iron), cream / yellow pigment, variation in zones of lysis
- Mannitol Salt: Selective and differential, high salt (7.5%), halo tolerant organisms, unique sugar (mannitol), ferment mannitol (yellow), cannot ferment mannitol (pink)
2
Q
Describe properties of staphylococcus aureus
A
- Beta haemolytic on blood plate leads to production of exotoxins, production of coagulase (free coagulase, clots), sodium chloride tolerance (7.5%), mannitol fermentation on MSA plate
- Presence of clumping factor (cell-bound coagulase), protection mechanism against immune recognition, covers itself with RBC and hence not recognised as foreign,
- Hyaluronidase production (positive decapsulation test with S.equi
3
Q
Describe properties of staphylococcus epidermis
A
- Forms white non haemolytic colonies on blood agar, facultative anaerobe, part of the normal human skin flora (not usually pathogenic), cannot lyse RBC, rely on nutrients in agar
- Opportunisticinfections, generally hospital-acquired (immunocompromised patients)
- Catalase-positive, coagulase negative (no clumping / clotting) and oxidase-negative
- Concern for people with catheters / implants, known to form biofilms that grow on these devices.
4
Q
Describe general properties of streptococcus
A
- Structure: Gram positive cocci, occur singly, pairs and short / long chains (characteristic), nonmotile, non-spore forming, catalase negative, some produce haemolysins (lyse RBC, ranges in ZoI)
- Classification: Alpha (green, capsule / quellung test, bile solubility test, ), beta (clear, group A / B / C or bacitracin / antibiotic susceptibility test) or gamma haemolytic
- Location: Transient colonisers of skin, resident colonisers of mucous membranes, normal flora in alimentary, respiratory and genital tract
- Physical Requirements: Facultative anaerobes
5
Q
What is lancefield typing and the CAMP test
A
- Lancefield Typing: A serologic classification of bacteria / haemolytic streptococci into groups based on specific carbohydrate antigen, groups A-O (group A = pathogenic to man), visual, simple, quick
- Antigenic cell wall carbohydrates, use of strain specific antibodies that differentiate cell wall CHOs
- CAMP Test: Identify group B β-haemolytic streptococci based on their formation of a substance that enlarges area of haemolysis formed by the β-haemolysin, beta-haemolytic streptococci often cause disease
- Production of CAMP factor reacts and causing increased haemolysis (arrow shaped)
6
Q
Describe properties of staphylococcus group A / B
A
Group A:
- Streptococci pyogenes, found in humans and primates
- Beta haemolytic, large colony forming unit, gray-whitish, glisten, lancefield group A antigen, numerous virulence factors allow S pyogenes to cause a wide array of serious infections
- Gram positive encapsulated diplococcus, prevented by conjugated vaccine, cause of pneumonia and otitis media (most life threatening in children and elderly)
- Positive PVR test is useful for preliminary identification of isolated strains (with negative catalase test)
- Identification can be confirmed by the latex agglutination test (serotyping test)
Group B:
- Colony morphology, gram stain, catalase reaction, lancefield, CAMP test, hippurate hydrolysis
7
Q
Describe 3 streptococcal skin infections
A
- Erysipelas: S. pyogenes infects the dermal layer of the skin, causes local tissue destruction and sepsis
- Necrotising Fasciitis: “Flesh-eating” disease, exotoxin A produced by S. pyogenes acts as a super antigen
- Streptococcal toxic shock syndrome: Similar to staphylococcal TSS
8
Q
Describe general properties of enterococcus
A
- Structure: Occurs singly, in pairs and in short chains, catalase negative, group D antigen positive
- Physical Requirements: Facultative anaerobes, optimum growth at 35ºC, growth in broth of 6.5% NaCl
- Species: 19 species (common in humans are E faecalis and E faecium), superbug (antibiotic resistance)
- Clinical Significance: 10% of all UTI’s, 16% of nosocomial UTI’s, intrabdominal or pelvic wounds (poly microbic), increasing rate in wound infection, leading cause of nosocomial bacteraemia
9
Q
Describe properties of enterococcus faecalis
A
- Gram-positive coccus, commensal bacterium inhabits the GI tracts of humans and other animals, ability to grow in the presence of high concentrations of bile salts and sodium chloride
- Can cause life-threatening infections in humans, especially in the nosocomial (hospital) environment, where the naturally high levels of antibiotic resistance found in E. faecalis contribute to its pathogenicity
- Severe virulence factors, colonies show beta haemolysis on blood agar plates, a plasmid-encoded haemolysin (cytolysin)
- High-level gentamicin resistance is associated with a five-fold increase in risk of death in human bacteremia patients, resistant to vancomycin (last resort antibiotic)
10
Q
List a number of gram positive aerobic and anaerobic bacteria
A
- Aerobic: Bacilli - listeria, erysipelothrix, nocardia, mycobacterium
- Anaerobic: Bacilli - clostridium, actinomyces, porphyromonas, lactobacillus, propionibacterium
11
Q
Describe general properties of listeria
A
- Structure: Non branching short gram positive rods (0.5-2 um by 0.4-0.5 um), occurs singly or in short chains, colonies 1-2mm, smooth, catalase positive, coccobacilli
- Physical Requirements: Facultatively anaerobic, active growth at 4ºC, motile at 28ºC
- Species: 7 species, including L. monocytogenes (pathogenic to humans) and L. ivanovii (frequent cause of abortions in animals)
12
Q
Describe properties of listeria monocytogenes
A
- Commonly found in soil, stream water, sewage, plants, and food, gram-positive non-sporing rod-shaped, pleomorphic (coccoid appearance), increased virulence
- Reproduces in phagocytes, reorganises actin filaments which allows ‘jumping’ to other cells, adapted itself to human cells
- Able to grow in temperatures ranging from 4°C (fridge) to 37°C (body’s internal temperature)
- Listeriosis: A rare but potentially lethal food-borne infection, virulence is attributed to listeria’s ability to spread to the nervous system (meningitis), infects pregnant women via placenta (stillbirth),
- Usually food borne and asymptomatic, meningitis more common in the immunocompromised, can invade the bloodstream (sepsis)
13
Q
Describe general properties of bacillus anthracis
A
- Structure: Gram positive rod (cause of anthrax), forms long chains, forms endospores, ‘biological weapon’ (spores, long-lasting, protection from environment)
- Physical Requirements: Aerobe, found in soils
- Clinical Significance: Severe and lethal respiratory infection (inhale spores), lethal infection of pneumonia
14
Q
Describe general properties of clostridium
A
- Structure: Gram positive rod shaped cells, form endospores that distend the shape of the cell, endospores are resistant to heat and chemicals (form in the absence of nutrients), tennis racket shape
- Physical Requirements: Obligate anaerobes
- Pathogen: C. tetani (tetanus), C. botulinum (botulism), C. perfringens (gas gangrene)
15
Q
What is tetanus
A
- Production of neurotoxin tetanospasmin, acts in CNS, binds to nerves that control contraction of skeletal muscles, blocks inhibiting pathway to prevent random contractions
- Causing convulsive symptoms
Infants are vaccinated against tetanus toxoid - Toxoid is a modified protein that retains immunogenicity but loses toxicity, denatured toxin (safe but generate innate immunity), ubiquitous
