Gram Positive Cocci: Staphylococci and Streptococci (trans 6) Flashcards

Question

Staphylococcus aureus - Pathogenesis

Answer 1

 Anterior nares: major reservoir of infection  The perineum is another carriage site  Lesions with draining pus on the hands of the patients: disseminates the organism via touching others  Abscess: characteristic of a Staph infection. Organisms penetrate a sebaceous gland or a hair shaft, multiply and produce the infection.  Skin/Mucous membrane is an excellent protective barrier. Destruction of the integrity of these surfaces predispose to staph infection. (ex. Burns)

Answer 2

```  FOLLICULITIS  FURUNCLE  CARBUNCLE  IMPETIGO  STAPHYLOCOCCAL SCALDED SKIN SYNDROME  STAPHYLOCOCCAL FOOD POISONING  OSTEOMYELITIS  SEPTICEMIA  ACUTE ENDOCARDITIS  PNEUMONIA  SEPTIC ARTHRITIS  TOXIC SHOCK SYNDROME ```

Answer 3

o Infection of hair follicles o Manifestations: yellow discoloration of skin near hair follicle, the base of the hair shaft is surrounded by redness and a greenish tinge o Treatment: during the old days - clamp and pull out the hair, the pus comes with it, no antibiotic needed

Answer 4

o When folliculitis extends to subcutaneous tissues o Focal suppurative lesion o Treatment: do incision and drainage

Answer 5

o Furuncle extending to the deeper fibrous tissues | o Multiple foci and multiple draining sinuses

Answer 6

o Encrusted pustules on the superficial skin of children and infants o Starts as vesicles/vesicular lesions then becomes pustular o When crusts are removed, a red denuded surface is exposed, most of the time with pus o Highly contagious o 2 causes: Staphylococcus aureus and Streptococcus pyogenes (classical impetigo --- produced more by S. pyogenes)

Answer 7

STAPHYLOCOCCAL SCALDED SKIN SYNDROME o Due to Exfoliative Toxin o Generalized painful erythema and bulbous desquamation of large areas of the skin

Answer 8

o Most common form of bacterial food poisoning o Preformed toxin in food contaminated by the hands of the food workers o The problem is food with enterotoxin are normal in odor and taste, you don’t know that it’s contaminated o Rapid onset of symptoms like abdominal pain, vomiting & diarrhea 2-6 days after infection o Immediate effect: 2-6 (1-8) hours for sign and symptoms to appear

Answer 9

o Follows a hematogenous spread usually from a furuncle or carbuncle o Or a contiguous spread because of the close proximity especially if it's on the shin bone because it'll just erode into the bone o Localizes at the diaphysis of long bones with accumulation of pus o Fever, chills, pain over the bone and on movement

Answer 10

o A generalized infection with sepsis or bacteremia that may be associated with a known focus like a septic joint or even an ordinary carbuncle o Pwedeng sa boils (pigsa) lang magsimula ang septicemia

Answer 11

o Acute = immediate o Associated with intravenous drug abuse caused by injection of contaminated preparations or by needles contaminated with S. aureus o S. aureus also colonizes the skin around the injection site, and if the skin is not sterilized before injection, the bacteria can be introduced into soft tissues and the bloodstream o Can kill you in a week or so o Differentiate this from subacute endocarditis caused by Streptococcus viridans (takes a long time, around 1 yr)

Answer 12

o Common in hospitalized patients; follows a viral influenza infection o Rapid destruction of lung parenchyma resulting in cavitations o The most common type of hospital acquired pneumonia o Common in malnourished post-measles children

Answer 13

o Invasion of synovial membrane resulting in a closed infection of the joint cavity o Staphylococcal lesion on a joint o Painful, red, swollen joint with a limited range of motion

Answer 14

o Common in menstruating women who use tampons or anybody who has a Staph wound infection o Women brought to the ER during that time (1970’s) forgot that they still have a tampon inside => cause of infection o TSS results in high fever, rash with diffuse erythema followed by desquamation, vomiting, diarrhea, hypotension and multi-organ involvement (especially GI, renal and/or hepatic damage)

Answer 15

 Catalase Test o Use 3% H2O2on specimen o Appearance of bubbles indicate that the test is (+)  Coagulase Test o Citrated plasma plus equal volume of broth. Incubated o Formation of a clot in 1-4 hours means the test is (+) Remember: “All Staph are Catalase (+), Strep is Catalase (-), only S. aureus is Coagulase (+), other staph are Coagulase (-)”

Answer 16

Methicillin Resistant Staphylococcus aureus (MRSA) o Resistant to all β-lactam antibiotics (Penicillins and Cephalosporins) o Vancomycin - choice for empiric treatment of life-threatening MRSA, 1g every 12 hours o Unfortunately, there’s already resistance to this drug so alternative drugs are used such as Quinupristin Dalfopristin, Linezolid, and Daptomycin

Answer 17

 Proper disposal of discharge of contaminated fomites o Wound-infected gauze – survive for 2 weeks  Washing of hands before and after the patient contact  Mupirocin to nasal and perineal carriage sites (Bactroban/Bactifree in the Phils)  Anti Staph Drug + Rifampicin – provides long term suppression (for long term suppression or abscess that do not respond to Cloxacillin)  2015 literatures: use Clindamycin + Ciprofloxacin

Answer 18

Pathogenesis o Low virulence for the normal host o Life threatening infection when host defenses are breached o Predilection for prosthetic devices (ex. total hip replacement) Treatment o Vancomycin 1g every 12 hours o Vancomycin sensitivity remains the rule, but resistant isolates have been reported

Answer 19

``` REMEMBER All Staph are CATALASE (+) Strep are CATALASE (-) Only Staphylococcus aureus is COAGULASE (+) Other Staph are COAGULASE (-) ```

Answer 20

Classification of Streptococci 1. Hemolysis Patterns on BAP 2. Lancefield Classification 3. Capsular Polysaccharides - Antigenic specificity is used to classify S. pneumoniae into 90 types, and to type Group B streptococci 4. Biochemical Reactions

Answer 21

 β-Hemolysis: clear zone of hemolysis due to complete lysis of RBC  α-Hemolysis: greenish discoloration (biliverdin) due to incomplete hemolysis  γ-Hemolysis: non-hemolytic

Answer 22

 System based on the antigenic composition of cell wall carbohydrates in β- hemolytic strep  Groups A,B,C,D,G: Associated with human infection (according to Jawetz, groups A, B, C, F and G are said to cause disease in humans)

Answer 23

 Streptococci ferment carbohydrates  CATALASE (-)  Optochin Sensitivity o Optochin is an antibiotic disk (derivative of quinine) o S. pneumoniae is sensitive, other Strep species are resistant  PYR Disk o Ability to hydrolyze pyrrolidonyl o (+) – red color (S. pyogenes and Group D Enterococci) o (-) – yellow (No Change)–Strep viridans  Oxidase Test (Doc said, “nevermind the oxidase test”) o Presence of cytochrome oxidase in certain bacteria. Organism will react to it and will change the color of the paper into: o (+) – violet blue (Neisseriae) o (-) – S. pyogenes

Answer 24

o Lancefield Group A, Gram-positive, spherical and in chains o Facultative anaerobe o CATALASE (-), OXIDASE (-), PYR (+) o Killed in 30 min at 60°C o Bacitracin sensitive – 95% accurate  Bacitracin test is performed to determine whether the organism is Streptococcus pyogenes or not  S. pyogenes is the only bacteria sensitive to bacitracin.  In Bacitracin test, a bacitracin disk is placed in a plate agar culture with S. pyogenes. After 18-24 hours of incubation, a zone of inhibition is seen around the disk.

Answer 25

o Blood Agar Plate (BAP): Primary isolation of S. pyogenes o Clinical specimens can be processed by POUR or STREAK PLATE methods o Optimal pH 7.4- 7.6 at 37°C o Since it is facultative, culture growth is enhanced by a low O2 tension or by increased CO2 o Discoid colonies, domed, grayish to opalescent with a zone of β-hemolysis o Utilizes glucose with lactic acid as product

Answer 26

1. M PROTEIN 2. LIPOTEICHOIC ACID 4. PROTEIN F (FIBRONECTIN-BINDING PROTEIN) 3. CAPSULAR POLYSACCHARIDE 4. PROTEIN F (FIBRONECTIN-BINDING PROTEIN)

Answer 27

 Hair-like projections of the streptococcal cell wall  Major virulence factor of S. pyogenes  Antiphagocytic: inhibits opsonization and the complement system  Immunity to Group A strep. is due to antibodies against M Protein (but there are 150 types of M Protein so a patient can have repeated infections with group A S. pyogenes of different M types)  A person may be immune to 1 M protein but may not be immune to another M proteins)  Type I-M induces antibodies that react with human cardiac muscle  There is a cross reaction with human cardiac muscle and heart bound antigens => destruction of heart valves => Rheumatic Fever and Rheumatic Heart Disease  Type II-M: no consequence

Answer 28

 Attached to peptidoglycans and covers the pili which consists partly of M proteins  It mediates buccal epithelial cell adherence (adheres to fibronectin on epithelial cells)  Cytotoxic to host cells

Answer 29

 Made up of hyaluronic acid capsules in some Group A streptococci  Hyaluronic acid mimics the ground substance of connective tissue, so the body will not act against it, and protects the organism from being phagocytosed (anti-phagocytic)

Answer 30

 Mediates attachment to fibronectin in the pharyngeal epithelium  M proteins and lipoteichoic acids also bind to fibronectin

Answer 31

1. HEMOLYSINS 2. PYROGENIC EXOTOXIN (ERYTHROGENIC TOXIN) 3. STREPTOKINASE 4. STREPTODORNASE 5. HYALURONIDASE 6. DIPHOSPHOPYRIDINE NUCLEOTIDASE - Ability to kill leukocytes

Answer 32

Streptolysin O (SLO) o Oxygen labile o Cytolytic activity against RBC, PMN, platelets, and the reason for β-hemolysis o Antigenic and stimulates production of the antibody, antistreptolysin O (ASO) in 10-14 days o ASO Titer: indicator of recent infection with S. pyogenes (>200iu = positive for S. pyogenes) Streptolysin S o Oxygen stable o Lytic for red and white blood cells o Non-antigenic o Responsible for the surface hemolysis seen on BAP

Answer 33

Exotoxin A o Associated with Toxic Shock Syndrome and Scarlet fever o Superantigen producing release of cytokines that mediate shock/tissue injury Exotoxin B o Necrosis of tissues (Necrotizing Fasciitis) Exotoxin C o Causes increased permeability of the blood-brain barrier to toxins and bacteria o Has a pyretic effect on the hypothalamus

Answer 34

 Transform plasminogen into plasmin, a proteolytic enzyme to digest fibrin and other proteins facilitating the rapid spread of the organism  Group C Streptokinase: Used as IV for treatment of pulmonary emboli, coronary artery and venous thrombosis

Answer 35

 Helps to liquefy exudates |  DNAses that degrade the viscous DNA in necrotizing tissue or exudates, aiding the spread of infection

Answer 36

 Splits hyaluronic acid in the ground substance of connective tissue thus aiding in the spread of the microorganism  Spreading factor

Answer 37

A. Diseases due to invasion by S. pyogenes 1. ERYSIPELAS 2. STREPTOCOCCAL CELLULITIS 3. NECROTIZING FASCIITIS (STREP. GANGRENE) 4. PUERPERAL FEVER/ CHILDBED FEVER 5. SCARLET FEVER 6. BACTEREMIA/SEPSIS/STREPTOCOCCAL TOXIC SHOCK SYNDROME B. Local infection with S. pyogenes 1. STREPTOCOCCAL SORE THROAT/PHARYNGITIS/TONSILLITIS 2. STREPTOCCOCUS PYODERMA / IMPETIGO C. Post-Streptococcal Diseases 1. ACUTE GLOMERULONEPHRITIS (AGN 2. RHEUMATIC FEVER

Answer 38

 Skin and subcutaneous tissue infection usually on the face or the lower extremities  With massive brawny/elevated edema and rapidly advancing margin of infection, which is well-delineated

Answer 39

 Rapidly spreading infection of the skin and subcutaneous tissues associated with trauma, burns, and wounds  Erythema, pain, tenderness, and swelling occur  Compared to erysipelas, the affected region in cellulitis is not raised/elevated; the line between involved and uninvolved tissue is indistinct  Not well demarcated

Answer 40

 Rapidly spreading necrosis of the skin, subcutaneous tissue, and fascia (usu within hours) due to M protein that blocks phagocytosis and Exotoxin B that causes the necrosis of the tissues, allowing bacteria to move rapidly through tissues  Symptoms may include a toxic shock-like syndrome, fever, hypotension, multi-organ involvement, as a sunburn-like rash, or a combination of these symptoms.  Must be recognized early. Surgically remove the fascia (Surgical debridement)  Called “Flesh-eating bacteria” due to necrotizing Exotoxin B

Answer 41

 Septicemia (blood poisoning) secondary to an endometritis occurring after birth  May be introduced by the healthcare workers (unhygienic practices) to the mother giving birth  Chills, fever, abdominal distention, and tenderness, serosanguinous vaginal discharge  High mortality rate before the advent of antibiotics  Ignatius Semmelweis – obstetrician in charge of 2 OB wards in Austria. One ward was run by midwives and it has a zero death rate, the other ward is run by medical students and the death rate was hitting the ceiling. The medical students during that time went straight from anatomy lab dissection and deliver babies.  Semmelweis instituted strict hand washing, and the death rate dropped to zero

Answer 42

 Associated with severe, purulent inflammation of the posterior oropharynx and tonsillar areas with a sunburn-like rash on the neck, trunk, and extremities, armpits and inguinal areas (called Pastia’s lines), in response to the release of pyrogenic/erythrogenic exotoxin to which the patient does not have antibodies  “Strawberry tongue” due to an erythrogenic toxin  Transmitted by droplets and fomites  Patient dies of toxic shock syndrome

Answer 43

 Isolation of Group A β-hemolytic streptococci from blood or another normally sterile body site in the presence of shock and multi-organ failure.  Mediated by the production of streptococcal pyrogenic exotoxins causing massive, nonspecific, T-cell activation and cytokine release  Flu-like symptoms, followed shortly by necrotizing soft tissue infection, shock, acute respiratory distress syndrome, and renal failure, followed by death

Answer 44

 Occurs as a subacute nasopharyngitis and sore throat with a thin serous discharge and little fever, and may extend to the middle ear and mastoid (otitis media)  In adults, there is intense nasopharyngitis, tonsillitis, intense redness, edema of the mucous membranes, with purulent exudate  Enlarged, tender cervical lymph nodes around the submandibular area and fever are also present  The pharynx may appear beefy red with exudates  Rheumatic Fever may be a sequelae o Caused by cross reactions between antigens of the heart and joint tissues, and the streptococcal antigen (M Protein) o Characterized by fever, rash, carditis, and arthritis

Answer 45

Treatment o Topical agent such as mupirocin ointment o Systemically treated with penicillin or a first generation cephalosporin o Doc: “Don’t use Silver sulfadiazine cream to your patients with impetigo because when it heals it will turn into black. Your patient will look like dalmatian. So use mupirocin instead.”  Impetigo is best prevented by attention to adequate hygiene!

Answer 46

 1-4 weeks after acute infection |  Not directly related to the bacteria but due to a hypersensitivity response

Answer 47

 Due to nephritogenic strains  Preceded more by skin infection rather than respiratory infection  Due to Ag-Ab complexes on the basement membrane, producing inflammation  Edema, hypertension, azotemia (elevated BUN and serum creatinine levels), hematuria, and proteinuria  Some go into chronicity leading to Renal Failure

Answer 48

Tendency to be reactivated by recurrent strep infection producing cumulative heart damage (valvular damage) o If a patient is getting 6 attacks of tonsillitis a year, for the last 3 or 4 years, it is best recommended that the tonsils be already taken out. o Prophylaxis is given: Benzathine Penicillin injection every 28 days, or Erythromycin tablets 250mg BID for life.

Answer 49

 Smears from Pus o Gram (+), usually seen as single cocci or pairs rather than definite chains  Smears from Throat swab o Rarely contributory o S. viridans has the same appearance as S. pyogenes so check the hemolysis!  Culture o Streptococcus pyogenes: β-hemolysis on BAP; PYR(+)(red color), Inhibited by Bacitracin o Streptococcus viridans: α-hemolytic on BAP; PYR(- (yellow color); Bacitracin (-)  Serologic Test o Estimates rise in antibody titer (ASO Titer)  Antigen Detection Test o Rapid detection of Group A antigen from a throat swab using agglutination o In a (+) test, the latex particles clump together, whereas in a (-) test, they stay separate, giving the suspension a milky appearance

Answer 50

 Penicillin, Erythromycin  Antimicrobials – no effect on established glomerulonephritis or rheumatic fever  Give antimicrobials to prevent post-streptococcal diseases and recurrence

Answer 51

 Nasal discharges are the most dangerous source for spread  Prophylactic antibiotics for surgery patients with known heart valve deformity or prosthetic heart valves  Eradicate Group A Strep in patient with respiratory or skin infections  Prophylaxis for those who have suffered an attack of rheumatic fever

Answer 52

 Can also cause puerperal infection  Immunocompromised/Elderly: can cause cellulitis, skin and soft tissue infections in diabetics, UTI, pneumonia, endocarditis and septic arthritis  Infection during first month of life may also present as Respiratory Distress Syndrome (RDS) o Neonates with meningitis do not present with a stiff neck, which is the classic sign seen in adults. They display nonspecific signs such as fever, vomiting, poor feeding, and irritability.  Penicillin G is the drug of choice: 12-24 million units/day  Other drugs: Erythromycin, Chloramphenicol, Cephalosporins, Vancomycin, Imipenem, Clindamycin

Answer 53

Characteristics o ALL are β-hemolytic EXCEPT S. dysgalactiae o Identified by reactions with specific antisera

Answer 54

Characteristics  Enterococcus o Can grow as diplococci or short chains o Part of the normal enteric flora, inhabits the skin & the upper repiratory, & the gut o Gamma to alpha hemolytic, PYR (+), o Grows in 40% bile o (+) Esculin hydrolysis (black) o (+) growth in 6.5% NaCl (distinguishing feature), halophilic  Streptococcus bovis o Lyse by 6.5% NaCl o The rest are same as enterococcus (grows in 40% biles, can hydrolyze esculin

Answer 55

``` Treatment  Enterococcus o Penicillin + Gentamycin/Streptomycin o Vancomycin + Erythromycin o E. faecium is more likely to be vancomycin or multiply resistant than E. faecalis  S. bovis o Penicillin G ``` **S. anginosus-milleri Group - S. constellatus, - S. intermedius, - S. anginosus, - S. milleri  Normal flora of oral cavity & gingival crevices  May be classified as Viridans strep because they are also α-hemolytic  Dental, brain, lung, and intra-abdominal abscess

Answer 56

Pathogenesis o May reach the bloodstream because of dental manipulation, trauma, or GI or GU instrumentation and cause subacute endocarditis. *(Subacute because it takes about a year before it is fatal. An underlying heart valve structural defect must be present) o Wound infection, meningitis, biliary and intra abdominal infections may occur o S. sanguis: most frequent single species causing Subacute bacterial endocarditis o S. mutans: dental caries & dental plaque

Answer 57

Morphology and Identification  Complex nutritional requirements  Has absolute nutritional requirement for choline  α-hemolytic on BAP  Facultative anaerobe  For primary isolation: o Trypticase soy agar or Tryptone soya agar (TSA) o Brain Heart Infusion broth enriched with 5% defibrinated blood - Young cultures of encapsulated pneumococci produce circular, glistening, dome shaped colonies 1mm in diameter; later center of colonies collapse - Unencapsulated strain produce rough colonies

Answer 58

 OPTOCHIN SENSITIVITY  BILE SOLUBILITY  QUELLUNG REACTION  Animal inoculation (not being done any more)

Answer 59

o Disc with a quinine derivative – inhibits the growth of pneumococci but NOT Viridans strep o Used to distinguish the two organisms because both are α-hemolytic

Answer 60

Two Methods: 1) Agar plate culture 2) Broth culture method

Answer 61

2) Broth culture method Requirements: o Young α-hemolytic colonies o Serum/digest/infusion broth o 10% sodium deoxycholate solution  Few identical alpha hemolytic colonies are picked up and inoculated into suitable broth. Two sets of broth are taken; one marked test and the other marked control. The tubes are incubated at 37˚C for a duration of18-24hrs.  0.5mL of 10% bile solution is added to the "test" tube and incubated at 37˚C for 15 minutes.  Result: A positive test is indicated by the clearance in the "test" tube but not in the "control" tube

Answer 62

o Pneumococci + Polyvalent antiserum = capsule swelling o Most useful and rapid method for identification of pneumococci in sputum, CSF, exudates o Polyvalent antiserum of “omniserum” contains antibodies for all types of Pneumococcus

Answer 63

o Adults: Types 1-8 –75% of Pneumonia cases | o Children: Types 6, 14, 19, 23

Answer 64

o CHOLINE BINDING PROTEIN A  Major adhesin allowing the pneumococcus to attach to carbohydrates on epithelial cells of the human nasopharynx o PNEUMOLYSIN O TOXIN (more important)  Hemolysin  Inhibits chemotaxis of PMN  Toxic effect on respiratory epithelium producing ciliary slowing and epithelial disruption.  Produces paralysis of mucociliary tract that’s why its hard to expectorate  This toxin stimulates production of pro-inflammatory cytokines

Answer 65

Pathology o Outpouring of edema fluid in the alveoli which facilitates microbial multiplication & spread to other alveoli o PMN’s & RBC’s accumulate in the alveoli => consolidation o Pneumococci reaches the bloodstream via the lymphatics, if it reaches the brain => meningitis o Later: phagocytes take up & digest the pneumococci

Answer 66

1. PNEUMOCOCCAL PNEUMONIA/ACUTE BACTERIAL PNEUMONIA 2. OTITIS MEDIA 3. MENINGITIS 4. BACTEREMIA/SEPSIS

Answer 67

 Rarely a primary infection, usually there is a prior infection by a virus  Results only when the normal defense barriers of the respiratory tract are breached  Pneumonia is frequently preceded by an upper or middle respiratory viral infection, which predisposes to S. pneumoniae infection of the pulmonary parenchyma  A leading cause of death, especially in older adults and those whose resistance is impaired  Acute fever, chills, severe pleuritic pain  Cough with “rusty” mucopurulent sputum – (compared to “currant jelly sputum,” which is Klebsiella pneumonia) Complications:  Pleural effusion that can lead to empyema (pus in the pleural space)  Meningitis, pericarditis, endocarditis

Answer 68

 The most common bacterial infection of children

Answer 69

 Pneumococcus is the most common cause of meningitis in adults and of recurrent meningitis in all age groups  Usually preceded by pulmonary infection, URTI, sinusitis, or otitis media

Answer 70

BACTEREMIA/SEPSIS |  Common in individuals who are functionally or anatomically asplenic (sickle cell disease) or immunocompromised

Answer 71

``` Specimens o Nasopharyngeal swab o Pus o Spinal fluid o Blood - Blood should be drawn for culture before antibiotics are given o Sputum  From the lungs, not saliva  Gram Stain: (+) lancet-shaped diplococci – a presumptive diagnosis of pneumococcal pneumonia can be made  Culture: brain heart infusion (BHI), Trypticase Soy agar and broth with 5% blood, BAP  α-hemolytic colonies on BAP  Bile-soluble  Optochin sensitive (+) Quellung reaction ```

Answer 72

o Penicillin G o Cephalosporin (Cefotaxime; Ceftriaxone) o Erythromycin o Chloramphenicol for meningitis (not used in the US because it can cross BBB even without inflammation) * Note that penicillin can penetrate the blood brain barrier only when there is an inflammation

Answer 73

o Vaccines provide 90% protection o Pneumococcal polysaccharide vaccine: PPV immunizes against 23 serotypes (adults); does not work for children, you have to conjugate it to a protein o Pneumococcal conjugate vaccine 13: PCV 13 – effective in infants and toddlers between 6 weeks to 5 years

Answer 74

Resistance to nafcillin (and to methicillin and oxacillin) is independent of β-lactamase production. Resistance to nafcillin is encoded and regulated by a sequence of genes found in a region of the chromosome called the staphylococcal cassette chromosome mec (SCCmec). Specifically, the mecA gene on this locus encodes a low-affinity penicillin binding protein (PBP2a) that is responsible for the resistance. There are 12 different SCCmec types. Types I, II, and III are associated with hospital-acquired infections and may contain genes that encode resistance to other antimicrobials as well. SCCmec type IV has principally been found in community-acquired methicillin-resistant S aureus (CA-MRSA)

Answer 75

Panton-Valentine Leukocidin has two components, and unlike the chromosomally encoded hemolysins, PVL is encoded on a mobile phage. It can kill white blood cells of humans and rabbits. The two components designated as S and F act synergistically on the white blood cell membrane as described for γ toxin. This toxin is an important virulence factor in CA-MRSA infections. Both groups of hemolysins are regulated by agr

Answer 76

Exfoliative toxin B is plasmid mediated and heat labile. These epidermolytic toxins yield the generalized desquamation of the staphylococcal scalded skin syndrome by dissolving the mucopolysaccharide matrix of the epidermis. The toxins are superantigens.

Answer 77

TSST-1 binds to major histocompatibility class (MHC) class II molecules, yielding T-cell stimulation, which promotes the protean manifestations of the toxic shock syndrome. The toxin is associated with fever, shock, and multisystem involvement, including a desquamative skin rash. The gene for TSST-1 is found in about 20% of S aureus isolates, including MRSA.

Answer 78

Binding of sensors to specific extracellular ligands or to a receptor results in a phosphorylation cascade that leads to binding of the regulator to specific DNA sequences. This ultimately leads to activation of transcription-regulating functions. There are several well-described two-component regulatory systems in S aureus. These include agr, the best described, sae RS, srrAB, arlSR, and lytRS.

Answer 79

At low cell density, the promoter P2 is off, and transcriptions of transmembrane protein, AgrB, peptide precursor, AgrD, transmembrane sensor, AgrC, and transcription regulator, Agr A, are at low levels. As cell density increases during stationary growth phase, the AgrC sensor activates the regulator AgrA. AgrA is a DNA-binding protein that activates promoter P2 and promoter P3. Promoter P3 initiates transcription of δ-hemolysin and an effector called RNAIII, which downregulates the expression of surface adhesins and activates secretion of exoproteins at both the transcriptional and translational levels. Agr is also positively controlled by a DNA-binding protein called SarA (encoded by sar)

Answer 80

Sae regulates gene expression at the transcriptional level and is essential for production of α-toxin, β-hemolysins, and coagulase. Its activity is independent from that of agr. SsrAB is important for regulation of virulence factor expression that is influenced by environmental oxygen. The arlSR locus is important to the control of autolysis and decreases the activation of the agr locus. The lytRS locus is also involved in autolysis.

Answer 81

For group A streptococci, this is rhamnose-N-acetylglucosamine; for group B, it is rhamnose-glucosamine polysaccharide; for group C, it is rhamnose- N -acetylgalactosamine; for group D, it is glycerol teichoic acid containing d-alanine and glucose; and for group F, it is glucopyranosyl-N-acetylgalactosamine.

Gram Positive Cocci: Staphylococci and Streptococci (trans 6) Flashcards

(105 cards)