Staphylococcus Aureus

•      STAPHYLOCOCCUS AUREUS 

Staphylococcus

•      Sir Alexander Ogston, a Scottish surgeon, first showed in 1880 that a number of human pyogenic diseases were associated with a cluster-forming micro-organism.

•      He introduced the name staphylococcus

   (Greek: staphyle = bunch of grapes; kokkos = berry),

•       now used as the genus name for a group of facultatively anaerobic, catalase-positive, Gram- positive cocci

•      Morphology

•      Gram positive cocci

•      Non sporing, non motile

•      Aerobic or facultative anaerobic

•      1 µm in diameter

•      Arranged singly and in pairs, short chains and irregular grape like clusters

•      Polysaccharide capsule is only rarely found on cells

•      Classification

•      Family    Micrococcaceae

•      Genus    Micrococcus and Staphylococcus

•      Species  S. aureus

              S. saprophyticus

              S. epidermidis

              M. luteus

•      Catalase-positive

        

Ø Catalase activity                 subdividing

Ø Catalase positive genera           Staphylococcus                                              Micrococcus

                                  Stomatococcus

                                  Alloiococcus

Ø Catalase negative genera          Streptococcus

                                  Enterococcus

•      Grow in 10 % NaCl medium @ 18-40 ^o C

•      Most common staph. causing disease

•      Structure of staphylococcal cell wall

•      Pathogenicity and virulence

Capsule

•      Not readily seen in vitro

•      11 types of S.aureus

•      Inhibit phagocytosis by polymorphonuclear leucocytes

Slime layer

•      Loose-boud, water soluble film

•      Facilitates bacterial adherence to tissues or foreign bodies and, consequently biofilm formation ( important for the pathogenesis of coagulase- negative staphylococci)

Peptidoglycan

•      Has endotoxin like activity

•      Induces production of cytokines

•      Activate complement

•      Induces aggregation of polymorphonuclear leucocytes

Teichoic acids and lipoteichoic acids

•      Is an antigenic component of cell wall

•      Bind covalently to peptidoglycan, species specific

•      Abs may be found in systemic staphylococcal disease, particularly endocarditis

Protein A

•      Present on the surface of S. aureus, but not other species

•      Binds to the Fc portion of IgG except IgG3

•      It interferes with opsonisation and phagocytosis of organism

Coagulase ( clumping factor)

•      Prodced by most S.aureus on the cell wall surface

•      Binds to fibrinogen and converts it to fibrin, resulting in aggregates of bacteria

Leukocidin (P-V toxin)

•      Similar to g-toxin in structure, kills WBCs of many animals and release the lysosomal enzymes.

•      Associated with severe pulmonary and cutaneous infections

•      Toxins

Cytotoxins

•      Also called haemolysin –attack cell membrane, lyses the erythrocytes

•      a-toxin:  pore-forming , cytotoxic to many types of cells including muscle          cells.

•      b-toxin:  degrades sphingomyelinase and is toxic for many kinds of cells,             including human RBCs.

•       b-toxin acts together with a-toxin and causes tissue distruction and                      abscess formation

•      g-toxin: biocomponent (slow eluting and fast eluting protein) toxins, pore-         forming.

•      d-toxin: has detergent-like activity on red cells – form channels on it and            leak the cellular contents.

Superantigen exotoxins

•      Exfoliative (epidermolytic) toxins:

•      proteases that split stratum corneum leading to seperation of superficial layer of the epidermis

•      produced by about 5-10% of S. aureus

•      ET-A is heat stable ,coded by chromosome & ET-B is heat labile, coded by plasmid

•      Causes the generalized desquamation of the staphylococcal scalded skin syndrome (SSSS) in children.

•      Toxic shock syndrome toxin-1 (TSST-1):

•      TSS is initiated with the localised growth of toxin producing strains of S.aureus in the vagina or a wound , followed by release of toxin in blood stream 

•      superantigen, associates with fever, shock, desquamative skin rash of toxic shock syndrome. 

•      Enterotoxins:

•      superantigens, at least 10 (A, B, C1, C2, C3, D, E, G, H, and I) soluble toxins produced by about 50% of S. aureus.

•      Heat-stable (100^oC, 30 min.) and resistant to the gastric acid and  gut enzymes.

•      Enterotoxins are produced in carbohydrate and protein foods.

•      Causing staphylococcal food poisoning

•      Enzymes

•      Coagulase:  bound and free forms. – causes clotting of rabbit and                      human plasma

•      Fibrinolysin (staphylokinase): dissolve fibrin clots, spread infection to                  contagious tissues

•      Catalase:   H₂O₂ (toxic)             H₂O

                                                 O₂

•      Hyaluronidase: hydrolyzes hyaluronic acid to facilitate spread of                            S. aureus in tissue.

•      Lipase: hydrolyze lipids and enable styphylococci to survive in                        sebaceous areas of body

             associated with superficial skin infection

•      Nuclease: produced only by S. aureus.( a marker of s.aureus)

•      Penicillinase (ß-lactamase)

•      Staphylococcal diseases

•      Skin infection can be caused by autoinoculation or spread from person-to-person (sometimes animal-to-person)

•      S. aureus is the most common cause of pyogenic skin infections

•      Superficial localised skin infection

•      Folliculitis

•      Furuncle

•      Carbuncle

•      Impetigo

•      Deep localised infection

•      Osteomyelitis

•      Septic arthritis

•      Pneumonia and Empyema

•      Bacteremia and endocarditis

•      Meningitis

•      Folliculitis

•      Small, superficial abscess involving hair follicle or sweat or sebaceous glands

•      Self limiting

•      Furuncle (boils)

•      Deeper seated than folliculitis

•      Pyogenic - abscess

•      Carbuncles

•      Furuncles fused together

•      Occur when multiple, interconnected abscesses involving hair follicles and surrounding deeper connective tissue coalesce

•      Multiple sinus tract

•      Pyoderma (Impetigo)

•      Vesicles filled with clear or yellowish fluid and later on scabs from the vesicles

•      Contagious

•      Usually on face

•      Saphylococcal Toxinosis

•      Staphylococcal Toxic shock syndrome (TSS)

•      Scalded skin syndrome or Ritter’s disease (exfoliative intoxications)

•      Staphylococcal Food poisoning (enterotoxicosis)

•      Staphylococcal Toxic shock syndrome (TSS)

•      Associated with tampon use in early 80’s - ignored for a while but is re-emerging

•      High fever, flushing, sloughing of skin on extremeties

•      Scalded Skin Syndrome

•      Usually in children and neonates

•      Erythema & sunburn-like rash

•      Desquamation due to exfoliatin toxin

•      Staphylococcal Food poisoning (enterotoxicosis)

•      Laboratory diagnosis

1. Specimens

•      Pus

•      CSF

•      Blood

•      Sputum

•      Suspected food, vomit or faeces

•      Nasal swab

•      Swabs from perineum, pieces of hair, umbilical stump

2. Smear

3. Culture

4. Catalase test

5. Coagulase test

6. Serological test

•      2. Smear

•      Microscopic

•      Gram stain

•      Gram positive cocci in cluster

•      3. Culture

•      4.Catalase test

•      H₂O₂ (toxic)                 H₂O

                                     O₂

•      5.Coagulase test

•      Serology: antibodies against teichoic acid can be detected in patients with staphylococcal endocarditis, but not those with osteomyelitis or wound infection. Elevated antibody titers is an indication for prolonged antibiotic treatment

•      Various subtyping methods (such as pulsed-field gel electrophoresis) are used for epidemiological purpose

•      Treatment

•      Antistaphylococcal antibiotics of first choice:

•      Oxacillin (methicillin)

•      Beta-lactam antibiotics (penicillins, cloxacillins, ampicillin, amoxycillin)

•      Cephalosporins of 1^st  generation (cefazolin, cephalotin)

•      Antistaphylococcal antibiotics of second choice:

•      Lincosamide (e.g clindamycin)

•      Glycopeptides (vancomycin, teicoplanin)

•      Linezolid

•      Tigecycline

•      Daptomycin

•      And others

•      MRSA

•      Methicillin resistant S. aureus

•      TOC- Vancomycin

•      Resistant to all penicillins, cephalosporins and

•      Usually multiple resistant

•      Vancomycin resistance is very rare – so far

•      Hospital acquired MRSA

•      Community acquired MRSA

•      Prevention

•      Hand antisepsis is the most important measure in preventing nosocomial infections

•      Sterilisation of instruments and disinfection of environment

•      Proper cleansing of wounds and surgical openings

•      Aseptic use of catheters or indwelling needles, an appropriate use of antiseptics

•      Coagulase negative Staphylococci

•      Common Skin Flora

•      Ubiquitous organism

•      75% of CONS are due to S.epidermidis

•      Has affinity to prosthetic devices (prosthetic heart valves, orthopaedic prostheses, vascular grafts)

•      Neonates, immunocompromised patients

•      Other CONS – S.lugdunensis, S.haemolyticus, S. hominis, S.saprophyticus

•      Micrococcus

•      Gram positive cocci

•      Free living in environment

•      Flora of skin

•      Appear in irregular clusters, tetrads or group of eight

•      Size : 1.0 – 1.8 µm

•      Differentiated from staphylococci by Hugh-Leifson’s oxidation-fermentation test

•      In immunocompromised pt – Pneumonia, meningitis, septic arthritis, bacteraemia, cather related sepsis and peritonitis

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