Staphylococcus

• • gram positive spherical cells, grape like irregular clusters
  • at least 40 species, most frequently species are Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus lugdunensis & Staphylococcus saprophyticus
  • Staphylococcus aureus is major pathogen and coagulase positive
  • coagulase negative staphylococci are normal human microbiota
  • 75% of infection caused by coagulase negative are due to Staphylococcus epidermidis, others Staphylococcus lugdunensis, Staphylococcus warneri, Staphylococcus hominis
  • Staphylococcus saprophyticus cause UTI in young women
• Aetiology

  (3)Culture

  • S.aureus forms grey to golden yellow colonies
  • S.epidermidis forms grey to white
  • haemolysis are produced by staphylococcus aureus
  • Peptostreptococcus and Peptoniphilus are anaerobic cocci often resemble staphylococci in morphology

  (4)Growth characteristics

  • staphylococci produces catalase, which differentiates them from streptococci
  • resistant to drying, heat, and 9% sodium chloride
  • inhibited by 3% hexachlorophene
• Antibiotic resistance in Staphylococcus aureus is often due to beta lactamase production under plasmid control, which is resistant to many penicillins.
• Staphylococcus aureus is also resistant to nafcillin (methicillin and oxacillin) independent of beta lactamase production.
• SCCmec and the mecA gene are associated with vancomycin resistant S.aureus (VRSA).
• Staphylococcus aureus has plasmid mediated resistance to tetracyclines, erythromycins and amynoglycosides.
• Staphylococcus aureus elicits the production of interleukin-1, opsonic antibodies by monocytes, chemoattractant for polymorphonuclear leukocytes, and activate complement system.
• Protein A

  • microbial surface components recognizing adhesive matrix molecules (MSCRAMMS) that attach to host cells.
• Protein A binds to the Fc portion of the IgG except IgG3.
• Staphylococcus aureus have a polysaccharide capsule which inhibit phagocytosis, and are targets for conjugate vaccine.
• Aetiology

  (8)Pathogenicity

  • able to multiply and spread widely
  • produces many extracellular substances
  • produces enzymes and toxins
• Aetiology

  (9)Enzymes - catalase, coagulase, clumping factor responsible for adherence, hyaluronidase or spreading factor, staphylokinase resulting in fibrinolysis, proteinases, lipases and beta lactamase
• Aetiology

  (9)Toxins

  -haemolysins

  -PVL (panton-valentine leukocidein)

  -Exfoliative toxins

  • two distinct proteins A and B act as superantigens
  • Toxin A associated with phage and its heat stable(resists to boiling for 20 mins)
  • Toxin B is plasmid mediated and heat labile
  • cause staphylococcal scalded skin syndrome

  -Toxic shock syndrome toxins

  • toxic shock syndrome toxins 1 (TSST-1) is superantigen yielding T cell stimulation.
  • associated with fever, shock, and multisystem involvement including desquamative skin rash
• Pathogenesis

  (2)Source of infection

  • patients with lesion
  • discharge from small lesions(pustule,paronychia)on the hands of doctors and nurses
  • healthy carrier(chronic carrier)
  • normal carriage of S.aureus

  Carrier sites - anterior nares

  (3)Mode of infection

  1. Contact transmission
  2. Air-borne transmission
  3. Cross transmission - between patients and from carriers(doctors,nurses and hospital workers) to patients.
• Differences(MOI)

  S.aureus - coagulase(+), haemolysis(+), yellow

  S.epidermidis- coagulase(-), haemolysis(-), white, novobiocin sensitive

  S.saprophyticus- non pigmented, haemolysis(-), novobiocin resistant
• Laboratory diagnosis

  (1)Nature of specimens - depends on the site

  (2)Microscopic examination of smear

  • gram stain, examined under oil immersion lens of ordinary light microscope
  • It is not possible to distinguish S. epidermidis and S. aureus

  (3)Culture

  • on milk agar, pigment is rapidly found
  • on blood agar, haemolysis may be found
  • also grow on selective culture media(mannitol salt agar) and enrichment media(salt meat broth)
• Laboratory diagnosis

  (4)Catalse test

  (5)Coagulase test

  (6)Deoxyribonuclease(DNase) test

  (7)Drug sensitivity test

  • disc diffusion methods
  • gene can be detected using polymerase chain reaction(PCR)

  (8)Serology and typing tests

  • serologic tests have little practical value
  • molecular typing techniques is used in epidemic disease