Staphylococcus spp.

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Overview

  • Commensals on skin and mucous membranes of animals and man
  • Enodgenous or exogenous infections
  • Opportunistic pyogenic infections associated with trauma, immusuppression, other infections
  • Coagulase-positive S. aureus and S. intermedius as well as S. hyicus are important pathogens of animals
  • Fairly stable in environment
  • Strains selective for particular species
  • Cause mastitis, tick pyaemia, exudative epidermitis, botryomycosis and pyoderma

Characteristics

  • Clusters of Gram-positive cocci
  • At least 30 species
  • Facultative anaerobes
  • Catalase positive, oxidase negative, non-motile
  • Virulent strains are coagulase positive
  • Grow on non-enriched media
  • White or golden colonies
  • S. aureus and S. intermedius produce double haemolysis; they produce alpha-haemolysin and beta-haemolysin
  • S. hyicus is non-haemolytic
  • Coagulase-negative strains vary in ability to cause haemolysis
  • Slide test to detect bound coagulase (clumping factor) on surface of bacteria; bacteria clump within 1-2 minutes
  • Tube test detects free coagulase, secreted by bactera; positive result indicated by clot formation in tube following 24-hour incubation; free coagulase converts prothrombin to thrombin which converts fibrinogen to fibrin
  • Biochemical tests to differentiate S. aureus and S. intermedius
  • PCR to differentiate species

Pathogenesis and pathogenicity

  • Cause suppurative lesions
  • Trauma or immunosuppression predispose to infection
  • Capsular polysaccharide, teichoic acids and potein A prevent opsonisation and therefore phagocytosis
  • Cell wall proteins bind fibronectin and fibrinogen, allowing bacteria to attched to damaged tissues
  • Coagulase, DNase and protein A production mark pathogenicity
  • Haemolysins made by S. aureus and S. intermedius act as toxins
  • Alpha-haemolysin prouces a narrow zone of complete haemolysis; this toxin causes necrosis and targets mast cells and plasma cells, whose contents cause damage
  • Beta-haemolysin produces a wide zone of incomplete haemolysis; damages membranes and causes leakage, contributing to necrosis
  • Also gamma and delta toxins
  • Toxic shock syndrome toxin acts as a superantigen, which causes T cell proliferation and production of cytokines, leading to cardiovascular shock, with microthrombus formation in capillaries
  • Enterotoxins produced by some strains of S. aureus cause food poisoning in humans
  • Proteases, hyaluronidases and lipases facilitate survival of bacteria and spread and tissue destruction

Diagnosis

  • Clusters of bacteria in Gram-stained smears of pus
  • Culture on selective blood agar and MacConkey agar
  • No growth on MacConkey
  • Colony characteristics, haemolysis, catalase and coagulase production
  • Phage typing for epidemiological studies


Bovine mastitis

  • S. aureus is a common cause of mastitisin cattle worldwide
  • Most infections subclinical
  • Systemic infection can occur with peracute and gangrenous forms
  • In gangrenous mastitis, the quarter may become necrotic and slough off; alpha toxin causes necrosis of smooth muscle in blood vessel walls, reducing blood flow to the affected quarter, and causes release of lysomal enzymes from leukocytes


Tick pyaemia

  • Infection of lambs with S. aureus in hill-grazing areas of the UK
  • Lambs carry S. aureus on their skin and nasal mucosa; infection via skin trauma including tick bites
  • Ixodes ricinus tick acts as a vector for Ehrlichia phagocytophila, which causes immunosuppression in lambs, predisposing to staphylococcal infection
  • Acute septicaemia and death or localised abscess formation in many organs
  • Arthritis, posterior paresis and ill-thrift
  • Microscopic identification of bacteria in pus and isolation of S. aureus
  • Prophylactic antibiotics e.g. tetracyclines initiated at 1 week of age may prevent infection
  • Tick control important


Exudative epidermitis (Greasy pig disease)

  • Worldwide disease of pigs under 3 months of age caused by S. hyicus
  • Highly contagious and persists in the environment for long periods
  • S. hyicus produces exfoliative toxin that causes separation of cells in stratum spinosum resulting in rapid intraepidermal spread of organisms
  • Similar to “scalded skin syndrome” in human neonates, due to skin infection by exfoliative toxin-producing Staphylococcus aureus
  • Excessive sebacious secretion and exudation on surface of skin
  • Pigs are anorexic, febrile, depressed and have a greasy, non-pruritic dermatitis
  • SKin thickened, wrinkled and scaley
  • Acute death in piglets under 3 weeks due to dehydration and septicaemia
  • 20-100% morbidity; up to 90% mortality
  • Organism enters skin via abrasions e.g. bite wounds
  • Carried in vaginal mucosa and skin of sows
  • Predisposed by lack of milk, weaning and other infections
  • Piglets from non-immune sows are predisposed
  • Passive transfer of antibodies from immune sows and development of immunity with age protect against disease
  • Isolation from skin lesions
  • Early systemic antibiotics amd topical antiseptics/antibiotics useful
  • Disinfection after outbreak
  • Wash sows before farrowing
  • Differential diagnosis:
    • Mange (Sarcoptes scabiei var. suis)
    • Swine parakeratosis (zinc and essential fatty acid deficiency)
    • Porcine juvenile pustular psoriasiform dermatitis (collarettes or rings typically on the ventrum of young pigs)
    • Dermatosis vegetans (associated with a giant cell pneumonia)
    • Dermatophytosis (most commonly Microsporum nanum)

Botryomycosis

  • Chronic, suppurative granulomatous condition
  • S. aureus
  • Occurs following castration of horses due to infection of stump of spermatic cors
  • Occurs in mammary tissues of sows
  • Mass of fibrous tissue containing pus and sinus tracts


Infections in dogs and cats


Other infections caused by S. aureus


Coagulase-negative staphylococci

  • Usually harmless commensals or secondary invaders
  • May adhere to indwelling catheters leading to urinary tract infections
  • Often display multiple antibiotic resistance