Category:Staphylococcus species

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

Infections in dogs and cats

• S. intermedius causes deep pyoderma, otitis externa, mastitis, endometritis, cystitis, osteomyelitis, wound infections, dyscospondylitis
• S. aureus may cause gastroenteritis

Coagulase-negative staphylococci

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

Enteritis

• 30% strains of Staphylococcus aureus produce potent enterotoxin. T
  • Protein and heat-stable
  • Responsible for staphylococcal food poisoning in man.
• Every reason to assume that acute gastro-intestinal disturbance in small animals may be caused by these enterotoxins BUT not well documented.
• Symptoms last 24-36 hours and include:
  • Acute vomiting
  • Diarrhoea
  • Pain
• The enterotoxins are superantigens.
  • Induce release of cytokines from lymphocytes