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

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

Other infections caused by S. aureus

• Impetigo in cattle and pigs
• Permeability types of pulmonary oedema
• Haemorrhagic disease by secondary thrombocytopenic disease
• May infect joints of sheep; arthritis in pigs
• Dermatitis in sheep and goats
• Arthritis and septicaemia in turkeys
• Bumblefoot and omphalitis in chickens

Coagulase-negative staphylococci

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